Handle ground. (#469) r=nalexander

query-algebrizer/Cargo.toml

@@ -15,4 +15,8 @@ path = "../query"
 
 # Only for tests.
 [dev-dependencies.mentat_query_parser]
-path = "../query-parser"
+path = "../query-parser"
+
+[dev-dependencies]
+itertools = "0.5"
+maplit = "0.1"

query-algebrizer/src/clauses/mod.rs

@@ -67,6 +67,7 @@ mod not;
 mod pattern;
 mod predicate;
 mod resolve;
+mod where_fn;
 
 use validate::{
     validate_not_join,
@@ -141,6 +142,7 @@ impl<K: Clone + Ord, V: Clone> Intersection<K> for BTreeMap<K, V> {
 ///
 /// - Ordinary pattern clauses turn into `FROM` parts and `WHERE` parts using `=`.
 /// - Predicate clauses turn into the same, but with other functions.
+/// - Function clauses turn into `WHERE` parts using function-specific comparisons.
 /// - `not` turns into `NOT EXISTS` with `WHERE` clauses inside the subquery to
 ///   bind it to the outer variables, or adds simple `WHERE` clauses to the outer
 ///   clause.
@@ -228,6 +230,7 @@ impl Debug for ConjoiningClauses {
         fmt.debug_struct("ConjoiningClauses")
             .field("empty_because", &self.empty_because)
             .field("from", &self.from)
+            .field("computed_tables", &self.computed_tables)
             .field("wheres", &self.wheres)
             .field("column_bindings", &self.column_bindings)
             .field("input_variables", &self.input_variables)
@@ -330,6 +333,12 @@ impl ConjoiningClauses {
 }
 
 impl ConjoiningClauses {
+    /// Be careful with this. It'll overwrite existing bindings.
+    pub fn bind_value(&mut self, var: &Variable, value: TypedValue) {
+        self.constrain_var_to_type(var.clone(), value.value_type());
+        self.value_bindings.insert(var.clone(), value);
+    }
+
     pub fn bound_value(&self, var: &Variable) -> Option<TypedValue> {
         self.value_bindings.get(var).cloned()
     }
@@ -350,6 +359,10 @@ impl ConjoiningClauses {
         }
     }
 
+    pub fn known_type_set(&self, var: &Variable) -> ValueTypeSet {
+        self.known_types.get(var).cloned().unwrap_or(ValueTypeSet::any())
+    }
+
     pub fn bind_column_to_var<C: Into<Column>>(&mut self, schema: &Schema, table: TableAlias, column: C, var: Variable) {
         let column = column.into();
         // Do we have an external binding for this?
@@ -469,14 +482,15 @@ impl ConjoiningClauses {
     /// Constrains the var if there's no existing type.
     /// Marks as known-empty if it's impossible for this type to apply because there's a conflicting
     /// type already known.
-    fn constrain_var_to_type(&mut self, variable: Variable, this_type: ValueType) {
+    fn constrain_var_to_type(&mut self, var: Variable, this_type: ValueType) {
         // Is there an existing mapping for this variable?
         // Any known inputs have already been added to known_types, and so if they conflict we'll
         // spot it here.
-        if let Some(existing) = self.known_types.insert(variable.clone(), ValueTypeSet::of_one(this_type)) {
+        let this_type_set = ValueTypeSet::of_one(this_type);
+        if let Some(existing) = self.known_types.insert(var.clone(), this_type_set) {
             // There was an existing mapping. Does this type match?
             if !existing.contains(this_type) {
-                self.mark_known_empty(EmptyBecause::TypeMismatch(variable, existing, this_type));
+                self.mark_known_empty(EmptyBecause::TypeMismatch { var, existing, desired: this_type_set });
             }
         }
     }
@@ -535,10 +549,9 @@ impl ConjoiningClauses {
             Entry::Occupied(mut e) => {
                 let intersected: ValueTypeSet = types.intersection(e.get());
                 if intersected.is_empty() {
-                    let mismatching_type = types.exemplar().expect("types isn't none");
-                    let reason = EmptyBecause::TypeMismatch(e.key().clone(),
-                                                            e.get().clone(),
-                                                            mismatching_type);
+                    let reason = EmptyBecause::TypeMismatch { var: e.key().clone(),
+                                                              existing: e.get().clone(),
+                                                              desired: types };
                     empty_because = Some(reason);
                 }
                 // Always insert, even if it's empty!
@@ -828,6 +841,9 @@ impl ConjoiningClauses {
             WhereClause::Pred(p) => {
                 self.apply_predicate(schema, p)
             },
+            WhereClause::WhereFn(f) => {
+                self.apply_where_fn(schema, f)
+            },
             WhereClause::OrJoin(o) => {
                 validate_or_join(&o)?;
                 self.apply_or_join(schema, o)
@@ -841,6 +857,18 @@ impl ConjoiningClauses {
     }
 }
 
+pub trait PushComputed {
+    fn push_computed(&mut self, item: ComputedTable) -> DatomsTable;
+}
+
+impl PushComputed for Vec<ComputedTable> {
+    fn push_computed(&mut self, item: ComputedTable) -> DatomsTable {
+        let next_index = self.len();
+        self.push(item);
+        DatomsTable::Computed(next_index)
+    }
+}
+
 // These are helpers that tests use to build Schema instances.
 #[cfg(test)]
 fn associate_ident(schema: &mut Schema, i: NamespacedKeyword, e: Entid) {

query-algebrizer/src/clauses/not.rs

@@ -32,7 +32,7 @@ impl ConjoiningClauses {
     pub fn apply_not_join(&mut self, schema: &Schema, not_join: NotJoin) -> Result<()> {
         let unified = match not_join.unify_vars {
             UnifyVars::Implicit => not_join.collect_mentioned_variables(),
-            UnifyVars::Explicit(vs) => vs.into_iter().collect(),
+            UnifyVars::Explicit(vs) => vs,
         };
 
         let mut template = self.use_as_template(&unified);

query-algebrizer/src/clauses/or.rs

@@ -29,7 +29,10 @@ use mentat_query::{
     WhereClause,
 };
 
-use clauses::ConjoiningClauses;
+use clauses::{
+    ConjoiningClauses,
+    PushComputed,
+};
 
 use errors::{
     Result,
@@ -74,18 +77,6 @@ fn _simply_matches_value_place(left: &PatternValuePlace, right: &PatternValuePla
     }
 }
 
-trait PushComputed {
-    fn push_computed(&mut self, item: ComputedTable) -> DatomsTable;
-}
-
-impl PushComputed for Vec<ComputedTable> {
-    fn push_computed(&mut self, item: ComputedTable) -> DatomsTable {
-        let next_index = self.len();
-        self.push(item);
-        DatomsTable::Computed(next_index)
-    }
-}
-
 pub enum DeconstructedOrJoin {
     KnownSuccess,
     KnownEmpty(EmptyBecause),
@@ -561,7 +552,7 @@ impl ConjoiningClauses {
         let (join_clauses, unify_vars, mentioned_vars) = or_join.dismember();
         let projected = match unify_vars {
             UnifyVars::Implicit => mentioned_vars.into_iter().collect(),
-            UnifyVars::Explicit(vs) => vs.into_iter().collect(),
+            UnifyVars::Explicit(vs) => vs,
         };
 
         let template = self.use_as_template(&projected);
@@ -822,6 +813,7 @@ mod testing {
         });
         schema
     }
+
     /// Test that if all the attributes in an `or` fail to resolve, the entire thing fails.
     #[test]
     fn test_schema_based_failure() {
@@ -1119,4 +1111,4 @@ mod testing {
                     [_ :foo/height ?x]]"#;
         compare_ccs(alg(&schema, query), alg(&schema, simple));
     }
-}
+}

query-algebrizer/src/clauses/pattern.rs

@@ -801,7 +801,11 @@ mod testing {
 
         assert!(cc.is_known_empty());
         assert_eq!(cc.empty_because.unwrap(),
-                   EmptyBecause::TypeMismatch(y.clone(), ValueTypeSet::of_one(ValueType::String), ValueType::Boolean));
+                   EmptyBecause::TypeMismatch {
+                       var: y.clone(),
+                       existing: ValueTypeSet::of_one(ValueType::String),
+                       desired: ValueTypeSet::of_one(ValueType::Boolean),
+                   });
     }
 
     #[test]
@@ -839,7 +843,11 @@ mod testing {
 
         assert!(cc.is_known_empty());
         assert_eq!(cc.empty_because.unwrap(),
-                   EmptyBecause::TypeMismatch(x.clone(), ValueTypeSet::of_one(ValueType::Ref), ValueType::Boolean));
+                   EmptyBecause::TypeMismatch {
+                       var: x.clone(),
+                       existing: ValueTypeSet::of_one(ValueType::Ref),
+                       desired: ValueTypeSet::of_one(ValueType::Boolean),
+                   });
     }
 
     #[test]

query-algebrizer/src/clauses/predicate.rs

@@ -30,15 +30,13 @@ use types::{
 
 /// Application of predicates.
 impl ConjoiningClauses {
-    /// There are several kinds of predicates/functions in our Datalog:
+    /// There are several kinds of predicates in our Datalog:
     /// - A limited set of binary comparison operators: < > <= >= !=.
     ///   These are converted into SQLite binary comparisons and some type constraints.
-    /// - A set of predicates like `fulltext` and `get-else` that are translated into
-    ///   SQL `MATCH`es or joins, yielding bindings.
     /// - In the future, some predicates that are implemented via function calls in SQLite.
     ///
     /// At present we have implemented only the five built-in comparison binary operators.
-    pub fn apply_predicate<'s, 'p>(&mut self, schema: &'s Schema, predicate: Predicate) -> Result<()> {
+    pub fn apply_predicate<'s>(&mut self, schema: &'s Schema, predicate: Predicate) -> Result<()> {
         // Because we'll be growing the set of built-in predicates, handling each differently,
         // and ultimately allowing user-specified predicates, we match on the predicate name first.
         if let Some(op) = NumericComparison::from_datalog_operator(predicate.operator.0.as_str()) {
@@ -53,7 +51,7 @@ impl ConjoiningClauses {
     /// - Ensures that the predicate functions name a known operator.
     /// - Accumulates a `NumericInequality` constraint into the `wheres` list.
     #[allow(unused_variables)]
-    pub fn apply_numeric_predicate<'s, 'p>(&mut self, schema: &'s Schema, comparison: NumericComparison, predicate: Predicate) -> Result<()> {
+    pub fn apply_numeric_predicate<'s>(&mut self, schema: &'s Schema, comparison: NumericComparison, predicate: Predicate) -> Result<()> {
         if predicate.args.len() != 2 {
             bail!(ErrorKind::InvalidNumberOfArguments(predicate.operator.clone(), predicate.args.len(), 2));
         }
@@ -224,8 +222,10 @@ mod testing {
 
         assert!(cc.is_known_empty());
         assert_eq!(cc.empty_because.unwrap(),
-                   EmptyBecause::TypeMismatch(y.clone(),
-                                              ValueTypeSet::of_numeric_types(),
-                                              ValueType::String));
+                   EmptyBecause::TypeMismatch {
+                       var: y.clone(),
+                       existing: ValueTypeSet::of_numeric_types(),
+                       desired: ValueTypeSet::of_one(ValueType::String),
+                   });
     }
-}
+}

query-algebrizer/src/clauses/resolve.rs

@@ -50,15 +50,16 @@ impl ConjoiningClauses {
             },
             // Can't be an entid.
             EntidOrInteger(i) => Ok(QueryValue::TypedValue(TypedValue::Long(i))),
-            Ident(_) |
+            IdentOrKeyword(_) |
             SrcVar(_) |
             Constant(NonIntegerConstant::Boolean(_)) |
             Constant(NonIntegerConstant::Text(_)) |
             Constant(NonIntegerConstant::Uuid(_)) |
             Constant(NonIntegerConstant::Instant(_)) |        // Instants are covered elsewhere.
-            Constant(NonIntegerConstant::BigInteger(_)) => {
+            Constant(NonIntegerConstant::BigInteger(_)) |
+            Vector(_) => {
                 self.mark_known_empty(EmptyBecause::NonNumericArgument);
-                bail!(ErrorKind::NonNumericArgument(function.clone(), position));
+                bail!(ErrorKind::InvalidArgument(function.clone(), "numeric", position));
             },
             Constant(NonIntegerConstant::Float(f)) => Ok(QueryValue::TypedValue(TypedValue::Double(f))),
         }
@@ -78,7 +79,7 @@ impl ConjoiningClauses {
                     .ok_or_else(|| Error::from_kind(ErrorKind::UnboundVariable(var.name())))
             },
             EntidOrInteger(i) => Ok(QueryValue::PrimitiveLong(i)),
-            Ident(_) => unimplemented!(),     // TODO
+            IdentOrKeyword(_) => unimplemented!(),     // TODO
             Constant(NonIntegerConstant::Boolean(val)) => Ok(QueryValue::TypedValue(TypedValue::Boolean(val))),
             Constant(NonIntegerConstant::Float(f)) => Ok(QueryValue::TypedValue(TypedValue::Double(f))),
             Constant(NonIntegerConstant::Text(s)) => Ok(QueryValue::TypedValue(TypedValue::typed_string(s.as_str()))),
@@ -86,6 +87,7 @@ impl ConjoiningClauses {
             Constant(NonIntegerConstant::Instant(u)) => Ok(QueryValue::TypedValue(TypedValue::Instant(u))),
             Constant(NonIntegerConstant::BigInteger(_)) => unimplemented!(),
             SrcVar(_) => unimplemented!(),
+            Vector(_) => unimplemented!(),    // TODO
         }
     }
 }

query-algebrizer/src/clauses/where_fn.rs

@@ -0,0 +1,570 @@
+// Copyright 2016 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+use std::rc::Rc;
+
+use mentat_core::{
+    Schema,
+    SQLValueType,
+    TypedValue,
+    ValueType,
+};
+
+use mentat_query::{
+    Binding,
+    FnArg,
+    NonIntegerConstant,
+    Variable,
+    VariableOrPlaceholder,
+    WhereFn,
+};
+
+use clauses::{
+    ConjoiningClauses,
+    PushComputed,
+};
+
+use errors::{
+    BindingError,
+    ErrorKind,
+    Result,
+};
+
+use super::QualifiedAlias;
+
+use types::{
+    ColumnConstraint,
+    ComputedTable,
+    EmptyBecause,
+    SourceAlias,
+    ValueTypeSet,
+    VariableColumn,
+};
+
+macro_rules! coerce_to_typed_value {
+    ($var: ident, $val: ident, $types: expr, $type: path, $constructor: path) => { {
+        Ok(if !$types.contains($type) {
+               Impossible(EmptyBecause::TypeMismatch {
+                   var: $var.clone(),
+                   existing: $types,
+                   desired: ValueTypeSet::of_one($type),
+               })
+           } else {
+               Val($constructor($val).into())
+           })
+    } }
+}
+
+enum ValueConversion {
+    Val(TypedValue),
+    Impossible(EmptyBecause),
+}
+
+/// Conversion of FnArgs to TypedValues.
+impl ConjoiningClauses {
+    /// Convert the provided `FnArg` to a `TypedValue`.
+    /// The conversion depends on, and can fail because of:
+    /// - Existing known types of a variable to which this arg will be bound.
+    /// - Existing bindings of a variable `FnArg`.
+    fn typed_value_from_arg<'s>(&self, schema: &'s Schema, var: &Variable, arg: FnArg, known_types: ValueTypeSet) -> Result<ValueConversion> {
+        use self::ValueConversion::*;
+        if known_types.is_empty() {
+            // If this happens, it likely means the pattern has already failed!
+            return Ok(Impossible(EmptyBecause::TypeMismatch {
+                var: var.clone(),
+                existing: known_types,
+                desired: ValueTypeSet::any(),
+            }));
+        }
+
+        match arg {
+            // Longs are potentially ambiguous: they might be longs or entids.
+            FnArg::EntidOrInteger(x) => {
+                match (ValueType::Ref.accommodates_integer(x),
+                       known_types.contains(ValueType::Ref),
+                       known_types.contains(ValueType::Long)) {
+                    (true, true, true) => {
+                        // Ambiguous: this arg could be an entid or a long.
+                        // We default to long.
+                        Ok(Val(TypedValue::Long(x)))
+                    },
+                    (true, true, false) => {
+                        // This can only be a ref.
+                        Ok(Val(TypedValue::Ref(x)))
+                    },
+                    (_, false, true) => {
+                        // This can only be a long.
+                        Ok(Val(TypedValue::Long(x)))
+                    },
+                    (false, true, _) => {
+                        // This isn't a valid ref, but that's the type to which this must conform!
+                        Ok(Impossible(EmptyBecause::TypeMismatch {
+                            var: var.clone(),
+                            existing: known_types,
+                            desired: ValueTypeSet::of_longs(),
+                        }))
+                    },
+                    (_, false, false) => {
+                        // Non-overlapping type sets.
+                        Ok(Impossible(EmptyBecause::TypeMismatch {
+                            var: var.clone(),
+                            existing: known_types,
+                            desired: ValueTypeSet::of_longs(),
+                        }))
+                    },
+                }
+            },
+
+            // If you definitely want to look up an ident, do it before running the query.
+            FnArg::IdentOrKeyword(x) => {
+                match (known_types.contains(ValueType::Ref),
+                       known_types.contains(ValueType::Keyword)) {
+                    (true, true) => {
+                        // Ambiguous: this could be a keyword or an ident.
+                        // Default to keyword.
+                        Ok(Val(TypedValue::Keyword(Rc::new(x))))
+                    },
+                    (true, false) => {
+                        // This can only be an ident. Look it up!
+                        match schema.get_entid(&x).map(TypedValue::Ref) {
+                            Some(e) => Ok(Val(e)),
+                            None => Ok(Impossible(EmptyBecause::UnresolvedIdent(x.clone()))),
+                        }
+                    },
+                    (false, true) => {
+                        Ok(Val(TypedValue::Keyword(Rc::new(x))))
+                    },
+                    (false, false) => {
+                        Ok(Impossible(EmptyBecause::TypeMismatch {
+                            var: var.clone(),
+                            existing: known_types,
+                            desired: ValueTypeSet::of_keywords(),
+                        }))
+                    },
+                }
+            },
+
+            FnArg::Variable(in_var) => {
+                // TODO: technically you could ground an existing variable inside the query….
+                if !self.input_variables.contains(&in_var) {
+                    bail!(ErrorKind::UnboundVariable((*in_var.0).clone()));
+                }
+                match self.bound_value(&in_var) {
+                    // The type is already known if it's a bound variable….
+                    Some(ref in_value) => Ok(Val(in_value.clone())),
+                    None => bail!(ErrorKind::UnboundVariable((*in_var.0).clone())),
+                }
+            },
+
+            // This isn't implemented yet.
+            FnArg::Constant(NonIntegerConstant::BigInteger(_)) => unimplemented!(),
+
+            // These don't make sense here.
+            FnArg::Vector(_) |
+            FnArg::SrcVar(_) => bail!(ErrorKind::InvalidGroundConstant),
+
+            // These are all straightforward.
+            FnArg::Constant(NonIntegerConstant::Boolean(x)) => {
+                coerce_to_typed_value!(var, x, known_types, ValueType::Boolean, TypedValue::Boolean)
+            },
+            FnArg::Constant(NonIntegerConstant::Instant(x)) => {
+                coerce_to_typed_value!(var, x, known_types, ValueType::Instant, TypedValue::Instant)
+            },
+            FnArg::Constant(NonIntegerConstant::Uuid(x)) => {
+                coerce_to_typed_value!(var, x, known_types, ValueType::Uuid, TypedValue::Uuid)
+            },
+            FnArg::Constant(NonIntegerConstant::Float(x)) => {
+                coerce_to_typed_value!(var, x, known_types, ValueType::Double, TypedValue::Double)
+            },
+            FnArg::Constant(NonIntegerConstant::Text(x)) => {
+                coerce_to_typed_value!(var, x, known_types, ValueType::String, TypedValue::String)
+            },
+        }
+    }
+}
+
+/// Application of `where` functions.
+impl ConjoiningClauses {
+    /// There are several kinds of functions binding variables in our Datalog:
+    /// - A set of functions like `ground`, fulltext` and `get-else` that are translated into SQL
+    ///   `VALUES`, `MATCH`, or `JOIN`, yielding bindings.
+    /// - In the future, some functions that are implemented via function calls in SQLite.
+    ///
+    /// At present we have implemented only a limited selection of functions.
+    pub fn apply_where_fn<'s>(&mut self, schema: &'s Schema, where_fn: WhereFn) -> Result<()> {
+        // Because we'll be growing the set of built-in functions, handling each differently, and
+        // ultimately allowing user-specified functions, we match on the function name first.
+        match where_fn.operator.0.as_str() {
+            "ground" => self.apply_ground(schema, where_fn),
+            _ => bail!(ErrorKind::UnknownFunction(where_fn.operator.clone())),
+        }
+    }
+
+    fn apply_ground_place<'s>(&mut self, schema: &'s Schema, var: VariableOrPlaceholder, arg: FnArg) -> Result<()> {
+        match var {
+            VariableOrPlaceholder::Placeholder => Ok(()),
+            VariableOrPlaceholder::Variable(var) => self.apply_ground_var(schema, var, arg),
+        }
+    }
+
+    /// Constrain the CC to associate the given var with the given ground argument.
+    /// Marks known-empty on failure.
+    fn apply_ground_var<'s>(&mut self, schema: &'s Schema, var: Variable, arg: FnArg) -> Result<()> {
+        let known_types = self.known_type_set(&var);
+        match self.typed_value_from_arg(schema, &var, arg, known_types)? {
+            ValueConversion::Val(value) => self.apply_ground_value(var, value),
+            ValueConversion::Impossible(because) => {
+                self.mark_known_empty(because);
+                Ok(())
+            },
+        }
+    }
+
+    /// Marks known-empty on failure.
+    fn apply_ground_value(&mut self, var: Variable, value: TypedValue) -> Result<()> {
+        if let Some(existing) = self.bound_value(&var) {
+            if existing != value {
+                self.mark_known_empty(EmptyBecause::ConflictingBindings {
+                    var: var.clone(),
+                    existing: existing.clone(),
+                    desired: value,
+                });
+                return Ok(())
+            }
+        } else {
+            self.bind_value(&var, value.clone());
+        }
+
+        let vt = value.value_type();
+
+        // Check to see whether this variable is already associated to a column.
+        // If so, we want to add an equality filter (or, in the future, redo the existing patterns).
+        if let Some(QualifiedAlias(table, column)) = self.column_bindings
+                                                         .get(&var)
+                                                         .and_then(|vec| vec.get(0).cloned()) {
+            self.constrain_column_to_constant(table, column, value);
+        }
+
+        // Are we also trying to figure out the type of the value when the query runs?
+        // If so, constrain that!
+        if let Some(table) = self.extracted_types.get(&var)
+                                                 .map(|qa| qa.0.clone()) {
+            self.wheres.add_intersection(ColumnConstraint::HasType(table, vt));
+        }
+
+        Ok(())
+    }
+
+    /// Take a relation: a matrix of values which will successively bind to named variables of
+    /// the provided types.
+    /// Construct a computed table to yield this relation.
+    /// This function will panic if some invariants are not met.
+    fn collect_named_bindings<'s>(&mut self, schema: &'s Schema, names: Vec<Variable>, types: Vec<ValueType>, values: Vec<TypedValue>) {
+        if values.is_empty() {
+            return;
+        }
+
+        assert!(!names.is_empty());
+        assert_eq!(names.len(), types.len());
+        assert!(values.len() >= names.len());
+        assert_eq!(values.len() % names.len(), 0);      // It's an exact multiple.
+
+        let named_values = ComputedTable::NamedValues {
+            names: names.clone(),
+            values: values,
+        };
+
+        let table = self.computed_tables.push_computed(named_values);
+        let alias = self.next_alias_for_table(table);
+
+        // Stitch the computed table into column_bindings, so we get cross-linking.
+        for (name, ty) in names.iter().zip(types.into_iter()) {
+            self.constrain_var_to_type(name.clone(), ty);
+            self.bind_column_to_var(schema, alias.clone(), VariableColumn::Variable(name.clone()), name.clone());
+        }
+
+        self.from.push(SourceAlias(table, alias));
+    }
+
+    pub fn apply_ground<'s>(&mut self, schema: &'s Schema, where_fn: WhereFn) -> Result<()> {
+        if where_fn.args.len() != 1 {
+            bail!(ErrorKind::InvalidNumberOfArguments(where_fn.operator.clone(), where_fn.args.len(), 1));
+        }
+
+        let mut args = where_fn.args.into_iter();
+
+        if where_fn.binding.is_empty() {
+            // The binding must introduce at least one bound variable.
+            bail!(ErrorKind::InvalidBinding(where_fn.operator.clone(), BindingError::NoBoundVariable));
+        }
+
+        if !where_fn.binding.is_valid() {
+            // The binding must not duplicate bound variables.
+            bail!(ErrorKind::InvalidBinding(where_fn.operator.clone(), BindingError::RepeatedBoundVariable));
+        }
+
+        // Scalar and tuple bindings are a little special: because there's only one value,
+        // we can immediately substitute the value as a known value in the CC, additionally
+        // generating a WHERE clause if columns have already been bound.
+        match (where_fn.binding, args.next().unwrap()) {
+            (Binding::BindScalar(var), constant) =>
+                self.apply_ground_var(schema, var, constant),
+
+            (Binding::BindTuple(places), FnArg::Vector(children)) => {
+                // Just the same, but we bind more than one column at a time.
+                if children.len() != places.len() {
+                    // Number of arguments don't match the number of values. TODO: better error message.
+                    bail!(ErrorKind::GroundBindingsMismatch);
+                }
+                for (place, arg) in places.into_iter().zip(children.into_iter()) {
+                    self.apply_ground_place(schema, place, arg)?  // TODO: short-circuit on impossible.
+                }
+                Ok(())
+            },
+
+            // Collection bindings and rel bindings are similar in that they are both
+            // implemented as a subquery with a projection list and a set of values.
+            // The difference is that BindColl has only a single variable, and its values
+            // are all in a single structure. That makes it substantially simpler!
+            (Binding::BindColl(var), FnArg::Vector(children)) => {
+                if children.is_empty() {
+                    bail!(ErrorKind::InvalidGroundConstant);
+                }
+
+                // Turn a collection of arguments into a Vec of `TypedValue`s of the same type.
+                let known_types = self.known_type_set(&var);
+                // Check that every value has the same type.
+                let mut accumulated_types = ValueTypeSet::none();
+                let mut skip: Option<EmptyBecause> = None;
+                let values = children.into_iter()
+                                     .filter_map(|arg| -> Option<Result<TypedValue>> {
+                                         // We need to get conversion errors out.
+                                         // We also want to mark known-empty on impossibilty, but
+                                         // still detect serious errors.
+                                         match self.typed_value_from_arg(schema, &var, arg, known_types) {
+                                             Ok(ValueConversion::Val(tv)) => {
+                                                 if accumulated_types.insert(tv.value_type()) &&
+                                                    !accumulated_types.is_unit() {
+                                                     // Values not all of the same type.
+                                                     Some(Err(ErrorKind::InvalidGroundConstant.into()))
+                                                 } else {
+                                                     Some(Ok(tv))
+                                                 }
+                                             },
+                                             Ok(ValueConversion::Impossible(because)) => {
+                                                 // Skip this value.
+                                                 skip = Some(because);
+                                                 None
+                                             },
+                                             Err(e) => Some(Err(e.into())),
+                                         }
+                                     })
+                                     .collect::<Result<Vec<TypedValue>>>()?;
+
+                if values.is_empty() {
+                    let because = skip.expect("we skipped all rows for a reason");
+                    self.mark_known_empty(because);
+                    return Ok(());
+                }
+
+                // Otherwise, we now have the values and the type.
+                let types = vec![accumulated_types.exemplar().unwrap()];
+                let names = vec![var.clone()];
+
+                self.collect_named_bindings(schema, names, types, values);
+                Ok(())
+            },
+
+            (Binding::BindRel(places), FnArg::Vector(rows)) => {
+                if rows.is_empty() {
+                    bail!(ErrorKind::InvalidGroundConstant);
+                }
+
+                // Grab the known types to which these args must conform, and track
+                // the places that won't be bound in the output.
+                let template: Vec<Option<(Variable, ValueTypeSet)>> =
+                    places.iter()
+                          .map(|x| match x {
+                              &VariableOrPlaceholder::Placeholder     => None,
+                              &VariableOrPlaceholder::Variable(ref v) => Some((v.clone(), self.known_type_set(v))),
+                          })
+                          .collect();
+
+                // The expected 'width' of the matrix is the number of named variables.
+                let full_width = places.len();
+                let names: Vec<Variable> = places.into_iter().filter_map(|x| x.into_var()).collect();
+                let expected_width = names.len();
+                let expected_rows = rows.len();
+
+                if expected_width == 0 {
+                    // They can't all be placeholders.
+                    bail!(ErrorKind::InvalidGroundConstant);
+                }
+
+                // Accumulate values into `matrix` and types into `a_t_f_c`.
+                // This representation of a rectangular matrix is more efficient than one composed
+                // of N separate vectors.
+                let mut matrix = Vec::with_capacity(expected_width * expected_rows);
+                let mut accumulated_types_for_columns = vec![ValueTypeSet::none(); expected_width];
+
+                // Loop so we can bail out.
+                let mut skipped_all: Option<EmptyBecause> = None;
+                for row in rows.into_iter() {
+                    match row {
+                        FnArg::Vector(cols) => {
+                            // Make sure that every row is the same length.
+                            if cols.len() != full_width {
+                                bail!(ErrorKind::InvalidGroundConstant);
+                            }
+
+                            // TODO: don't accumulate twice.
+                            let mut vals = Vec::with_capacity(expected_width);
+                            let mut skip: Option<EmptyBecause> = None;
+                            for (col, pair) in cols.into_iter().zip(template.iter()) {
+                                // Now we have (val, Option<(name, known_types)>). Silly,
+                                // but this is how we iter!
+                                // Convert each item in the row.
+                                // If any value in the row is impossible, then skip the row.
+                                // If all rows are impossible, fail the entire CC.
+                                if let &Some(ref pair) = pair {
+                                    match self.typed_value_from_arg(schema, &pair.0, col, pair.1)? {
+                                        ValueConversion::Val(tv) => vals.push(tv),
+                                        ValueConversion::Impossible(because) => {
+                                            // Skip this row. It cannot produce bindings.
+                                            skip = Some(because);
+                                            break;
+                                        },
+                                    }
+                                }
+                            }
+
+                            if skip.is_some() {
+                                // Skip this row and record why, in case we skip all.
+                                skipped_all = skip;
+                                continue;
+                            }
+
+                            // Accumulate the values into the matrix and the types into the type set.
+                            for (val, acc) in vals.into_iter().zip(accumulated_types_for_columns.iter_mut()) {
+                                let inserted = acc.insert(val.value_type());
+                                if inserted && !acc.is_unit() {
+                                    // Heterogeneous types.
+                                    bail!(ErrorKind::InvalidGroundConstant);
+                                }
+                                matrix.push(val);
+                            }
+
+                        },
+                        _ => bail!(ErrorKind::InvalidGroundConstant),
+                    }
+                }
+
+                // Do we have rows? If not, the CC cannot succeed.
+                if matrix.is_empty() {
+                    // We will either have bailed or will have accumulated *something* into the matrix,
+                    // so we can safely unwrap here.
+                    self.mark_known_empty(skipped_all.expect("we skipped for a reason"));
+                    return Ok(());
+                }
+
+                // Take the single type from each set. We know there's only one: we got at least one
+                // type, 'cos we bailed out for zero rows, and we also bailed out each time we
+                // inserted a second type.
+                // By restricting to homogeneous columns, we greatly simplify projection. In the
+                // future, we could loosen this restriction, at the cost of projecting (some) value
+                // type tags. If and when we want to algebrize in two phases and allow for
+                // late-binding input variables, we'll probably be able to loosen this restriction
+                // with little penalty.
+                let types = accumulated_types_for_columns.into_iter()
+                                                         .map(|x| x.exemplar().unwrap())
+                                                         .collect();
+                self.collect_named_bindings(schema, names, types, matrix);
+                Ok(())
+            },
+            (_, _) => bail!(ErrorKind::InvalidGroundConstant),
+        }
+    }
+}
+
+#[cfg(test)]
+mod testing {
+    use super::*;
+
+    use mentat_core::{
+        Attribute,
+        ValueType,
+    };
+
+    use mentat_query::{
+        Binding,
+        FnArg,
+        NamespacedKeyword,
+        PlainSymbol,
+        Variable,
+    };
+
+    use clauses::{
+        add_attribute,
+        associate_ident,
+    };
+
+    use types::{
+        ValueTypeSet,
+    };
+
+    #[test]
+    fn test_apply_ground() {
+        let vz = Variable::from_valid_name("?z");
+
+        let mut cc = ConjoiningClauses::default();
+        let mut schema = Schema::default();
+
+        associate_ident(&mut schema, NamespacedKeyword::new("foo", "fts"), 100);
+        add_attribute(&mut schema, 100, Attribute {
+            value_type: ValueType::String,
+            index: true,
+            fulltext: true,
+            ..Default::default()
+        });
+
+        // It's awkward enough to write these expansions that we give the details for the simplest
+        // case only.  See the tests of the translator for more extensive (albeit looser) coverage.
+        let op = PlainSymbol::new("ground");
+        cc.apply_ground(&schema, WhereFn {
+            operator: op,
+            args: vec![
+                FnArg::EntidOrInteger(10),
+            ],
+            binding: Binding::BindScalar(vz.clone()),
+        }).expect("to be able to apply_ground");
+
+        assert!(!cc.is_known_empty());
+
+        // Finally, expand column bindings.
+        cc.expand_column_bindings();
+        assert!(!cc.is_known_empty());
+
+        let clauses = cc.wheres;
+        assert_eq!(clauses.len(), 0);
+
+        let column_bindings = cc.column_bindings;
+        assert_eq!(column_bindings.len(), 0);           // Scalar doesn't need this.
+
+        let known_types = cc.known_types;
+        assert_eq!(known_types.len(), 1);
+        assert_eq!(known_types.get(&vz).expect("to know the type of ?z"),
+                   &ValueTypeSet::of_one(ValueType::Long));
+
+        let value_bindings = cc.value_bindings;
+        assert_eq!(value_bindings.len(), 1);
+        assert_eq!(value_bindings.get(&vz).expect("to have a value for ?z"),
+                   &TypedValue::Long(10));        // We default to Long instead of entid.
+    }
+}

query-algebrizer/src/errors.rs

@@ -10,12 +10,20 @@
 
 extern crate mentat_query;
 
-use mentat_core::ValueType;
+use mentat_core::{
+    ValueType,
+};
 
 use self::mentat_query::{
     PlainSymbol,
 };
 
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub enum BindingError {
+    NoBoundVariable,
+    RepeatedBoundVariable, // TODO: include repeated variable(s).
+}
+
 error_chain! {
     types {
         Error, ErrorKind, ResultExt, Result;
@@ -32,9 +40,9 @@ error_chain! {
             display("no function named {}", name)
         }
     
-        InvalidNumberOfArguments(name: PlainSymbol, number: usize, expected: usize) {
+        InvalidNumberOfArguments(function: PlainSymbol, number: usize, expected: usize) {
             description("invalid number of arguments")
-            display("invalid number of arguments to {}: expected {}, got {}.", name, expected, number)
+            display("invalid number of arguments to {}: expected {}, got {}.", function, expected, number)
         }
 
         UnboundVariable(name: PlainSymbol) {
@@ -42,9 +50,25 @@ error_chain! {
             display("unbound variable: {}", name)
         }
 
-        NonNumericArgument(function: PlainSymbol, position: usize) {
+        InvalidBinding(function: PlainSymbol, binding_error: BindingError) {
+            description("invalid binding")
+            display("invalid binding for {}: {:?}.", function, binding_error)
+        }
+
+        GroundBindingsMismatch {
+            description("mismatched bindings in ground")
+            display("mismatched bindings in ground")
+        }
+
+        InvalidGroundConstant {
+            // TODO: flesh this out.
+            description("invalid expression in ground constant")
+            display("invalid expression in ground constant")
+        }
+
+        InvalidArgument(function: PlainSymbol, expected_type: &'static str, position: usize) {
             description("invalid argument")
-            display("invalid argument to {}: expected numeric in position {}.", function, position)
+            display("invalid argument to {}: expected {} in position {}.", function, expected_type, position)
         }
 
         InvalidLimit(val: String, kind: ValueType) {

query-algebrizer/src/lib.rs

@@ -13,6 +13,10 @@ extern crate enum_set;
 #[macro_use]
 extern crate error_chain;
 
+#[cfg(test)]
+#[macro_use]
+extern crate maplit;
+
 extern crate mentat_core;
 extern crate mentat_query;
 
@@ -42,6 +46,7 @@ use mentat_query::{
 };
 
 pub use errors::{
+    BindingError,
     Error,
     ErrorKind,
     Result,
@@ -51,7 +56,7 @@ pub use clauses::{
     QueryInputs,
 };
 
-#[allow(dead_code)]
+#[derive(Debug)]
 pub struct AlgebraicQuery {
     default_source: SrcVar,
     pub find_spec: FindSpec,

query-algebrizer/src/types.rs

@@ -53,6 +53,10 @@ pub enum ComputedTable {
         type_extraction: BTreeSet<Variable>,
         arms: Vec<::clauses::ConjoiningClauses>,
     },
+    NamedValues {
+        names: Vec<Variable>,
+        values: Vec<TypedValue>,
+    },
 }
 
 impl DatomsTable {
@@ -228,6 +232,7 @@ impl Debug for QueryValue {
 
 /// Represents an entry in the ORDER BY list: a variable or a variable's type tag.
 /// (We require order vars to be projected, so we can simply use a variable here.)
+#[derive(Debug)]
 pub struct OrderBy(pub Direction, pub VariableColumn);
 
 impl From<Order> for OrderBy {
@@ -418,8 +423,8 @@ impl Debug for ColumnConstraint {
 
 #[derive(PartialEq, Clone)]
 pub enum EmptyBecause {
-    // Var, existing, desired.
-    TypeMismatch(Variable, ValueTypeSet, ValueType),
+    ConflictingBindings { var: Variable, existing: TypedValue, desired: TypedValue },
+    TypeMismatch { var: Variable, existing: ValueTypeSet, desired: ValueTypeSet },
     NoValidTypes(Variable),
     NonNumericArgument,
     NonStringFulltextValue,
@@ -435,7 +440,11 @@ impl Debug for EmptyBecause {
     fn fmt(&self, f: &mut Formatter) -> ::std::fmt::Result {
         use self::EmptyBecause::*;
         match self {
-            &TypeMismatch(ref var, ref existing, ref desired) => {
+            &ConflictingBindings { ref var, ref existing, ref desired } => {
+                write!(f, "Var {:?} can't be {:?} because it's already bound to {:?}",
+                       var, desired, existing)
+            },
+            &TypeMismatch { ref var, ref existing, ref desired } => {
                 write!(f, "Type mismatch: {:?} can't be {:?}, because it's already {:?}",
                        var, desired, existing)
             },
@@ -496,7 +505,7 @@ impl<T: CLike + Clone> EnumSetExtensions<T> for EnumSet<T> {
     }
 }
 
-#[derive(Clone, Debug, Eq, PartialEq)]
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
 pub struct ValueTypeSet(pub EnumSet<ValueType>);
 
 impl Default for ValueTypeSet {
@@ -525,9 +534,27 @@ impl ValueTypeSet {
     pub fn of_numeric_types() -> ValueTypeSet {
         ValueTypeSet(EnumSet::of_both(ValueType::Double, ValueType::Long))
     }
+
+    /// Return a set containing `Ref` and `Keyword`.
+    pub fn of_keywords() -> ValueTypeSet {
+        ValueTypeSet(EnumSet::of_both(ValueType::Ref, ValueType::Keyword))
+    }
+
+    /// Return a set containing `Ref` and `Long`.
+    pub fn of_longs() -> ValueTypeSet {
+        ValueTypeSet(EnumSet::of_both(ValueType::Ref, ValueType::Long))
+    }
 }
 
 impl ValueTypeSet {
+    pub fn insert(&mut self, vt: ValueType) -> bool {
+        self.0.insert(vt)
+    }
+
+    pub fn len(&self) -> usize {
+        self.0.len()
+    }
+
     /// Returns a set containing all the types in this set and `other`.
     pub fn union(&self, other: &ValueTypeSet) -> ValueTypeSet {
         ValueTypeSet(self.0.union(other.0))
@@ -554,4 +581,4 @@ impl ValueTypeSet {
     pub fn is_unit(&self) -> bool {
         self.0.len() == 1
     }
-}
+}

query-algebrizer/src/validate.rs

@@ -212,7 +212,7 @@ mod tests {
                                    (and [?artist :artist/type ?type]
                                         [?type :artist/role :artist.role/parody]))]"#;
         let parsed = parse_find_string(query).expect("expected successful parse");
-        let clauses = valid_or_join(parsed, UnifyVars::Explicit(vec![Variable::from_valid_name("?artist")]));
+        let clauses = valid_or_join(parsed, UnifyVars::Explicit(btreeset!{Variable::from_valid_name("?artist")}));
 
         // Let's do some detailed parse checks.
         let mut arms = clauses.into_iter();
@@ -322,7 +322,7 @@ mod tests {
                                    [?release :release/artists ?artist]
                                    [?release :release/year 1970])]"#;
         let parsed = parse_find_string(query).expect("expected successful parse");
-        let clauses = valid_not_join(parsed, UnifyVars::Explicit(vec![Variable::from_valid_name("?artist")]));
+        let clauses = valid_not_join(parsed, UnifyVars::Explicit(btreeset!{Variable::from_valid_name("?artist")}));
 
         let release = PatternNonValuePlace::Variable(Variable::from_valid_name("?release"));
         let artist = PatternValuePlace::Variable(Variable::from_valid_name("?artist"));
@@ -375,4 +375,4 @@ mod tests {
             _ => panic!(),
         }
     }    
-}
+}

query-algebrizer/tests/ground.rs

@@ -0,0 +1,315 @@
+// Copyright 2016 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+extern crate mentat_core;
+extern crate mentat_query;
+extern crate mentat_query_algebrizer;
+extern crate mentat_query_parser;
+
+use mentat_core::{
+    Attribute,
+    Entid,
+    Schema,
+    ValueType,
+    TypedValue,
+};
+
+use mentat_query_parser::{
+    parse_find_string,
+};
+
+use mentat_query::{
+    NamespacedKeyword,
+    PlainSymbol,
+    Variable,
+};
+
+use mentat_query_algebrizer::{
+    BindingError,
+    ConjoiningClauses,
+    ComputedTable,
+    Error,
+    ErrorKind,
+    algebrize,
+};
+
+// These are helpers that tests use to build Schema instances.
+#[cfg(test)]
+fn associate_ident(schema: &mut Schema, i: NamespacedKeyword, e: Entid) {
+    schema.entid_map.insert(e, i.clone());
+    schema.ident_map.insert(i.clone(), e);
+}
+
+#[cfg(test)]
+fn add_attribute(schema: &mut Schema, e: Entid, a: Attribute) {
+    schema.schema_map.insert(e, a);
+}
+
+fn prepopulated_schema() -> Schema {
+    let mut schema = Schema::default();
+    associate_ident(&mut schema, NamespacedKeyword::new("foo", "name"), 65);
+    associate_ident(&mut schema, NamespacedKeyword::new("foo", "knows"), 66);
+    associate_ident(&mut schema, NamespacedKeyword::new("foo", "parent"), 67);
+    associate_ident(&mut schema, NamespacedKeyword::new("foo", "age"), 68);
+    associate_ident(&mut schema, NamespacedKeyword::new("foo", "height"), 69);
+    add_attribute(&mut schema, 65, Attribute {
+        value_type: ValueType::String,
+        multival: false,
+        ..Default::default()
+    });
+    add_attribute(&mut schema, 66, Attribute {
+        value_type: ValueType::Ref,
+        multival: true,
+        ..Default::default()
+    });
+    add_attribute(&mut schema, 67, Attribute {
+        value_type: ValueType::String,
+        multival: true,
+        ..Default::default()
+    });
+    add_attribute(&mut schema, 68, Attribute {
+        value_type: ValueType::Long,
+        multival: false,
+        ..Default::default()
+    });
+    add_attribute(&mut schema, 69, Attribute {
+        value_type: ValueType::Long,
+        multival: false,
+        ..Default::default()
+    });
+    schema
+}
+
+fn bails(schema: &Schema, input: &str) -> Error {
+    let parsed = parse_find_string(input).expect("query input to have parsed");
+    algebrize(schema.into(), parsed).expect_err("algebrize to have failed")
+}
+
+fn alg(schema: &Schema, input: &str) -> ConjoiningClauses {
+    let parsed = parse_find_string(input).expect("query input to have parsed");
+    algebrize(schema.into(), parsed).expect("algebrizing to have succeeded").cc
+}
+
+#[test]
+fn test_ground_doesnt_bail_for_type_conflicts() {
+    // We know `?x` to be a ref, but we're attempting to ground it to a Double.
+    // The query can return no results.
+    let q = r#"[:find ?x :where [?x :foo/knows ?p] [(ground 9.95) ?x]]"#;
+    let schema = prepopulated_schema();
+    let cc = alg(&schema, &q);
+    assert!(cc.empty_because.is_some());
+}
+
+#[test]
+fn test_ground_tuple_fails_impossible() {
+    let q = r#"[:find ?x :where [?x :foo/knows ?p] [(ground [5 9.95]) [?x ?p]]]"#;
+    let schema = prepopulated_schema();
+    let cc = alg(&schema, &q);
+    assert!(cc.empty_because.is_some());
+}
+
+#[test]
+fn test_ground_scalar_fails_impossible() {
+    let q = r#"[:find ?x :where [?x :foo/knows ?p] [(ground true) ?p]]"#;
+    let schema = prepopulated_schema();
+    let cc = alg(&schema, &q);
+    assert!(cc.empty_because.is_some());
+}
+
+#[test]
+fn test_ground_coll_skips_impossible() {
+    // We know `?x` to be a ref, but we're attempting to ground it to a Double.
+    // The query can return no results.
+    let q = r#"[:find ?x :where [?x :foo/knows ?p] [(ground [5 9.95 11]) [?x ...]]]"#;
+    let schema = prepopulated_schema();
+    let cc = alg(&schema, &q);
+    assert!(cc.empty_because.is_none());
+    assert_eq!(cc.computed_tables[0], ComputedTable::NamedValues {
+        names: vec![Variable::from_valid_name("?x")],
+        values: vec![TypedValue::Ref(5), TypedValue::Ref(11)],
+    });
+}
+
+#[test]
+fn test_ground_coll_fails_if_all_impossible() {
+    let q = r#"[:find ?x :where [?x :foo/knows ?p] [(ground [5.1 5.2]) [?p ...]]]"#;
+    let schema = prepopulated_schema();
+    let cc = alg(&schema, &q);
+    assert!(cc.empty_because.is_some());
+}
+
+#[test]
+fn test_ground_rel_skips_impossible() {
+    let q = r#"[:find ?x :where [?x :foo/knows ?p] [(ground [[8 "foo"] [5 7] [9.95 9] [11 12]]) [[?x ?p]]]]"#;
+    let schema = prepopulated_schema();
+    let cc = alg(&schema, &q);
+    assert!(cc.empty_because.is_none());
+    assert_eq!(cc.computed_tables[0], ComputedTable::NamedValues {
+        names: vec![Variable::from_valid_name("?x"), Variable::from_valid_name("?p")],
+        values: vec![TypedValue::Ref(5), TypedValue::Ref(7), TypedValue::Ref(11), TypedValue::Ref(12)],
+    });
+}
+
+#[test]
+fn test_ground_rel_fails_if_all_impossible() {
+    let q = r#"[:find ?x :where [?x :foo/knows ?p] [(ground [[11 5.1] [12 5.2]]) [[?x ?p]]]]"#;
+    let schema = prepopulated_schema();
+    let cc = alg(&schema, &q);
+    assert!(cc.empty_because.is_some());
+}
+
+#[test]
+fn test_ground_tuple_rejects_all_placeholders() {
+    let q = r#"[:find ?x :where [?x :foo/knows ?p] [(ground [8 "foo" 3]) [_ _ _]]]"#;
+    let schema = prepopulated_schema();
+    bails(&schema, &q);
+}
+
+#[test]
+fn test_ground_rel_rejects_all_placeholders() {
+    let q = r#"[:find ?x :where [?x :foo/knows ?p] [(ground [[8 "foo"]]) [[_ _]]]]"#;
+    let schema = prepopulated_schema();
+    bails(&schema, &q);
+}
+
+#[test]
+fn test_ground_tuple_placeholders() {
+    let q = r#"[:find ?x :where [?x :foo/knows ?p] [(ground [8 "foo" 3]) [?x _ ?p]]]"#;
+    let schema = prepopulated_schema();
+    let cc = alg(&schema, &q);
+    assert!(cc.empty_because.is_none());
+    assert_eq!(cc.bound_value(&Variable::from_valid_name("?x")), Some(TypedValue::Ref(8)));
+    assert_eq!(cc.bound_value(&Variable::from_valid_name("?p")), Some(TypedValue::Ref(3)));
+}
+
+#[test]
+fn test_ground_rel_placeholders() {
+    let q = r#"[:find ?x :where [?x :foo/knows ?p] [(ground [[8 "foo" 3] [5 false 7] [5 9.95 9]]) [[?x _ ?p]]]]"#;
+    let schema = prepopulated_schema();
+    let cc = alg(&schema, &q);
+    assert!(cc.empty_because.is_none());
+    assert_eq!(cc.computed_tables[0], ComputedTable::NamedValues {
+        names: vec![Variable::from_valid_name("?x"), Variable::from_valid_name("?p")],
+        values: vec![
+            TypedValue::Ref(8),
+            TypedValue::Ref(3),
+            TypedValue::Ref(5),
+            TypedValue::Ref(7),
+            TypedValue::Ref(5),
+            TypedValue::Ref(9),
+        ],
+    });
+}
+
+// Nothing to do with ground, but while we're here…
+#[test]
+fn test_multiple_reference_type_failure() {
+    let q = r#"[:find ?x :where [?x :foo/age ?y] [?x :foo/knows ?y]]"#;
+    let schema = prepopulated_schema();
+    let cc = alg(&schema, &q);
+    assert!(cc.empty_because.is_some());
+}
+
+#[test]
+fn test_ground_tuple_infers_types() {
+    let q = r#"[:find ?x :where [?x :foo/age ?v] [(ground [8 10]) [?x ?v]]]"#;
+    let schema = prepopulated_schema();
+    let cc = alg(&schema, &q);
+    assert!(cc.empty_because.is_none());
+    assert_eq!(cc.bound_value(&Variable::from_valid_name("?x")), Some(TypedValue::Ref(8)));
+    assert_eq!(cc.bound_value(&Variable::from_valid_name("?v")), Some(TypedValue::Long(10)));
+}
+
+#[test]
+fn test_ground_rel_infers_types() {
+    let q = r#"[:find ?x :where [?x :foo/age ?v] [(ground [[8 10]]) [[?x ?v]]]]"#;
+    let schema = prepopulated_schema();
+    let cc = alg(&schema, &q);
+    assert!(cc.empty_because.is_none());
+    assert_eq!(cc.computed_tables[0], ComputedTable::NamedValues {
+        names: vec![Variable::from_valid_name("?x"), Variable::from_valid_name("?v")],
+        values: vec![TypedValue::Ref(8), TypedValue::Long(10)],
+    });
+}
+
+#[test]
+fn test_ground_coll_heterogeneous_types() {
+    let q = r#"[:find ?x :where [?x _ ?v] [(ground [false 8.5]) [?v ...]]]"#;
+    let schema = prepopulated_schema();
+    let e = bails(&schema, &q);
+    match e {
+        Error(ErrorKind::InvalidGroundConstant, _) => {
+        },
+        _ => {
+            panic!();
+        },
+    }
+}
+
+#[test]
+fn test_ground_rel_heterogeneous_types() {
+    let q = r#"[:find ?x :where [?x _ ?v] [(ground [[false] [5]]) [[?v]]]]"#;
+    let schema = prepopulated_schema();
+    let e = bails(&schema, &q);
+    match e {
+        Error(ErrorKind::InvalidGroundConstant, _) => {
+        },
+        _ => {
+            panic!();
+        },
+    }
+}
+
+#[test]
+fn test_ground_tuple_duplicate_vars() {
+    let q = r#"[:find ?x :where [?x :foo/age ?v] [(ground [8 10]) [?x ?x]]]"#;
+    let schema = prepopulated_schema();
+    let e = bails(&schema, &q);
+    match e {
+        Error(ErrorKind::InvalidBinding(v, e), _) => {
+            assert_eq!(v, PlainSymbol::new("ground"));
+            assert_eq!(e, BindingError::RepeatedBoundVariable);
+        },
+        _ => {
+            panic!();
+        },
+    }
+}
+
+#[test]
+fn test_ground_rel_duplicate_vars() {
+    let q = r#"[:find ?x :where [?x :foo/age ?v] [(ground [[8 10]]) [[?x ?x]]]]"#;
+    let schema = prepopulated_schema();
+    let e = bails(&schema, &q);
+    match e {
+        Error(ErrorKind::InvalidBinding(v, e), _) => {
+            assert_eq!(v, PlainSymbol::new("ground"));
+            assert_eq!(e, BindingError::RepeatedBoundVariable);
+        },
+        _ => {
+            panic!();
+        },
+    }
+}
+
+#[test]
+fn test_ground_nonexistent_variable_invalid() {
+    let q = r#"[:find ?x ?e :where [?e _ ?x] (not [(ground 17) ?v])]"#;
+    let schema = prepopulated_schema();
+    let e = bails(&schema, &q);
+    match e {
+        Error(ErrorKind::UnboundVariable(PlainSymbol(v)), _) => {
+            assert_eq!(v, "?v".to_string());
+        },
+        _ => {
+            panic!();
+        },
+    }
+}

query-parser/Cargo.toml

@@ -6,6 +6,7 @@ workspace = ".."
 [dependencies]
 combine = "2.3.2"
 error-chain = "0.8.1"
+maplit = "0.1"
 matches = "0.1"
 
 [dependencies.edn]

query-parser/src/lib.rs

@@ -10,6 +10,9 @@
 
 #![allow(unused_imports)]
 
+#[macro_use]
+extern crate maplit;
+
 #[macro_use]
 extern crate error_chain;
 

query-parser/src/parse.rs

@@ -38,6 +38,7 @@ use self::mentat_parser_utils::value_and_span::{
 };
 
 use self::mentat_query::{
+    Binding,
     Direction,
     Element,
     FindQuery,
@@ -57,7 +58,9 @@ use self::mentat_query::{
     SrcVar,
     UnifyVars,
     Variable,
+    VariableOrPlaceholder,
     WhereClause,
+    WhereFn,
 };
 
 error_chain! {
@@ -134,7 +137,7 @@ def_parser!(Query, predicate_fn, PredicateFn, {
 });
 
 def_parser!(Query, fn_arg, FnArg, {
-    satisfy_map(FnArg::from_value)
+    satisfy_map(FnArg::from_value).or(vector().of_exactly(many::<Vec<FnArg>, _>(Query::fn_arg())).map(FnArg::Vector))
 });
 
 def_parser!(Query, arguments, Vec<FnArg>, {
@@ -197,9 +200,9 @@ def_matches_plain_symbol!(Where, not, "not");
 
 def_matches_plain_symbol!(Where, not_join, "not-join");
 
-def_parser!(Where, rule_vars, Vec<Variable>, {
+def_parser!(Where, rule_vars, BTreeSet<Variable>, {
     seq()
-        .of_exactly(many1(Query::variable()))
+        .of_exactly(many1(Query::variable()).and_then(unique_vars))
 });
 
 def_parser!(Where, or_pattern_clause, OrWhereClause, {
@@ -279,6 +282,25 @@ def_parser!(Where, pred, WhereClause, {
                 })))
 });
 
+/// A vector containing a parenthesized function expression and a binding.
+def_parser!(Where, where_fn, WhereClause, {
+    // Accept either a nested list or a nested vector here:
+    // `[(foo ?x ?y) binding]` or `[[foo ?x ?y] binding]`
+    vector()
+        .of_exactly(
+            (seq().of_exactly(
+                (Query::predicate_fn(), Query::arguments())),
+             Bind::binding())
+                    .map(|((f, args), binding)| {
+                        WhereClause::WhereFn(
+                            WhereFn {
+                                operator: f.0,
+                                args: args,
+                                binding: binding,
+                            })
+                    }))
+});
+
 def_parser!(Where, pattern, WhereClause, {
     vector()
         .of_exactly(
@@ -331,6 +353,7 @@ def_parser!(Where, clause, WhereClause, {
             try(Where::not_clause()),
 
             try(Where::pred()),
+            try(Where::where_fn()),
     ])
 });
 
@@ -345,6 +368,8 @@ def_matches_plain_symbol!(Find, period, ".");
 
 def_matches_plain_symbol!(Find, ellipsis, "...");
 
+def_matches_plain_symbol!(Find, placeholder, "_");
+
 def_parser!(Find, find_scalar, FindSpec, {
     Query::variable()
         .skip(Find::period())
@@ -392,18 +417,20 @@ def_parser!(Find, spec, FindSpec, {
           &mut try(Find::find_rel())])
 });
 
+fn unique_vars<T, E>(vars: Vec<Variable>) -> std::result::Result<BTreeSet<Variable>, combine::primitives::Error<T, E>> {
+    let given = vars.len();
+    let set: BTreeSet<Variable> = vars.into_iter().collect();
+    if given != set.len() {
+        // TODO: find out what the variable is!
+        let e = Box::new(Error::from_kind(ErrorKind::DuplicateVariableError));
+        Err(combine::primitives::Error::Other(e))
+    } else {
+        Ok(set)
+    }
+}
+
 def_parser!(Find, vars, BTreeSet<Variable>, {
-    many(Query::variable()).and_then(|vars: Vec<Variable>| {
-        let given = vars.len();
-        let set: BTreeSet<Variable> = vars.into_iter().collect();
-        if given != set.len() {
-            // TODO: find out what the variable is!
-            let e = Box::new(Error::from_kind(ErrorKind::DuplicateVariableError));
-            Err(combine::primitives::Error::Other(e))
-        } else {
-            Ok(set)
-        }
-    })
+    many(Query::variable()).and_then(unique_vars)
 });
 
 /// This is awkward, but will do for now.  We use `keyword_map()` to optionally accept vector find
@@ -446,6 +473,48 @@ def_parser!(Find, query, FindQuery, {
         })
 });
 
+pub struct Bind<'a>(std::marker::PhantomData<&'a ()>);
+
+def_parser!(Bind, bind_scalar, Binding, {
+    Query::variable()
+        .skip(eof())
+        .map(|var| Binding::BindScalar(var))
+});
+
+def_parser!(Bind, variable_or_placeholder, VariableOrPlaceholder, {
+    Query::variable().map(VariableOrPlaceholder::Variable)
+        .or(Find::placeholder().map(|_| VariableOrPlaceholder::Placeholder))
+});
+
+def_parser!(Bind, bind_coll, Binding, {
+    vector()
+        .of_exactly(Query::variable()
+            .skip(Find::ellipsis()))
+            .map(Binding::BindColl)
+});
+
+def_parser!(Bind, bind_rel, Binding, {
+    vector().of_exactly(
+        vector().of_exactly(
+            many1::<Vec<VariableOrPlaceholder>, _>(Bind::variable_or_placeholder())
+                .map(Binding::BindRel)))
+});
+
+def_parser!(Bind, bind_tuple, Binding, {
+    vector().of_exactly(
+        many1::<Vec<VariableOrPlaceholder>, _>(Bind::variable_or_placeholder())
+            .map(Binding::BindTuple))
+});
+
+def_parser!(Bind, binding, Binding, {
+    // Any one of the four binding types might apply, so we combine them with `choice`.  Our parsers
+    // consume input, so we need to wrap them in `try` so that they operate independently.
+    choice([try(Bind::bind_scalar()),
+            try(Bind::bind_coll()),
+            try(Bind::bind_tuple()),
+            try(Bind::bind_rel())])
+});
+
 pub fn parse_find_string(string: &str) -> Result<FindQuery> {
     let expr = edn::parse::value(string)?;
     Find::query()
@@ -465,6 +534,7 @@ mod test {
     use self::combine::Parser;
     use self::edn::OrderedFloat;
     use self::mentat_query::{
+        Binding,
         Element,
         FindSpec,
         NonIntegerConstant,
@@ -473,6 +543,7 @@ mod test {
         PatternValuePlace,
         SrcVar,
         Variable,
+        VariableOrPlaceholder,
     };
 
     use super::*;
@@ -573,7 +644,7 @@ mod test {
         let e = edn::PlainSymbol::new("?e");
         let input = edn::Value::Vector(vec![edn::Value::PlainSymbol(e.clone())]);
         assert_parses_to!(Where::rule_vars, input,
-                          vec![variable(e.clone())]);
+                          btreeset!{variable(e.clone())});
     }
 
     #[test]
@@ -583,7 +654,7 @@ mod test {
         let input = edn::Value::Vector(vec![edn::Value::PlainSymbol(e.clone()),
                                             edn::Value::PlainSymbol(f.clone()),]);
         assert_parses_to!(|| vector().of_exactly(Find::vars()), input,
-                          vec![variable(e.clone()), variable(f.clone())].into_iter().collect());
+                          btreeset!{variable(e.clone()), variable(f.clone())});
 
         let g = edn::PlainSymbol::new("?g");
         let input = edn::Value::Vector(vec![edn::Value::PlainSymbol(g.clone()),
@@ -642,7 +713,7 @@ mod test {
                                          edn::Value::PlainSymbol(v.clone())])].into_iter().collect());
         assert_parses_to!(Where::or_join_clause, input,
                           WhereClause::OrJoin(
-                              OrJoin::new(UnifyVars::Explicit(vec![variable(e.clone())]),
+                              OrJoin::new(UnifyVars::Explicit(btreeset!{variable(e.clone())}),
                                           vec![OrWhereClause::Clause(
                                               WhereClause::Pattern(Pattern {
                                                   source: None,
@@ -685,7 +756,7 @@ mod test {
                               "(not-join [?e] [?e ?a ?v])",
                               WhereClause::NotJoin(
                               NotJoin {
-                                  unify_vars: UnifyVars::Explicit(vec![variable(e.clone())]),
+                                  unify_vars: UnifyVars::Explicit(btreeset!{variable(e.clone())}),
                                   clauses: vec![WhereClause::Pattern(Pattern {
                                           source: None,
                                           entity: PatternNonValuePlace::Variable(variable(e)),
@@ -777,4 +848,100 @@ mod test {
         let mut par = Query::natural_number();
         assert_eq!(None, par.parse(input.atom_stream()).err());
     }
+
+    #[test]
+    fn test_fn_arg_collections() {
+        let vx = edn::PlainSymbol::new("?x");
+        let vy = edn::PlainSymbol::new("?y");
+        let input = edn::Value::Vector(vec![edn::Value::Vector(vec![edn::Value::PlainSymbol(vx.clone()),
+                                            edn::Value::PlainSymbol(vy.clone())])]);
+
+        assert_parses_to!(|| vector().of_exactly(Query::fn_arg()),
+                          input,
+                          FnArg::Vector(vec![FnArg::Variable(variable(vx)),
+                                             FnArg::Variable(variable(vy)),
+                          ]));
+    }
+
+    #[test]
+    fn test_bind_scalar() {
+        let vx = edn::PlainSymbol::new("?x");
+        assert_edn_parses_to!(|| list().of_exactly(Bind::binding()),
+                              "(?x)",
+                              Binding::BindScalar(variable(vx)));
+    }
+
+    #[test]
+    fn test_bind_coll() {
+        let vx = edn::PlainSymbol::new("?x");
+        assert_edn_parses_to!(|| list().of_exactly(Bind::binding()),
+                              "([?x ...])",
+                              Binding::BindColl(variable(vx)));
+    }
+
+    #[test]
+    fn test_bind_rel() {
+        let vx = edn::PlainSymbol::new("?x");
+        let vy = edn::PlainSymbol::new("?y");
+        let vw = edn::PlainSymbol::new("?w");
+        assert_edn_parses_to!(|| list().of_exactly(Bind::binding()),
+                              "([[?x ?y _ ?w]])",
+                              Binding::BindRel(vec![VariableOrPlaceholder::Variable(variable(vx)),
+                                                    VariableOrPlaceholder::Variable(variable(vy)),
+                                                    VariableOrPlaceholder::Placeholder,
+                                                    VariableOrPlaceholder::Variable(variable(vw)),
+                              ]));
+    }
+
+    #[test]
+    fn test_bind_tuple() {
+        let vx = edn::PlainSymbol::new("?x");
+        let vy = edn::PlainSymbol::new("?y");
+        let vw = edn::PlainSymbol::new("?w");
+        assert_edn_parses_to!(|| list().of_exactly(Bind::binding()),
+                              "([?x ?y _ ?w])",
+                              Binding::BindTuple(vec![VariableOrPlaceholder::Variable(variable(vx)),
+                                                      VariableOrPlaceholder::Variable(variable(vy)),
+                                                      VariableOrPlaceholder::Placeholder,
+                                                      VariableOrPlaceholder::Variable(variable(vw)),
+                              ]));
+    }
+
+    #[test]
+    fn test_where_fn() {
+        assert_edn_parses_to!(Where::where_fn,
+                              "[(f ?x 1) ?y]",
+                              WhereClause::WhereFn(WhereFn {
+                                  operator: edn::PlainSymbol::new("f"),
+                                  args: vec![FnArg::Variable(Variable::from_valid_name("?x")),
+                                             FnArg::EntidOrInteger(1)],
+                                  binding: Binding::BindScalar(Variable::from_valid_name("?y")),
+                              }));
+
+        assert_edn_parses_to!(Where::where_fn,
+                              "[(f ?x) [?y ...]]",
+                              WhereClause::WhereFn(WhereFn {
+                                  operator: edn::PlainSymbol::new("f"),
+                                  args: vec![FnArg::Variable(Variable::from_valid_name("?x"))],
+                                  binding: Binding::BindColl(Variable::from_valid_name("?y")),
+                              }));
+
+        assert_edn_parses_to!(Where::where_fn,
+                              "[(f) [?y _]]",
+                              WhereClause::WhereFn(WhereFn {
+                                  operator: edn::PlainSymbol::new("f"),
+                                  args: vec![],
+                                  binding: Binding::BindTuple(vec![VariableOrPlaceholder::Variable(Variable::from_valid_name("?y")),
+                                                                   VariableOrPlaceholder::Placeholder]),
+                              }));
+
+        assert_edn_parses_to!(Where::where_fn,
+                              "[(f) [[_ ?y]]]",
+                              WhereClause::WhereFn(WhereFn {
+                                  operator: edn::PlainSymbol::new("f"),
+                                  args: vec![],
+                                  binding: Binding::BindRel(vec![VariableOrPlaceholder::Placeholder,
+                                                                 VariableOrPlaceholder::Variable(Variable::from_valid_name("?y"))]),
+                              }));
+    }
 }

query-parser/tests/find_tests.rs

@@ -8,6 +8,9 @@
 // CONDITIONS OF ANY KIND, either express or implied. See the License for the
 // specific language governing permissions and limitations under the License.
 
+#[macro_use]
+extern crate maplit;
+
 extern crate edn;
 extern crate mentat_core;
 extern crate mentat_query;
@@ -111,7 +114,7 @@ fn can_parse_unit_or_join() {
     assert_eq!(p.where_clauses,
                vec![
                    WhereClause::OrJoin(OrJoin::new(
-                       UnifyVars::Explicit(vec![Variable::from_valid_name("?x")]),
+                       UnifyVars::Explicit(btreeset!{Variable::from_valid_name("?x")}),
                        vec![
                            OrWhereClause::Clause(
                                WhereClause::Pattern(Pattern {
@@ -136,7 +139,7 @@ fn can_parse_simple_or_join() {
     assert_eq!(p.where_clauses,
                vec![
                    WhereClause::OrJoin(OrJoin::new(
-                       UnifyVars::Explicit(vec![Variable::from_valid_name("?x")]),
+                       UnifyVars::Explicit(btreeset!{Variable::from_valid_name("?x")}),
                        vec![
                            OrWhereClause::Clause(
                                WhereClause::Pattern(Pattern {

query-projector/src/lib.rs

@@ -28,6 +28,7 @@ use rusqlite::{
 use mentat_core::{
     SQLValueType,
     TypedValue,
+    ValueType,
 };
 
 use mentat_db::{
@@ -44,6 +45,7 @@ use mentat_query::{
 use mentat_query_algebrizer::{
     AlgebraicQuery,
     ColumnName,
+    ConjoiningClauses,
     VariableColumn,
 };
 
@@ -157,29 +159,43 @@ impl TypedIndex {
     }
 }
 
-fn candidate_column(query: &AlgebraicQuery, var: &Variable) -> (ColumnOrExpression, Name) {
+fn candidate_column(cc: &ConjoiningClauses, var: &Variable) -> (ColumnOrExpression, Name) {
     // Every variable should be bound by the top-level CC to at least
     // one column in the query. If that constraint is violated it's a
     // bug in our code, so it's appropriate to panic here.
-    let columns = query.cc
-                       .column_bindings
-                       .get(var)
-                       .expect("Every variable has a binding");
+    let columns = cc.column_bindings
+                    .get(var)
+                    .expect(format!("Every variable should have a binding, but {:?} does not", var).as_str());
 
     let qa = columns[0].clone();
     let name = VariableColumn::Variable(var.clone()).column_name();
     (ColumnOrExpression::Column(qa), name)
 }
 
-fn candidate_type_column(query: &AlgebraicQuery, var: &Variable) -> (ColumnOrExpression, Name) {
-    let extracted_alias = query.cc
-                               .extracted_types
-                               .get(var)
-                               .expect("Every variable has a known type or an extracted type");
+fn candidate_type_column(cc: &ConjoiningClauses, var: &Variable) -> (ColumnOrExpression, Name) {
+    let extracted_alias = cc.extracted_types
+                            .get(var)
+                            .expect("Every variable has a known type or an extracted type");
     let type_name = VariableColumn::VariableTypeTag(var.clone()).column_name();
     (ColumnOrExpression::Column(extracted_alias.clone()), type_name)
 }
 
+/// Return the projected column -- that is, a value or SQL column and an associated name -- for a
+/// given variable. Also return the type, if known.
+/// Callers are expected to determine whether to project a type tag as an additional SQL column.
+pub fn projected_column_for_var(var: &Variable, cc: &ConjoiningClauses) -> (ProjectedColumn, Option<ValueType>) {
+    if let Some(value) = cc.bound_value(&var) {
+        // If we already know the value, then our lives are easy.
+        let tag = value.value_type();
+        let name = VariableColumn::Variable(var.clone()).column_name();
+        (ProjectedColumn(ColumnOrExpression::Value(value.clone()), name), Some(tag))
+    } else {
+        // If we don't, then the CC *must* have bound the variable.
+        let (column, name) = candidate_column(cc, var);
+        (ProjectedColumn(column, name), cc.known_type(var))
+    }
+}
+
 /// Walk an iterator of `Element`s, collecting projector templates and columns.
 ///
 /// Returns a pair: the SQL projection (which should always be a `Projection::Columns`)
@@ -211,10 +227,10 @@ fn project_elements<'a, I: IntoIterator<Item = &'a Element>>(
                 // If we're projecting this, we don't need it in :with.
                 with.remove(var);
 
-                let (column, name) = candidate_column(query, var);
-                cols.push(ProjectedColumn(column, name));
-                if let Some(t) = query.cc.known_type(var) {
-                    let tag = t.value_type_tag();
+                let (projected_column, maybe_type) = projected_column_for_var(&var, &query.cc);
+                cols.push(projected_column);
+                if let Some(ty) = maybe_type {
+                    let tag = ty.value_type_tag();
                     templates.push(TypedIndex::Known(i, tag));
                     i += 1;     // We used one SQL column.
                 } else {
@@ -222,7 +238,7 @@ fn project_elements<'a, I: IntoIterator<Item = &'a Element>>(
                     i += 2;     // We used two SQL columns.
 
                     // Also project the type from the SQL query.
-                    let (type_column, type_name) = candidate_type_column(query, &var);
+                    let (type_column, type_name) = candidate_type_column(&query.cc, &var);
                     cols.push(ProjectedColumn(type_column, type_name));
                 }
             }
@@ -233,10 +249,10 @@ fn project_elements<'a, I: IntoIterator<Item = &'a Element>>(
         // We need to collect these into the SQL column list, but they don't affect projection.
         // If a variable is of a non-fixed type, also project the type tag column, so we don't
         // accidentally unify across types when considering uniqueness!
-        let (column, name) = candidate_column(query, &var);
+        let (column, name) = candidate_column(&query.cc, &var);
         cols.push(ProjectedColumn(column, name));
         if query.cc.known_type(&var).is_none() {
-            let (type_column, type_name) = candidate_type_column(query, &var);
+            let (type_column, type_name) = candidate_type_column(&query.cc, &var);
             cols.push(ProjectedColumn(type_column, type_name));
         }
     }
@@ -496,4 +512,4 @@ pub fn query_projection(query: &AlgebraicQuery) -> CombinedProjection {
             },
         }
     }
-}
+}

query-sql/src/lib.rs

@@ -152,6 +152,18 @@ pub enum TableOrSubquery {
     Table(SourceAlias),
     Union(Vec<SelectQuery>, TableAlias),
     Subquery(Box<SelectQuery>),
+    Values(Values, TableAlias),
+}
+
+pub enum Values {
+    /// Like "VALUES (0, 1), (2, 3), ...".
+    /// The vector must be of a length that is a multiple of the given size.
+    Unnamed(usize, Vec<TypedValue>),
+
+    /// Like "SELECT 0 AS x, SELECT 0 AS y WHERE 0 UNION ALL VALUES (0, 1), (2, 3), ...".
+    /// The vector of values must be of a length that is a multiple of the length
+    /// of the vector of names.
+    Named(Vec<Variable>, Vec<TypedValue>),
 }
 
 pub enum FromClause {
@@ -212,7 +224,13 @@ fn push_column(qb: &mut QueryBuilder, col: &Column) -> BuildQueryResult {
 /// without producing an intermediate string sequence.
 macro_rules! interpose {
     ( $name: pat, $across: expr, $body: block, $inter: block ) => {
-        let mut seq = $across.iter();
+        interpose_iter!($name, $across.iter(), $body, $inter)
+    }
+}
+
+macro_rules! interpose_iter {
+    ( $name: pat, $across: expr, $body: block, $inter: block ) => {
+        let mut seq = $across;
         if let Some($name) = seq.next() {
             $body;
             for $name in seq {
@@ -222,6 +240,7 @@ macro_rules! interpose {
         }
     }
 }
+
 impl QueryFragment for ColumnOrExpression {
     fn push_sql(&self, out: &mut QueryBuilder) -> BuildQueryResult {
         use self::ColumnOrExpression::*;
@@ -391,13 +410,60 @@ impl QueryFragment for TableOrSubquery {
                 out.push_identifier(table_alias.as_str())
             },
             &Subquery(ref subquery) => {
-                subquery.push_sql(out)?;
-                Ok(())
+                subquery.push_sql(out)
+            },
+            &Values(ref values, ref table_alias) => {
+                // XXX: does this work for Values::Unnamed?
+                out.push_sql("(");
+                values.push_sql(out)?;
+                out.push_sql(") AS ");
+                out.push_identifier(table_alias.as_str())
             },
         }
     }
 }
 
+impl QueryFragment for Values {
+    fn push_sql(&self, out: &mut QueryBuilder) -> BuildQueryResult {
+        // There are at least 3 ways to name the columns of a VALUES table:
+        // 1) the columns are named "", ":1", ":2", ... -- but this is undocumented.  See
+        //    http://stackoverflow.com/a/40921724.
+        // 2) A CTE ("WITH" statement) can declare the shape of the table, like "WITH
+        //    table_name(column_name, ...) AS (VALUES ...)".
+        // 3) We can "UNION ALL" a dummy "SELECT" statement in place.
+        //
+        // We don't want to use an undocumented SQLite quirk, and we're a little concerned that some
+        // SQL systems will not optimize WITH statements well.  It's also convenient to have an in
+        // place table to query, so for now we implement option 3.
+        if let &Values::Named(ref names, _) = self {
+            out.push_sql("SELECT ");
+            interpose!(alias, names,
+                       { out.push_sql("0 AS ");
+                         out.push_identifier(alias.as_str())? },
+                       { out.push_sql(", ") });
+
+            out.push_sql(" WHERE 0 UNION ALL ");
+        }
+
+        let values = match self {
+            &Values::Named(ref names, ref values) => values.chunks(names.len()),
+            &Values::Unnamed(ref size, ref values) => values.chunks(*size),
+        };
+
+        out.push_sql("VALUES ");
+
+        interpose_iter!(outer, values,
+                        { out.push_sql("(");
+                          interpose!(inner, outer,
+                                     { out.push_typed_value(inner)? },
+                                     { out.push_sql(", ") });
+                          out.push_sql(")");
+                        },
+                        { out.push_sql(", ") });
+        Ok(())
+    }
+}
+
 impl QueryFragment for FromClause {
     fn push_sql(&self, out: &mut QueryBuilder) -> BuildQueryResult {
         use self::FromClause::*;
@@ -494,7 +560,7 @@ mod tests {
         DatomsTable,
     };
 
-    fn build_constraint(c: Constraint) -> String {
+    fn build(c: &QueryFragment) -> String {
         let mut builder = SQLiteQueryBuilder::new();
         c.push_sql(&mut builder)
          .map(|_| builder.finish())
@@ -524,9 +590,9 @@ mod tests {
             ],
         };
 
-        assert_eq!("`datoms01`.v IN ()", build_constraint(none));
-        assert_eq!("`datoms01`.v IN (123)", build_constraint(one));
-        assert_eq!("`datoms01`.v IN (123, 456, 789)", build_constraint(three));
+        assert_eq!("`datoms01`.v IN ()", build(&none));
+        assert_eq!("`datoms01`.v IN (123)", build(&one));
+        assert_eq!("`datoms01`.v IN (123, 456, 789)", build(&three));
     }
 
     #[test]
@@ -552,7 +618,37 @@ mod tests {
 
         // Two sets of parens: the outermost AND only has one child,
         // but still contributes parens.
-        assert_eq!("((123 = 456 AND 789 = 246))", build_constraint(c));
+        assert_eq!("((123 = 456 AND 789 = 246))", build(&c));
+    }
+
+    #[test]
+    fn test_unnamed_values() {
+        let build = |len, values| build(&Values::Unnamed(len, values));
+
+        assert_eq!(build(1, vec![TypedValue::Long(1)]),
+                   "VALUES (1)");
+
+        assert_eq!(build(2, vec![TypedValue::Boolean(false), TypedValue::Long(1)]),
+                   "VALUES (0, 1)");
+
+        assert_eq!(build(2, vec![TypedValue::Boolean(false), TypedValue::Long(1),
+                                 TypedValue::Boolean(true), TypedValue::Long(2)]),
+                   "VALUES (0, 1), (1, 2)");
+    }
+
+    #[test]
+    fn test_named_values() {
+        let build = |names: Vec<_>, values| build(&Values::Named(names.into_iter().map(Variable::from_valid_name).collect(), values));
+        assert_eq!(build(vec!["?a"], vec![TypedValue::Long(1)]),
+                   "SELECT 0 AS `?a` WHERE 0 UNION ALL VALUES (1)");
+
+        assert_eq!(build(vec!["?a", "?b"], vec![TypedValue::Boolean(false), TypedValue::Long(1)]),
+                   "SELECT 0 AS `?a`, 0 AS `?b` WHERE 0 UNION ALL VALUES (0, 1)");
+
+        assert_eq!(build(vec!["?a", "?b"],
+                         vec![TypedValue::Boolean(false), TypedValue::Long(1),
+                              TypedValue::Boolean(true), TypedValue::Long(2)]),
+                   "SELECT 0 AS `?a`, 0 AS `?b` WHERE 0 UNION ALL VALUES (0, 1), (1, 2)");
     }
 
     #[test]

query-translator/src/translate.rs

@@ -38,6 +38,7 @@ use mentat_query_algebrizer::{
 use mentat_query_projector::{
     CombinedProjection,
     Projector,
+    projected_column_for_var,
     query_projection,
 };
 
@@ -51,6 +52,7 @@ use mentat_query_sql::{
     SelectQuery,
     TableList,
     TableOrSubquery,
+    Values,
 };
 
 trait ToConstraint {
@@ -201,33 +203,33 @@ fn table_for_computed(computed: ComputedTable, alias: TableAlias) -> TableOrSubq
                         // project it as the variable name.
                         // E.g., SELECT datoms03.v AS `?x`.
                         for var in projection.iter() {
-                            let col = cc.column_bindings.get(&var).unwrap()[0].clone();
-                            let proj = ProjectedColumn(ColumnOrExpression::Column(col), var.to_string());
-                            columns.push(proj);
-                        }
+                            let (projected_column, maybe_type) = projected_column_for_var(var, &cc);
+                            columns.push(projected_column);
 
-                        // Similarly, project type tags if they're not known conclusively in the
-                        // outer query.
-                        for var in type_extraction.iter() {
-                            let expression =
-                                if let Some(known) = cc.known_type(var) {
-                                    // If we know the type for sure, just project the constant.
-                                    // SELECT datoms03.v AS `?x`, 10 AS `?x_value_type_tag`
-                                    ColumnOrExpression::Integer(known.value_type_tag())
-                                } else {
-                                    // Otherwise, we'll have an established type binding! This'll be
-                                    // either a datoms table or, recursively, a subquery. Project
-                                    // this:
-                                    // SELECT datoms03.v AS `?x`,
-                                    //        datoms03.value_type_tag AS `?x_value_type_tag`
-                                    let extract = cc.extracted_types
-                                                    .get(var)
-                                                    .expect("Expected variable to have a known type or an extracted type");
-                                    ColumnOrExpression::Column(extract.clone())
-                                };
-                            let type_column = VariableColumn::VariableTypeTag(var.clone());
-                            let proj = ProjectedColumn(expression, type_column.column_name());
-                            columns.push(proj);
+                            // Similarly, project type tags if they're not known conclusively in the
+                            // outer query.
+                            // Assumption: we'll never need to project a tag without projecting the value of a variable.
+                            if type_extraction.contains(var) {
+                                let expression =
+                                    if let Some(ty) = maybe_type {
+                                        // If we know the type for sure, just project the constant.
+                                        // SELECT datoms03.v AS `?x`, 10 AS `?x_value_type_tag`
+                                        ColumnOrExpression::Integer(ty.value_type_tag())
+                                    } else {
+                                        // Otherwise, we'll have an established type binding! This'll be
+                                        // either a datoms table or, recursively, a subquery. Project
+                                        // this:
+                                        // SELECT datoms03.v AS `?x`,
+                                        //        datoms03.value_type_tag AS `?x_value_type_tag`
+                                        let extract = cc.extracted_types
+                                                        .get(var)
+                                                        .expect("Expected variable to have a known type or an extracted type");
+                                        ColumnOrExpression::Column(extract.clone())
+                                    };
+                                let type_column = VariableColumn::VariableTypeTag(var.clone());
+                                let proj = ProjectedColumn(expression, type_column.column_name());
+                                columns.push(proj);
+                            }
                         }
 
                         // Each arm simply turns into a subquery.
@@ -239,7 +241,13 @@ fn table_for_computed(computed: ComputedTable, alias: TableAlias) -> TableOrSubq
         },
         ComputedTable::Subquery(subquery) => {
             TableOrSubquery::Subquery(Box::new(cc_to_exists(subquery)))
-        }
+        },
+        ComputedTable::NamedValues {
+            names, values,
+        } => {
+            // We assume column homogeneity, so we won't have any type tag columns.
+            TableOrSubquery::Values(Values::Named(names, values), alias)
+        },
     }
 }
 

query-translator/tests/translate.rs

@@ -550,3 +550,180 @@ fn test_complex_nested_or_join_type_projection() {
                      LIMIT 1");
     assert_eq!(args, vec![]);
 }
+
+#[test]
+fn test_ground_scalar() {
+    let schema = prepopulated_schema();
+
+    // Verify that we accept inline constants.
+    let query = r#"[:find ?x . :where [(ground "yyy") ?x]]"#;
+    let SQLQuery { sql, args } = translate(&schema, query);
+    assert_eq!(sql, "SELECT $v0 AS `?x` LIMIT 1");
+    assert_eq!(args, vec![make_arg("$v0", "yyy")]);
+
+    // Verify that we accept bound input constants.
+    let query = r#"[:find ?x . :in ?v :where [(ground ?v) ?x]]"#;
+    let inputs = QueryInputs::with_value_sequence(vec![(Variable::from_valid_name("?v"), TypedValue::String(Rc::new("aaa".into())))]);
+    let SQLQuery { sql, args } = translate_with_inputs(&schema, query, inputs);
+    assert_eq!(sql, "SELECT $v0 AS `?x` LIMIT 1");
+    assert_eq!(args, vec![make_arg("$v0", "aaa"),]);
+}
+
+#[test]
+fn test_ground_tuple() {
+    let schema = prepopulated_schema();
+
+    // Verify that we accept inline constants.
+    let query = r#"[:find ?x ?y :where [(ground [1 "yyy"]) [?x ?y]]]"#;
+    let SQLQuery { sql, args } = translate(&schema, query);
+    assert_eq!(sql, "SELECT DISTINCT 1 AS `?x`, $v0 AS `?y`");
+    assert_eq!(args, vec![make_arg("$v0", "yyy")]);
+
+    // Verify that we accept bound input constants.
+    let query = r#"[:find [?x ?y] :in ?u ?v :where [(ground [?u ?v]) [?x ?y]]]"#;
+    let inputs = QueryInputs::with_value_sequence(vec![(Variable::from_valid_name("?u"), TypedValue::Long(2)),
+                                                       (Variable::from_valid_name("?v"), TypedValue::String(Rc::new("aaa".into()))),]);
+    let SQLQuery { sql, args } = translate_with_inputs(&schema, query, inputs);
+    // TODO: treat 2 as an input variable that could be bound late, rather than eagerly binding it.
+    assert_eq!(sql, "SELECT 2 AS `?x`, $v0 AS `?y` LIMIT 1");
+    assert_eq!(args, vec![make_arg("$v0", "aaa"),]);
+}
+
+#[test]
+fn test_ground_coll() {
+    let schema = prepopulated_schema();
+
+    // Verify that we accept inline constants.
+    let query = r#"[:find ?x :where [(ground ["xxx" "yyy"]) [?x ...]]]"#;
+    let SQLQuery { sql, args } = translate(&schema, query);
+    assert_eq!(sql, "SELECT DISTINCT `c00`.`?x` AS `?x` FROM \
+                         (SELECT 0 AS `?x` WHERE 0 UNION ALL VALUES ($v0), ($v1)) AS `c00`");
+    assert_eq!(args, vec![make_arg("$v0", "xxx"),
+                          make_arg("$v1", "yyy")]);
+
+    // Verify that we accept bound input constants.
+    let query = r#"[:find ?x :in ?u ?v :where [(ground [?u ?v]) [?x ...]]]"#;
+    let inputs = QueryInputs::with_value_sequence(vec![(Variable::from_valid_name("?u"), TypedValue::Long(2)),
+                                                       (Variable::from_valid_name("?v"), TypedValue::Long(3)),]);
+    let SQLQuery { sql, args } = translate_with_inputs(&schema, query, inputs);
+    // TODO: treat 2 and 3 as input variables that could be bound late, rather than eagerly binding.
+    assert_eq!(sql, "SELECT DISTINCT `c00`.`?x` AS `?x` FROM \
+                         (SELECT 0 AS `?x` WHERE 0 UNION ALL VALUES (2), (3)) AS `c00`");
+    assert_eq!(args, vec![]);
+}
+
+#[test]
+fn test_ground_rel() {
+    let schema = prepopulated_schema();
+
+    // Verify that we accept inline constants.
+    let query = r#"[:find ?x ?y :where [(ground [[1 "xxx"] [2 "yyy"]]) [[?x ?y]]]]"#;
+    let SQLQuery { sql, args } = translate(&schema, query);
+    assert_eq!(sql, "SELECT DISTINCT `c00`.`?x` AS `?x`, `c00`.`?y` AS `?y` FROM \
+                         (SELECT 0 AS `?x`, 0 AS `?y` WHERE 0 UNION ALL VALUES (1, $v0), (2, $v1)) AS `c00`");
+    assert_eq!(args, vec![make_arg("$v0", "xxx"),
+                          make_arg("$v1", "yyy")]);
+
+    // Verify that we accept bound input constants.
+    let query = r#"[:find ?x ?y :in ?u ?v :where [(ground [[?u 1] [?v 2]]) [[?x ?y]]]]"#;
+    let inputs = QueryInputs::with_value_sequence(vec![(Variable::from_valid_name("?u"), TypedValue::Long(3)),
+                                                       (Variable::from_valid_name("?v"), TypedValue::Long(4)),]);
+    let SQLQuery { sql, args } = translate_with_inputs(&schema, query, inputs);
+    // TODO: treat 3 and 4 as input variables that could be bound late, rather than eagerly binding.
+    assert_eq!(sql, "SELECT DISTINCT `c00`.`?x` AS `?x`, `c00`.`?y` AS `?y` FROM \
+                         (SELECT 0 AS `?x`, 0 AS `?y` WHERE 0 UNION ALL VALUES (3, 1), (4, 2)) AS `c00`");
+    assert_eq!(args, vec![]);
+}
+
+#[test]
+fn test_compound_with_ground() {
+    let schema = prepopulated_schema();
+
+    // Verify that we can use the resulting CCs as children in compound CCs.
+    let query = r#"[:find ?x :where (or [(ground "yyy") ?x]
+                                        [(ground "zzz") ?x])]"#;
+    let SQLQuery { sql, args } = translate(&schema, query);
+
+    // This is confusing because the computed tables (like `c00`) are numbered sequentially in each
+    // arm of the `or` rather than numbered globally.  But SQLite scopes the names correctly, so it
+    // works.  In the future, we might number the computed tables globally to make this more clear.
+    assert_eq!(sql, "SELECT DISTINCT `c00`.`?x` AS `?x` FROM (\
+                         SELECT $v0 AS `?x` UNION \
+                         SELECT $v1 AS `?x`) AS `c00`");
+    assert_eq!(args, vec![make_arg("$v0", "yyy"),
+                          make_arg("$v1", "zzz"),]);
+
+    // Verify that we can use ground to constrain the bindings produced by earlier clauses.
+    let query = r#"[:find ?x . :where [_ :foo/bar ?x] [(ground "yyy") ?x]]"#;
+    let SQLQuery { sql, args } = translate(&schema, query);
+    assert_eq!(sql, "SELECT $v0 AS `?x` FROM `datoms` AS `datoms00` \
+                     WHERE `datoms00`.a = 99 AND `datoms00`.v = $v0 LIMIT 1");
+
+    assert_eq!(args, vec![make_arg("$v0", "yyy")]);
+
+    // Verify that we can further constrain the bindings produced by our clause.
+    let query = r#"[:find ?x . :where [(ground "yyy") ?x] [_ :foo/bar ?x]]"#;
+    let SQLQuery { sql, args } = translate(&schema, query);
+    assert_eq!(sql, "SELECT $v0 AS `?x` FROM `datoms` AS `datoms00` \
+                     WHERE `datoms00`.a = 99 AND `datoms00`.v = $v0 LIMIT 1");
+
+    assert_eq!(args, vec![make_arg("$v0", "yyy")]);
+}
+
+#[test]
+fn test_unbound_attribute_with_ground_entity() {
+    let query = r#"[:find ?x ?v :where [?x _ ?v] (not [(ground 17) ?x])]"#;
+    let schema = prepopulated_schema();
+    let SQLQuery { sql, .. } = translate(&schema, query);
+    assert_eq!(sql, "SELECT DISTINCT `all_datoms00`.e AS `?x`, \
+                                     `all_datoms00`.v AS `?v`, \
+                                     `all_datoms00`.value_type_tag AS `?v_value_type_tag` \
+                     FROM `all_datoms` AS `all_datoms00` \
+                     WHERE NOT EXISTS (SELECT 1 WHERE `all_datoms00`.e = 17)");
+}
+
+#[test]
+fn test_unbound_attribute_with_ground() {
+    let query = r#"[:find ?x ?v :where [?x _ ?v] (not [(ground 17) ?v])]"#;
+    let schema = prepopulated_schema();
+    let SQLQuery { sql, .. } = translate(&schema, query);
+    assert_eq!(sql, "SELECT DISTINCT `all_datoms00`.e AS `?x`, \
+                                     `all_datoms00`.v AS `?v`, \
+                                     `all_datoms00`.value_type_tag AS `?v_value_type_tag` \
+                     FROM `all_datoms` AS `all_datoms00` \
+                     WHERE NOT EXISTS (SELECT 1 WHERE `all_datoms00`.v = 17 AND \
+                                                      `all_datoms00`.value_type_tag = 5)");
+}
+
+
+#[test]
+fn test_not_with_ground() {
+    let mut schema = prepopulated_schema();
+    associate_ident(&mut schema, NamespacedKeyword::new("db", "valueType"), 7);
+    associate_ident(&mut schema, NamespacedKeyword::new("db.type", "ref"), 23);
+    associate_ident(&mut schema, NamespacedKeyword::new("db.type", "bool"), 28);
+    associate_ident(&mut schema, NamespacedKeyword::new("db.type", "instant"), 29);
+    add_attribute(&mut schema, 7, Attribute {
+        value_type: ValueType::Ref,
+        multival: false,
+        ..Default::default()
+    });
+
+    // Scalar.
+    // TODO: this kind of simple `not` should be implemented without the subquery. #476.
+    let query = r#"[:find ?x :where [?x :db/valueType ?v] (not [(ground :db.type/instant) ?v])]"#;
+    let SQLQuery { sql, .. } = translate(&schema, query);
+    assert_eq!(sql,
+               "SELECT DISTINCT `datoms00`.e AS `?x` FROM `datoms` AS `datoms00` WHERE `datoms00`.a = 7 AND NOT \
+                EXISTS (SELECT 1 WHERE `datoms00`.v = 29)");
+
+    // Coll.
+    // TODO: we can generate better SQL for this, too. #476.
+    let query = r#"[:find ?x :where [?x :db/valueType ?v] (not [(ground [:db.type/bool :db.type/instant]) [?v ...]])]"#;
+    let SQLQuery { sql, .. } = translate(&schema, query);
+    assert_eq!(sql,
+               "SELECT DISTINCT `datoms00`.e AS `?x` FROM `datoms` AS `datoms00` \
+                WHERE `datoms00`.a = 7 AND NOT EXISTS \
+                (SELECT 1 FROM (SELECT 0 AS `?v` WHERE 0 UNION ALL VALUES (28), (29)) AS `c00` \
+                 WHERE `datoms00`.v = `c00`.`?v`)");
+}

query/src/lib.rs

@@ -35,6 +35,7 @@ extern crate mentat_core;
 
 use std::collections::{
     BTreeSet,
+    HashSet,
 };
 
 use std::fmt;
@@ -59,7 +60,7 @@ use mentat_core::{
 
 pub type SrcVarName = String;          // Do not include the required syntactic '$'.
 
-#[derive(Clone, PartialEq, Eq, PartialOrd, Ord)]
+#[derive(Clone, PartialEq, Eq, Hash, PartialOrd, Ord)]
 pub struct Variable(pub Rc<PlainSymbol>);
 
 impl Variable {
@@ -174,7 +175,11 @@ impl FromValue<SrcVar> for SrcVar {
 impl SrcVar {
     pub fn from_symbol(sym: &PlainSymbol) -> Option<SrcVar> {
         if sym.is_src_symbol() {
-            Some(SrcVar::NamedSrc(sym.plain_name().to_string()))
+            if sym.0 == "$" {
+                Some(SrcVar::DefaultSrc)
+            } else {
+                Some(SrcVar::NamedSrc(sym.plain_name().to_string()))
+            }
         } else {
             None
         }
@@ -210,22 +215,47 @@ pub enum FnArg {
     Variable(Variable),
     SrcVar(SrcVar),
     EntidOrInteger(i64),
-    Ident(NamespacedKeyword),
+    IdentOrKeyword(NamespacedKeyword),
     Constant(NonIntegerConstant),
+    // The collection values representable in EDN.  There's no advantage to destructuring up front,
+    // since consumers will need to handle arbitrarily nested EDN themselves anyway.
+    Vector(Vec<FnArg>),
 }
 
 impl FromValue<FnArg> for FnArg {
     fn from_value(v: &edn::ValueAndSpan) -> Option<FnArg> {
-        // TODO: support SrcVars.
-        Variable::from_value(v)
-                 .and_then(|v| Some(FnArg::Variable(v)))
-                 .or_else(|| {
-                          println!("from_value {}", v.inner);
-            match v.inner {
-                edn::SpannedValue::Integer(i) => Some(FnArg::EntidOrInteger(i)),
-                edn::SpannedValue::Float(f) => Some(FnArg::Constant(NonIntegerConstant::Float(f))),
-                _ => unimplemented!(),
-            }})
+        use edn::SpannedValue::*;
+        match v.inner {
+            Integer(x) =>
+                Some(FnArg::EntidOrInteger(x)),
+            PlainSymbol(ref x) if x.is_src_symbol() =>
+                SrcVar::from_symbol(x).map(FnArg::SrcVar),
+            PlainSymbol(ref x) if x.is_var_symbol() =>
+                Variable::from_symbol(x).map(FnArg::Variable),
+            PlainSymbol(_) => None,
+            NamespacedKeyword(ref x) =>
+                Some(FnArg::IdentOrKeyword(x.clone())),
+            Instant(x) =>
+                Some(FnArg::Constant(NonIntegerConstant::Instant(x))),
+            Uuid(x) =>
+                Some(FnArg::Constant(NonIntegerConstant::Uuid(x))),
+            Boolean(x) =>
+                Some(FnArg::Constant(NonIntegerConstant::Boolean(x))),
+            Float(x) =>
+                Some(FnArg::Constant(NonIntegerConstant::Float(x))),
+            BigInteger(ref x) =>
+                Some(FnArg::Constant(NonIntegerConstant::BigInteger(x.clone()))),
+            Text(ref x) =>
+                // TODO: intern strings. #398.
+                Some(FnArg::Constant(NonIntegerConstant::Text(Rc::new(x.clone())))),
+            Nil |
+            NamespacedSymbol(_) |
+            Keyword(_) |
+            Vector(_) |
+            List(_) |
+            Set(_) |
+            Map(_) => None,
+        }
     }
 }
 
@@ -505,6 +535,94 @@ impl FindSpec {
     }
 }
 
+// Datomic accepts variable or placeholder.  DataScript accepts recursive bindings.  Mentat sticks
+// to the non-recursive form Datomic accepts, which is much simpler to process.
+#[derive(Clone, Debug, Eq, Hash, PartialEq)]
+pub enum VariableOrPlaceholder {
+    Placeholder,
+    Variable(Variable),
+}
+
+impl VariableOrPlaceholder {
+    pub fn into_var(self) -> Option<Variable> {
+        match self {
+            VariableOrPlaceholder::Placeholder => None,
+            VariableOrPlaceholder::Variable(var) => Some(var),
+        }
+    }
+
+    pub fn var(&self) -> Option<&Variable> {
+        match self {
+            &VariableOrPlaceholder::Placeholder => None,
+            &VariableOrPlaceholder::Variable(ref var) => Some(var),
+        }
+    }
+}
+
+#[derive(Clone,Debug,Eq,PartialEq)]
+pub enum Binding {
+    BindScalar(Variable),
+    BindColl(Variable),
+    BindRel(Vec<VariableOrPlaceholder>),
+    BindTuple(Vec<VariableOrPlaceholder>),
+}
+
+impl Binding {
+    /// Return each variable or `None`, in order.
+    pub fn variables(&self) -> Vec<Option<Variable>> {
+        match self {
+            &Binding::BindScalar(ref var) | &Binding::BindColl(ref var) => vec![Some(var.clone())],
+            &Binding::BindRel(ref vars) | &Binding::BindTuple(ref vars) => vars.iter().map(|x| x.var().cloned()).collect(),
+        }
+    }
+
+    /// Return `true` if no variables are bound, i.e., all binding entries are placeholders.
+    pub fn is_empty(&self) -> bool {
+        match self {
+            &Binding::BindScalar(_) | &Binding::BindColl(_) => false,
+            &Binding::BindRel(ref vars) | &Binding::BindTuple(ref vars) => vars.iter().all(|x| x.var().is_none()),
+        }
+    }
+
+    /// Return `true` if no variable is bound twice, i.e., each binding entry is either a
+    /// placeholder or unique.
+    ///
+    /// ```
+    /// extern crate mentat_query;
+    /// use std::rc::Rc;
+    ///
+    /// let v = mentat_query::Variable::from_valid_name("?foo");
+    /// let vv = mentat_query::VariableOrPlaceholder::Variable(v);
+    /// let p = mentat_query::VariableOrPlaceholder::Placeholder;
+    ///
+    /// let e = mentat_query::Binding::BindTuple(vec![p.clone()]);
+    /// let b = mentat_query::Binding::BindTuple(vec![p.clone(), vv.clone()]);
+    /// let d = mentat_query::Binding::BindTuple(vec![vv.clone(), p, vv]);
+    /// assert!(b.is_valid());          // One var, one placeholder: OK.
+    /// assert!(!e.is_valid());         // Empty: not OK.
+    /// assert!(!d.is_valid());         // Duplicate var: not OK.
+    /// ```
+    pub fn is_valid(&self) -> bool {
+        match self {
+            &Binding::BindScalar(_) | &Binding::BindColl(_) => true,
+            &Binding::BindRel(ref vars) | &Binding::BindTuple(ref vars) => {
+                let mut acc = HashSet::<Variable>::new();
+                for var in vars {
+                    if let &VariableOrPlaceholder::Variable(ref var) = var {
+                        if !acc.insert(var.clone()) {
+                            // It's invalid if there was an equal var already present in the set --
+                            // i.e., we have a duplicate var.
+                            return false;
+                        }
+                    }
+                }
+                // We're not valid if every place is a placeholder!
+                !acc.is_empty()
+            }
+        }
+    }
+}
+
 // Note that the "implicit blank" rule applies.
 // A pattern with a reversed attribute — :foo/_bar — is reversed
 // at the point of parsing. These `Pattern` instances only represent
@@ -560,6 +678,13 @@ pub struct Predicate {
     pub args: Vec<FnArg>,
 }
 
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub struct WhereFn {
+    pub operator: PlainSymbol,
+    pub args: Vec<FnArg>,
+    pub binding: Binding,
+}
+
 #[derive(Clone, Debug, Eq, PartialEq)]
 pub enum UnifyVars {
     /// `Implicit` means the variables in an `or` or `not` are derived from the enclosed pattern.
@@ -586,7 +711,7 @@ pub enum UnifyVars {
     /// Only the named variables will be unified with the enclosing query.
     ///
     /// Every 'arm' in an `or-join` must mention the entire set of explicit vars.
-    Explicit(Vec<Variable>),
+    Explicit(BTreeSet<Variable>),
 }
 
 impl WhereClause {
@@ -635,7 +760,7 @@ pub enum WhereClause {
     NotJoin(NotJoin),
     OrJoin(OrJoin),
     Pred(Predicate),
-    WhereFn,
+    WhereFn(WhereFn),
     RuleExpr,
     Pattern(Pattern),
 }
@@ -701,7 +826,7 @@ impl ContainsVariables for WhereClause {
             &Pred(ref p)    => p.accumulate_mentioned_variables(acc),
             &Pattern(ref p) => p.accumulate_mentioned_variables(acc),
             &NotJoin(ref n) => n.accumulate_mentioned_variables(acc),
-            &WhereFn        => (),
+            &WhereFn(ref f) => f.accumulate_mentioned_variables(acc),
             &RuleExpr       => (),
         }
     }
@@ -765,6 +890,34 @@ impl ContainsVariables for Predicate {
     }
 }
 
+impl ContainsVariables for Binding {
+    fn accumulate_mentioned_variables(&self, acc: &mut BTreeSet<Variable>) {
+        match self {
+            &Binding::BindScalar(ref v) | &Binding::BindColl(ref v) => {
+                acc_ref(acc, v)
+            },
+            &Binding::BindRel(ref vs) | &Binding::BindTuple(ref vs) => {
+                for v in vs {
+                    if let &VariableOrPlaceholder::Variable(ref v) = v {
+                        acc_ref(acc, v);
+                    }
+                }
+            },
+        }
+    }
+}
+
+impl ContainsVariables for WhereFn {
+    fn accumulate_mentioned_variables(&self, acc: &mut BTreeSet<Variable>) {
+        for arg in &self.args {
+            if let &FnArg::Variable(ref v) = arg {
+                acc_ref(acc, v)
+            }
+        }
+        self.binding.accumulate_mentioned_variables(acc);
+    }
+}
+
 fn acc_ref<T: Clone + Ord>(acc: &mut BTreeSet<T>, v: &T) {
     // Roll on, reference entries!
     if !acc.contains(v) {