diff --git a/core/src/lib.rs b/core/src/lib.rs
index b8285137..73e28858 100644
--- a/core/src/lib.rs
+++ b/core/src/lib.rs
@@ -18,7 +18,11 @@ extern crate edn;
 
 pub mod values;
 
-use std::collections::BTreeMap;
+use std::collections::{
+    BTreeMap,
+    BTreeSet,
+};
+
 use std::fmt;
 use std::rc::Rc;
 
@@ -64,6 +68,8 @@ pub enum ValueType {
     Uuid,
 }
 
+pub type ValueTypeTag = i32;
+
 impl ValueType {
     pub fn all_enums() -> EnumSet<ValueType> {
         // TODO: lazy_static.
@@ -229,6 +235,178 @@ impl SQLValueType for ValueType {
     }
 }
 
+trait EnumSetExtensions<T: enum_set::CLike + Clone> {
+    /// Return a set containing both `x` and `y`.
+    fn of_both(x: T, y: T) -> EnumSet<T>;
+
+    /// Return a clone of `self` with `y` added.
+    fn with(&self, y: T) -> EnumSet<T>;
+}
+
+impl<T: enum_set::CLike + Clone> EnumSetExtensions<T> for EnumSet<T> {
+    /// Return a set containing both `x` and `y`.
+    fn of_both(x: T, y: T) -> Self {
+        let mut o = EnumSet::new();
+        o.insert(x);
+        o.insert(y);
+        o
+    }
+
+    /// Return a clone of `self` with `y` added.
+    fn with(&self, y: T) -> EnumSet<T> {
+        let mut o = self.clone();
+        o.insert(y);
+        o
+    }
+}
+
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub struct ValueTypeSet(pub EnumSet<ValueType>);
+
+impl Default for ValueTypeSet {
+    fn default() -> ValueTypeSet {
+        ValueTypeSet::any()
+    }
+}
+
+impl ValueTypeSet {
+    pub fn any() -> ValueTypeSet {
+        ValueTypeSet(ValueType::all_enums())
+    }
+
+    pub fn none() -> ValueTypeSet {
+        ValueTypeSet(EnumSet::new())
+    }
+
+    /// Return a set containing only `t`.
+    pub fn of_one(t: ValueType) -> ValueTypeSet {
+        let mut s = EnumSet::new();
+        s.insert(t);
+        ValueTypeSet(s)
+    }
+
+    /// Return a set containing `Double` and `Long`.
+    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))
+    }
+
+    pub fn intersection(&self, other: &ValueTypeSet) -> ValueTypeSet {
+        ValueTypeSet(self.0.intersection(other.0))
+    }
+
+    /// Return an arbitrary type that's part of this set.
+    /// For a set containing a single type, this will be that type.
+    pub fn exemplar(&self) -> Option<ValueType> {
+        self.0.iter().next()
+    }
+
+    pub fn is_subset(&self, other: &ValueTypeSet) -> bool {
+        self.0.is_subset(&other.0)
+    }
+
+    pub fn contains(&self, vt: ValueType) -> bool {
+        self.0.contains(&vt)
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+
+    pub fn is_unit(&self) -> bool {
+        self.0.len() == 1
+    }
+}
+
+impl IntoIterator for ValueTypeSet {
+    type Item = ValueType;
+    type IntoIter = ::enum_set::Iter<ValueType>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.0.into_iter()
+    }
+}
+
+impl ::std::iter::FromIterator<ValueType> for ValueTypeSet {
+    fn from_iter<I: IntoIterator<Item = ValueType>>(iterator: I) -> Self {
+        let mut ret = Self::none();
+        ret.0.extend(iterator);
+        ret
+    }
+}
+
+impl ::std::iter::Extend<ValueType> for ValueTypeSet {
+    fn extend<I: IntoIterator<Item = ValueType>>(&mut self, iter: I) {
+        for element in iter {
+            self.0.insert(element);
+        }
+    }
+}
+
+pub trait SQLValueTypeSet {
+    fn value_type_tags(&self) -> BTreeSet<ValueTypeTag>;
+    fn has_unique_type_code(&self) -> bool;
+    fn unique_type_code(&self) -> Option<ValueTypeTag>;
+}
+
+impl SQLValueTypeSet for ValueTypeSet {
+    // This is inefficient, but it'll do for now.
+    fn value_type_tags(&self) -> BTreeSet<ValueTypeTag> {
+        let mut out = BTreeSet::new();
+        for t in self.0.iter() {
+            out.insert(t.value_type_tag());
+        }
+        out
+    }
+
+    fn unique_type_code(&self) -> Option<ValueTypeTag> {
+        if self.is_unit() || self.has_unique_type_code() {
+            self.exemplar().map(|t| t.value_type_tag())
+        } else {
+            None
+        }
+    }
+
+    fn has_unique_type_code(&self) -> bool {
+        if self.is_unit() {
+            return true;
+        }
+
+        let mut acc = BTreeSet::new();
+        for t in self.0.iter() {
+            if acc.insert(t.value_type_tag()) && acc.len() > 1 {
+                // We inserted a second or subsequent value.
+                return false;
+            }
+        }
+        !acc.is_empty()
+    }
+}
+
 #[test]
 fn test_typed_value() {
     assert!(TypedValue::Boolean(false).is_congruent_with(None));
diff --git a/query-algebrizer/Cargo.toml b/query-algebrizer/Cargo.toml
index 7df1b243..310af20d 100644
--- a/query-algebrizer/Cargo.toml
+++ b/query-algebrizer/Cargo.toml
@@ -4,7 +4,6 @@ version = "0.0.1"
 workspace = ".."
 
 [dependencies]
-enum-set = { git = "https://github.com/rnewman/enum-set" }
 error-chain = { git = "https://github.com/rnewman/error-chain", branch = "rnewman/sync" }
 
 [dependencies.mentat_core]
diff --git a/query-algebrizer/src/clauses/convert.rs b/query-algebrizer/src/clauses/convert.rs
index f232ef64..bffb9783 100644
--- a/query-algebrizer/src/clauses/convert.rs
+++ b/query-algebrizer/src/clauses/convert.rs
@@ -15,6 +15,7 @@ use mentat_core::{
     SQLValueType,
     TypedValue,
     ValueType,
+    ValueTypeSet,
 };
 
 use mentat_query::{
@@ -34,7 +35,6 @@ use errors::{
 
 use types::{
     EmptyBecause,
-    ValueTypeSet,
 };
 
 macro_rules! coerce_to_typed_value {
diff --git a/query-algebrizer/src/clauses/ground.rs b/query-algebrizer/src/clauses/ground.rs
index d7ad7557..f6d81283 100644
--- a/query-algebrizer/src/clauses/ground.rs
+++ b/query-algebrizer/src/clauses/ground.rs
@@ -12,6 +12,7 @@ use mentat_core::{
     Schema,
     TypedValue,
     ValueType,
+    ValueTypeSet,
 };
 
 use mentat_query::{
@@ -39,7 +40,6 @@ use types::{
     ComputedTable,
     EmptyBecause,
     SourceAlias,
-    ValueTypeSet,
     VariableColumn,
 };
 
@@ -335,10 +335,6 @@ mod testing {
         associate_ident,
     };
 
-    use types::{
-        ValueTypeSet,
-    };
-
     #[test]
     fn test_apply_ground() {
         let vz = Variable::from_valid_name("?z");
diff --git a/query-algebrizer/src/clauses/mod.rs b/query-algebrizer/src/clauses/mod.rs
index 7ee93032..2e26a07f 100644
--- a/query-algebrizer/src/clauses/mod.rs
+++ b/query-algebrizer/src/clauses/mod.rs
@@ -28,6 +28,7 @@ use mentat_core::{
     Schema,
     TypedValue,
     ValueType,
+    ValueTypeSet,
 };
 
 use mentat_core::counter::RcCounter;
@@ -59,7 +60,6 @@ use types::{
     QueryValue,
     SourceAlias,
     TableAlias,
-    ValueTypeSet,
 };
 
 mod convert;              // Converting args to values.
@@ -365,8 +365,17 @@ impl ConjoiningClauses {
         self.value_bindings.get(var).cloned()
     }
 
+    pub fn is_value_bound(&self, var: &Variable) -> bool {
+        self.value_bindings.contains_key(var)
+    }
+
+    /// Return an interator over the variables externally bound to values.
+    pub fn value_bound_variables(&self) -> ::std::collections::btree_map::Keys<Variable, TypedValue> {
+        self.value_bindings.keys()
+    }
+
     /// Return a set of the variables externally bound to values.
-    pub fn value_bound_variables(&self) -> BTreeSet<Variable> {
+    pub fn value_bound_variable_set(&self) -> BTreeSet<Variable> {
         self.value_bindings.keys().cloned().collect()
     }
 
diff --git a/query-algebrizer/src/clauses/not.rs b/query-algebrizer/src/clauses/not.rs
index 308cf28a..30a089e0 100644
--- a/query-algebrizer/src/clauses/not.rs
+++ b/query-algebrizer/src/clauses/not.rs
@@ -81,6 +81,7 @@ mod testing {
         Attribute, 
         TypedValue, 
         ValueType,
+        ValueTypeSet,
     };
 
     use mentat_query::{
@@ -113,7 +114,6 @@ mod testing {
         QualifiedAlias,
         QueryValue, 
         SourceAlias, 
-        ValueTypeSet,
     };
 
     use {
diff --git a/query-algebrizer/src/clauses/or.rs b/query-algebrizer/src/clauses/or.rs
index ed1cc239..6f47ec6a 100644
--- a/query-algebrizer/src/clauses/or.rs
+++ b/query-algebrizer/src/clauses/or.rs
@@ -16,6 +16,7 @@ use std::collections::{
 
 use mentat_core::{
     Schema,
+    ValueTypeSet,
 };
 
 use mentat_query::{
@@ -47,7 +48,6 @@ use types::{
     EmptyBecause,
     QualifiedAlias,
     SourceAlias,
-    ValueTypeSet,
     VariableColumn,
 };
 
diff --git a/query-algebrizer/src/clauses/pattern.rs b/query-algebrizer/src/clauses/pattern.rs
index 516816b6..6bba9d44 100644
--- a/query-algebrizer/src/clauses/pattern.rs
+++ b/query-algebrizer/src/clauses/pattern.rs
@@ -294,6 +294,7 @@ mod testing {
     use mentat_core::attribute::Unique;
     use mentat_core::{
         Attribute,
+        ValueTypeSet,
     };
 
     use mentat_query::{
@@ -320,7 +321,6 @@ mod testing {
         QualifiedAlias,
         QueryValue,
         SourceAlias,
-        ValueTypeSet,
     };
 
     use algebrize;
diff --git a/query-algebrizer/src/clauses/predicate.rs b/query-algebrizer/src/clauses/predicate.rs
index 0a77ac21..a7b22549 100644
--- a/query-algebrizer/src/clauses/predicate.rs
+++ b/query-algebrizer/src/clauses/predicate.rs
@@ -11,6 +11,7 @@
 use mentat_core::{
     Schema,
     ValueType,
+    ValueTypeSet,
 };
 
 use mentat_query::{
@@ -31,7 +32,6 @@ use types::{
     ColumnConstraint,
     EmptyBecause,
     Inequality,
-    ValueTypeSet,
 };
 
 /// Application of predicates.
@@ -177,7 +177,6 @@ mod testing {
         ColumnConstraint,
         EmptyBecause,
         QueryValue,
-        ValueTypeSet,
     };
 
     #[test]
diff --git a/query-algebrizer/src/lib.rs b/query-algebrizer/src/lib.rs
index b25b221d..f81e55e8 100644
--- a/query-algebrizer/src/lib.rs
+++ b/query-algebrizer/src/lib.rs
@@ -8,8 +8,6 @@
 // CONDITIONS OF ANY KIND, either express or implied. See the License for the
 // specific language governing permissions and limitations under the License.
 
-extern crate enum_set;
-
 #[macro_use]
 extern crate error_chain;
 
@@ -58,7 +56,6 @@ pub use clauses::{
 
 pub use types::{
     EmptyBecause,
-    ValueTypeSet,
 };
 
 #[derive(Debug)]
@@ -81,7 +78,7 @@ impl AlgebraicQuery {
     /// Return a set of the input variables mentioned in the `:in` clause that have not yet been
     /// bound. We do this by looking at the CC.
     pub fn unbound_variables(&self) -> BTreeSet<Variable> {
-        self.cc.input_variables.sub(&self.cc.value_bound_variables())
+        self.cc.input_variables.sub(&self.cc.value_bound_variable_set())
     }
 }
 
diff --git a/query-algebrizer/src/types.rs b/query-algebrizer/src/types.rs
index 970404a1..57132a99 100644
--- a/query-algebrizer/src/types.rs
+++ b/query-algebrizer/src/types.rs
@@ -15,15 +15,11 @@ use std::fmt::{
     Result,
 };
 
-use enum_set::{
-    CLike,
-    EnumSet,
-};
-
 use mentat_core::{
     Entid,
     TypedValue,
     ValueType,
+    ValueTypeSet,
 };
 
 use mentat_query::{
@@ -540,135 +536,3 @@ impl Debug for EmptyBecause {
         }
     }
 }
-
-trait EnumSetExtensions<T: CLike + Clone> {
-    /// Return a set containing both `x` and `y`.
-    fn of_both(x: T, y: T) -> EnumSet<T>;
-
-    /// Return a clone of `self` with `y` added.
-    fn with(&self, y: T) -> EnumSet<T>;
-}
-
-impl<T: CLike + Clone> EnumSetExtensions<T> for EnumSet<T> {
-    /// Return a set containing both `x` and `y`.
-    fn of_both(x: T, y: T) -> Self {
-        let mut o = EnumSet::new();
-        o.insert(x);
-        o.insert(y);
-        o
-    }
-
-    /// Return a clone of `self` with `y` added.
-    fn with(&self, y: T) -> EnumSet<T> {
-        let mut o = self.clone();
-        o.insert(y);
-        o
-    }
-}
-
-#[derive(Clone, Copy, Debug, Eq, PartialEq)]
-pub struct ValueTypeSet(pub EnumSet<ValueType>);
-
-impl Default for ValueTypeSet {
-    fn default() -> ValueTypeSet {
-        ValueTypeSet::any()
-    }
-}
-
-impl ValueTypeSet {
-    pub fn any() -> ValueTypeSet {
-        ValueTypeSet(ValueType::all_enums())
-    }
-
-    pub fn none() -> ValueTypeSet {
-        ValueTypeSet(EnumSet::new())
-    }
-
-    /// Return a set containing only `t`.
-    pub fn of_one(t: ValueType) -> ValueTypeSet {
-        let mut s = EnumSet::new();
-        s.insert(t);
-        ValueTypeSet(s)
-    }
-
-    /// Return a set containing `Double` and `Long`.
-    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))
-    }
-
-    pub fn intersection(&self, other: &ValueTypeSet) -> ValueTypeSet {
-        ValueTypeSet(self.0.intersection(other.0))
-    }
-
-    /// Return an arbitrary type that's part of this set.
-    /// For a set containing a single type, this will be that type.
-    pub fn exemplar(&self) -> Option<ValueType> {
-        self.0.iter().next()
-    }
-
-    pub fn is_subset(&self, other: &ValueTypeSet) -> bool {
-        self.0.is_subset(&other.0)
-    }
-
-    pub fn contains(&self, vt: ValueType) -> bool {
-        self.0.contains(&vt)
-    }
-
-    pub fn is_empty(&self) -> bool {
-        self.0.is_empty()
-    }
-
-    pub fn is_unit(&self) -> bool {
-        self.0.len() == 1
-    }
-}
-
-impl IntoIterator for ValueTypeSet {
-    type Item = ValueType;
-    type IntoIter = ::enum_set::Iter<ValueType>;
-
-    fn into_iter(self) -> Self::IntoIter {
-        self.0.into_iter()
-    }
-}
-
-impl ::std::iter::FromIterator<ValueType> for ValueTypeSet {
-    fn from_iter<I: IntoIterator<Item = ValueType>>(iterator: I) -> Self {
-        let mut ret = Self::none();
-        ret.0.extend(iterator);
-        ret
-    }
-}
-
-impl ::std::iter::Extend<ValueType> for ValueTypeSet {
-    fn extend<I: IntoIterator<Item = ValueType>>(&mut self, iter: I) {
-        for element in iter {
-            self.0.insert(element);
-        }
-    }
-}
diff --git a/query-algebrizer/tests/predicate.rs b/query-algebrizer/tests/predicate.rs
index 4c34d4d9..2ecf74ed 100644
--- a/query-algebrizer/tests/predicate.rs
+++ b/query-algebrizer/tests/predicate.rs
@@ -18,6 +18,7 @@ use mentat_core::{
     Entid,
     Schema,
     ValueType,
+    ValueTypeSet,
 };
 
 use mentat_query_parser::{
@@ -35,7 +36,6 @@ use mentat_query_algebrizer::{
     EmptyBecause,
     Error,
     ErrorKind,
-    ValueTypeSet,
     algebrize,
 };
 
diff --git a/query-parser/src/parse.rs b/query-parser/src/parse.rs
index f671489c..79614105 100644
--- a/query-parser/src/parse.rs
+++ b/query-parser/src/parse.rs
@@ -54,7 +54,7 @@ use self::mentat_query::{
     PatternNonValuePlace,
     PatternValuePlace,
     Predicate,
-    PredicateFn,
+    QueryFunction,
     SrcVar,
     UnifyVars,
     Variable,
@@ -132,8 +132,8 @@ def_parser!(Query, source_var, SrcVar, {
 });
 
 // TODO: interning.
-def_parser!(Query, predicate_fn, PredicateFn, {
-    satisfy_map(PredicateFn::from_value)
+def_parser!(Query, query_function, QueryFunction, {
+    satisfy_map(QueryFunction::from_value)
 });
 
 def_parser!(Query, fn_arg, FnArg, {
@@ -266,13 +266,17 @@ def_parser!(Where, not_join_clause, WhereClause, {
             }))
 });
 
+def_parser!(Query, func, (QueryFunction, Vec<FnArg>), {
+    (Query::query_function(), Query::arguments())
+});
+
 /// A vector containing just a parenthesized filter expression.
 def_parser!(Where, pred, WhereClause, {
     // Accept either a nested list or a nested vector here:
     // `[(foo ?x ?y)]` or `[[foo ?x ?y]]`
     vector()
         .of_exactly(seq()
-            .of_exactly((Query::predicate_fn(), Query::arguments())
+            .of_exactly(Query::func()
                 .map(|(f, args)| {
                     WhereClause::Pred(
                         Predicate {
@@ -289,7 +293,7 @@ def_parser!(Where, where_fn, WhereClause, {
     vector()
         .of_exactly(
             (seq().of_exactly(
-                (Query::predicate_fn(), Query::arguments())),
+             Query::func()),
              Bind::binding())
                     .map(|((f, args), binding)| {
                         WhereClause::WhereFn(
@@ -370,21 +374,22 @@ def_matches_plain_symbol!(Find, ellipsis, "...");
 
 def_matches_plain_symbol!(Find, placeholder, "_");
 
+def_parser!(Find, elem, Element, {
+    Query::variable().map(Element::Variable)
+});
+
 def_parser!(Find, find_scalar, FindSpec, {
-    Query::variable()
-        .skip(Find::period())
-        .map(|var| FindSpec::FindScalar(Element::Variable(var)))
+    Find::elem().skip(Find::period())
+                .map(FindSpec::FindScalar)
 });
 
 def_parser!(Find, find_coll, FindSpec, {
-    vector()
-        .of_exactly(Query::variable()
-            .skip(Find::ellipsis()))
-            .map(|var| FindSpec::FindColl(Element::Variable(var)))
+    vector().of_exactly(Find::elem().skip(Find::ellipsis()))
+            .map(FindSpec::FindColl)
 });
 
 def_parser!(Find, elements, Vec<Element>, {
-    many1::<Vec<Element>, _>(Query::variable().map(Element::Variable))
+    many1::<Vec<Element>, _>(Find::elem())
 });
 
 def_parser!(Find, find_rel, FindSpec, {
@@ -392,8 +397,8 @@ def_parser!(Find, find_rel, FindSpec, {
 });
 
 def_parser!(Find, find_tuple, FindSpec, {
-    vector()
-        .of_exactly(Find::elements().map(FindSpec::FindTuple))
+    vector().of_exactly(Find::elements())
+            .map(FindSpec::FindTuple)
 });
 
 /// Parse a stream of values into one of four find specs.
@@ -462,7 +467,7 @@ def_parser!(Find, query, FindQuery, {
 
             Ok(FindQuery {
                 default_source: SrcVar::DefaultSrc,
-                find_spec: find_spec.clone().ok_or(combine::primitives::Error::Unexpected("expected :find".into()))?,
+                find_spec: find_spec.ok_or(combine::primitives::Error::Unexpected("expected :find".into()))?,
                 in_sources: BTreeSet::default(),    // TODO
                 in_vars: in_vars,
                 limit: limit,
diff --git a/query-projector/src/lib.rs b/query-projector/src/lib.rs
index b9922731..f7e963f0 100644
--- a/query-projector/src/lib.rs
+++ b/query-projector/src/lib.rs
@@ -27,8 +27,11 @@ use rusqlite::{
 
 use mentat_core::{
     SQLValueType,
+    SQLValueTypeSet,
     TypedValue,
     ValueType,
+    ValueTypeTag,
+    ValueTypeSet,
 };
 
 use mentat_db::{
@@ -121,7 +124,6 @@ impl QueryResults {
 }
 
 type Index = i32;            // See rusqlite::RowIndex.
-type ValueTypeTag = i32;
 enum TypedIndex {
     Known(Index, ValueTypeTag),
     Unknown(Index, Index),
@@ -211,7 +213,7 @@ pub fn projected_column_for_var(var: &Variable, cc: &ConjoiningClauses) -> (Proj
 fn project_elements<'a, I: IntoIterator<Item = &'a Element>>(
     count: usize,
     elements: I,
-    query: &AlgebraicQuery) -> (Projection, Vec<TypedIndex>) {
+    query: &AlgebraicQuery) -> Result<(Projection, Vec<TypedIndex>)> {
 
     let mut cols = Vec::with_capacity(count);
     let mut i: i32 = 0;
@@ -257,7 +259,7 @@ fn project_elements<'a, I: IntoIterator<Item = &'a Element>>(
         }
     }
 
-    (Projection::Columns(cols), templates)
+    Ok((Projection::Columns(cols), templates))
 }
 
 pub trait Projector {
@@ -293,13 +295,13 @@ impl ScalarProjector {
         }
     }
 
-    fn combine(sql: Projection, mut templates: Vec<TypedIndex>) -> CombinedProjection {
+    fn combine(sql: Projection, mut templates: Vec<TypedIndex>) -> Result<CombinedProjection> {
         let template = templates.pop().expect("Expected a single template");
-        CombinedProjection {
+        Ok(CombinedProjection {
             sql_projection: sql,
             datalog_projector: Box::new(ScalarProjector::with_template(template)),
             distinct: false,
-        }
+        })
     }
 }
 
@@ -338,13 +340,13 @@ impl TupleProjector {
             .collect::<Result<Vec<TypedValue>>>()
     }
 
-    fn combine(column_count: usize, sql: Projection, templates: Vec<TypedIndex>) -> CombinedProjection {
+    fn combine(column_count: usize, sql: Projection, templates: Vec<TypedIndex>) -> Result<CombinedProjection> {
         let p = TupleProjector::with_templates(column_count, templates);
-        CombinedProjection {
+        Ok(CombinedProjection {
             sql_projection: sql,
             datalog_projector: Box::new(p),
             distinct: false,
-        }
+        })
     }
 }
 
@@ -386,13 +388,13 @@ impl RelProjector {
             .collect::<Result<Vec<TypedValue>>>()
     }
 
-    fn combine(column_count: usize, sql: Projection, templates: Vec<TypedIndex>) -> CombinedProjection {
+    fn combine(column_count: usize, sql: Projection, templates: Vec<TypedIndex>) -> Result<CombinedProjection> {
         let p = RelProjector::with_templates(column_count, templates);
-        CombinedProjection {
+        Ok(CombinedProjection {
             sql_projection: sql,
             datalog_projector: Box::new(p),
             distinct: true,
-        }
+        })
     }
 }
 
@@ -421,13 +423,13 @@ impl CollProjector {
         }
     }
 
-    fn combine(sql: Projection, mut templates: Vec<TypedIndex>) -> CombinedProjection {
+    fn combine(sql: Projection, mut templates: Vec<TypedIndex>) -> Result<CombinedProjection> {
         let template = templates.pop().expect("Expected a single template");
-        CombinedProjection {
+        Ok(CombinedProjection {
             sql_projection: sql,
             datalog_projector: Box::new(CollProjector::with_template(template)),
             distinct: true,
-        }
+        })
     }
 }
 
@@ -475,39 +477,39 @@ impl CombinedProjection {
 /// - The bindings established by the topmost CC.
 /// - The types known at algebrizing time.
 /// - The types extracted from the store for unknown attributes.
-pub fn query_projection(query: &AlgebraicQuery) -> CombinedProjection {
+pub fn query_projection(query: &AlgebraicQuery) -> Result<CombinedProjection> {
     use self::FindSpec::*;
 
     if query.is_known_empty() {
         // Do a few gyrations to produce empty results of the right kind for the query.
         let empty = QueryResults::empty_factory(&query.find_spec);
         let constant_projector = ConstantProjector::new(empty);
-        CombinedProjection {
+        Ok(CombinedProjection {
             sql_projection: Projection::One,
             datalog_projector: Box::new(constant_projector),
             distinct: false,
-        }
+        })
     } else {
         match query.find_spec {
             FindColl(ref element) => {
-                let (cols, templates) = project_elements(1, iter::once(element), query);
-                CollProjector::combine(cols, templates).flip_distinct_for_limit(&query.limit)
+                let (cols, templates) = project_elements(1, iter::once(element), query)?;
+                CollProjector::combine(cols, templates).map(|p| p.flip_distinct_for_limit(&query.limit))
             },
 
             FindScalar(ref element) => {
-                let (cols, templates) = project_elements(1, iter::once(element), query);
+                let (cols, templates) = project_elements(1, iter::once(element), query)?;
                 ScalarProjector::combine(cols, templates)
             },
 
             FindRel(ref elements) => {
                 let column_count = query.find_spec.expected_column_count();
-                let (cols, templates) = project_elements(column_count, elements, query);
-                RelProjector::combine(column_count, cols, templates).flip_distinct_for_limit(&query.limit)
+                let (cols, templates) = project_elements(column_count, elements, query)?;
+                RelProjector::combine(column_count, cols, templates).map(|p| p.flip_distinct_for_limit(&query.limit))
             },
 
             FindTuple(ref elements) => {
                 let column_count = query.find_spec.expected_column_count();
-                let (cols, templates) = project_elements(column_count, elements, query);
+                let (cols, templates) = project_elements(column_count, elements, query)?;
                 TupleProjector::combine(column_count, cols, templates)
             },
         }
diff --git a/query-translator/Cargo.toml b/query-translator/Cargo.toml
index e2a8f141..1edc8728 100644
--- a/query-translator/Cargo.toml
+++ b/query-translator/Cargo.toml
@@ -4,6 +4,8 @@ version = "0.0.1"
 workspace = ".."
 
 [dependencies]
+error-chain = { git = "https://github.com/rnewman/error-chain", branch = "rnewman/sync" }
+
 [dependencies.mentat_core]
 path = "../core"
 
diff --git a/query-translator/src/lib.rs b/query-translator/src/lib.rs
index 30a8395c..003e187a 100644
--- a/query-translator/src/lib.rs
+++ b/query-translator/src/lib.rs
@@ -8,6 +8,8 @@
 // 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 error_chain;
 extern crate mentat_core;
 extern crate mentat_query;
 extern crate mentat_query_algebrizer;
@@ -25,3 +27,16 @@ pub use translate::{
     cc_to_exists,
     query_to_select,
 };
+
+error_chain! {
+    types {
+        Error, ErrorKind, ResultExt, Result;
+    }
+
+    foreign_links {
+    }
+
+    links {
+        ProjectorError(mentat_query_projector::Error, mentat_query_projector::ErrorKind);
+    }
+}
diff --git a/query-translator/src/translate.rs b/query-translator/src/translate.rs
index 52170413..47019a1e 100644
--- a/query-translator/src/translate.rs
+++ b/query-translator/src/translate.rs
@@ -10,6 +10,7 @@
 
 use mentat_core::{
     SQLValueType,
+    SQLValueTypeSet,
     TypedValue,
     ValueType,
 };
@@ -55,6 +56,8 @@ use mentat_query_sql::{
     Values,
 };
 
+use super::Result;
+
 trait ToConstraint {
     fn to_constraint(self) -> Constraint;
 }
@@ -329,12 +332,12 @@ pub fn cc_to_exists(cc: ConjoiningClauses) -> SelectQuery {
 
 /// Consume a provided `AlgebraicQuery` to yield a new
 /// `ProjectedSelect`.
-pub fn query_to_select(query: AlgebraicQuery) -> ProjectedSelect {
+pub fn query_to_select(query: AlgebraicQuery) -> Result<ProjectedSelect> {
     // TODO: we can't pass `query.limit` here if we aggregate during projection.
     // SQL-based aggregation -- `SELECT SUM(datoms00.e)` -- is fine.
-    let CombinedProjection { sql_projection, datalog_projector, distinct } = query_projection(&query);
-    ProjectedSelect {
+    let CombinedProjection { sql_projection, datalog_projector, distinct } = query_projection(&query)?;
+    Ok(ProjectedSelect {
         query: cc_to_select_query(sql_projection, query.cc, distinct, query.order, query.limit),
         projector: datalog_projector,
-    }
+    })
 }
diff --git a/query-translator/tests/translate.rs b/query-translator/tests/translate.rs
index fb9d0aec..605a4e71 100644
--- a/query-translator/tests/translate.rs
+++ b/query-translator/tests/translate.rs
@@ -54,9 +54,9 @@ fn add_attribute(schema: &mut Schema, e: Entid, a: Attribute) {
 }
 
 fn translate_with_inputs(schema: &Schema, query: &'static str, inputs: QueryInputs) -> SQLQuery {
-    let parsed = parse_find_string(query).expect("parse failed");
-    let algebrized = algebrize_with_inputs(schema, parsed, 0, inputs).expect("algebrize failed");
-    let select = query_to_select(algebrized);
+    let parsed = parse_find_string(query).expect("parse to succeed");
+    let algebrized = algebrize_with_inputs(schema, parsed, 0, inputs).expect("algebrize to succeed");
+    let select = query_to_select(algebrized).expect("translate to succeed");
     select.query.to_sql_query().unwrap()
 }
 
@@ -191,7 +191,7 @@ fn test_bound_variable_limit_affects_types() {
     assert_eq!(Some(ValueType::Long),
                algebrized.cc.known_type(&Variable::from_valid_name("?limit")));
 
-    let select = query_to_select(algebrized);
+    let select = query_to_select(algebrized).expect("query to translate");
     let SQLQuery { sql, args } = select.query.to_sql_query().unwrap();
 
     // TODO: this query isn't actually correct -- we don't yet algebrize for variables that are
@@ -281,7 +281,7 @@ fn test_unknown_ident() {
     assert!(algebrized.is_known_empty());
 
     // If you insist…
-    let select = query_to_select(algebrized);
+    let select = query_to_select(algebrized).expect("query to translate");
     let sql = select.query.to_sql_query().unwrap().sql;
     assert_eq!("SELECT 1 LIMIT 0", sql);
 }
diff --git a/query/src/lib.rs b/query/src/lib.rs
index 85703534..1dc7168b 100644
--- a/query/src/lib.rs
+++ b/query/src/lib.rs
@@ -126,23 +126,23 @@ impl fmt::Debug for Variable {
     }
 }
 
-#[derive(Clone, PartialEq, Eq, PartialOrd, Ord)]
-pub struct PredicateFn(pub PlainSymbol);
+#[derive(Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
+pub struct QueryFunction(pub PlainSymbol);
 
-impl FromValue<PredicateFn> for PredicateFn {
-    fn from_value(v: &edn::ValueAndSpan) -> Option<PredicateFn> {
+impl FromValue<QueryFunction> for QueryFunction {
+    fn from_value(v: &edn::ValueAndSpan) -> Option<QueryFunction> {
         if let edn::SpannedValue::PlainSymbol(ref s) = v.inner {
-            PredicateFn::from_symbol(s)
+            QueryFunction::from_symbol(s)
         } else {
             None
         }
     }
 }
 
-impl PredicateFn {
-    pub fn from_symbol(sym: &PlainSymbol) -> Option<PredicateFn> {
+impl QueryFunction {
+    pub fn from_symbol(sym: &PlainSymbol) -> Option<QueryFunction> {
         // TODO: validate the acceptable set of function names.
-        Some(PredicateFn(sym.clone()))
+        Some(QueryFunction(sym.clone()))
     }
 }
 
@@ -435,7 +435,7 @@ pub struct Aggregate {
 }
 */
 
-#[derive(Clone, Debug, Eq, PartialEq)]
+#[derive(Debug, Eq, PartialEq)]
 pub enum Element {
     Variable(Variable),
     // Aggregate(Aggregate),   // TODO
@@ -480,7 +480,7 @@ pub enum Limit {
 /// # }
 /// ```
 ///
-#[derive(Clone, Debug, Eq, PartialEq)]
+#[derive(Debug, Eq, PartialEq)]
 pub enum FindSpec {
     /// Returns an array of arrays.
     FindRel(Vec<Element>),
@@ -775,7 +775,7 @@ pub enum WhereClause {
 }
 
 #[allow(dead_code)]
-#[derive(Clone, Debug, Eq, PartialEq)]
+#[derive(Debug, Eq, PartialEq)]
 pub struct FindQuery {
     pub find_spec: FindSpec,
     pub default_source: SrcVar,
diff --git a/src/errors.rs b/src/errors.rs
index 96587dc8..17c9f35a 100644
--- a/src/errors.rs
+++ b/src/errors.rs
@@ -19,6 +19,7 @@ use mentat_db;
 use mentat_query_algebrizer;
 use mentat_query_parser;
 use mentat_query_projector;
+use mentat_query_translator;
 use mentat_sql;
 use mentat_tx_parser;
 
@@ -37,6 +38,7 @@ error_chain! {
         QueryError(mentat_query_algebrizer::Error, mentat_query_algebrizer::ErrorKind);   // Let's not leak the term 'algebrizer'.
         QueryParseError(mentat_query_parser::Error, mentat_query_parser::ErrorKind);
         ProjectorError(mentat_query_projector::Error, mentat_query_projector::ErrorKind);
+        TranslatorError(mentat_query_translator::Error, mentat_query_translator::ErrorKind);
         SqlError(mentat_sql::Error, mentat_sql::ErrorKind);
         TxParseError(mentat_tx_parser::Error, mentat_tx_parser::ErrorKind);
     }
diff --git a/src/query.rs b/src/query.rs
index 010f4485..da12f601 100644
--- a/src/query.rs
+++ b/src/query.rs
@@ -81,7 +81,7 @@ pub fn q_once<'sqlite, 'schema, 'query, T>
     if !unbound.is_empty() {
         bail!(ErrorKind::UnboundVariables(unbound.into_iter().map(|v| v.to_string()).collect()));
     }
-    let select = query_to_select(algebrized);
+    let select = query_to_select(algebrized)?;
     let SQLQuery { sql, args } = select.query.to_sql_query()?;
 
     let mut statement = sqlite.prepare(sql.as_str())?;