Implement a FromValue trait for SrcVar and Variable. (#227) r=nalexander

query-parser/src/parse.rs

@@ -15,7 +15,7 @@ extern crate mentat_query;
 use self::combine::{eof, many1, parser, satisfy_map, Parser, ParseResult, Stream};
 use self::combine::combinator::{Expected, FnParser, choice, try};
 use self::edn::Value::PlainSymbol;
-use self::mentat_query::{Element, FindSpec, Variable};
+use self::mentat_query::{Element, FromValue, FindSpec, Variable};
 
 use super::error::{FindParseError, FindParseResult};
 
@@ -45,7 +45,7 @@ impl<I> FindSp<I>
     }
 
     fn variable_(input: I) -> ParseResult<Variable, I> {
-        satisfy_map(|x: edn::Value| super::util::value_to_variable(&x)).parse_stream(input)
+        satisfy_map(|x: edn::Value| Variable::from_value(&x)).parse_stream(input)
     }
 
     fn period() -> FindSpParser<(), I> {

query-parser/src/util.rs

@@ -13,28 +13,16 @@ extern crate mentat_query;
 
 use std::collections::BTreeMap;
 
-use self::edn::Value::PlainSymbol;
-use self::mentat_query::Variable;
+use self::mentat_query::{FromValue, Variable};
 use super::error::NotAVariableError;
 
-/// If the provided EDN value is a PlainSymbol beginning with '?', return
-/// it wrapped in a Variable. If not, return None.
-pub fn value_to_variable(v: &edn::Value) -> Option<Variable> {
-    if let PlainSymbol(ref sym) = *v {
-        if sym.0.starts_with('?') {
-            return Some(Variable(sym.clone()));
-        }
-    }
-    return None;
-}
-
 /// If the provided slice of EDN values are all variables as
 /// defined by `value_to_variable`, return a Vec of Variables.
 /// Otherwise, return the unrecognized Value.
 pub fn values_to_variables(vals: &[edn::Value]) -> Result<Vec<Variable>, NotAVariableError> {
     let mut out: Vec<Variable> = Vec::with_capacity(vals.len());
     for v in vals {
-        if let Some(var) = value_to_variable(v) {
+        if let Some(var) = Variable::from_value(v) {
             out.push(var);
             continue;
         }

query/src/lib.rs

@@ -43,12 +43,58 @@ pub type SrcVarName = String;          // Do not include the required syntactic
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub struct Variable(pub PlainSymbol);
 
-#[derive(Clone,Debug,Eq,PartialEq)]
+pub trait FromValue<T> {
+    fn from_value(v: &edn::Value) -> Option<T>;
+}
+
+/// If the provided EDN value is a PlainSymbol beginning with '?', return
+/// it wrapped in a Variable. If not, return None.
+impl FromValue<Variable> for Variable {
+    fn from_value(v: &edn::Value) -> Option<Variable> {
+        if let edn::Value::PlainSymbol(ref s) = *v {
+            Variable::from_symbol(s)
+        } else {
+            None
+        }
+    }
+}
+
+impl Variable {
+    fn from_symbol(sym: &PlainSymbol) -> Option<Variable> {
+        if sym.is_var_symbol() {
+            Some(Variable(sym.clone()))
+        } else {
+            None
+        }
+    }
+}
+
+#[derive(Clone, Debug, Eq, PartialEq)]
 pub enum SrcVar {
     DefaultSrc,
     NamedSrc(SrcVarName),
 }
 
+impl FromValue<SrcVar> for SrcVar {
+    fn from_value(v: &edn::Value) -> Option<SrcVar> {
+        if let edn::Value::PlainSymbol(ref s) = *v {
+            SrcVar::from_symbol(s)
+        } else {
+            None
+        }
+    }
+}
+
+impl SrcVar {
+    pub fn from_symbol(sym: &PlainSymbol) -> Option<SrcVar> {
+        if sym.is_src_symbol() {
+            Some(SrcVar::NamedSrc(sym.plain_name().to_string()))
+        } else {
+            None
+        }
+    }
+}
+
 /// These are the scalar values representable in EDN.
 #[derive(Clone,Debug,Eq,PartialEq)]
 pub enum NonIntegerConstant {