Added `null` as a first class token (#29)

An expression may evaluate to Null but an equality test evaluates to UndefinedIdentifier

jexl-eval/src/lib.rs

@@ -160,6 +160,7 @@ impl<'a> Evaluator<'a> {
             Expression::Boolean(b) => Ok(value!(b)),
             Expression::String(s) => Ok(value!(s)),
             Expression::Array(xs) => xs.into_iter().map(|x| self.eval_ast(*x, context)).collect(),
+            Expression::Null => Ok(Value::Null),
 
             Expression::Object(items) => {
                 let mut map = serde_json::Map::with_capacity(items.len());
@@ -834,6 +835,7 @@ mod tests {
         test("BOOLEAN", "true", true);
         test("OBJECT", "OBJECT", true);
         test("ARRAY", "[ 'string' ]", true);
+        test("NULL", "null", true);
 
         test("OBJECT", "{ 'x': 1, 'y': 2 }", false);
 

jexl-parser/src/ast.rs

@@ -10,6 +10,7 @@ pub enum Expression {
     Array(Vec<Box<Expression>>),
     Object(Vec<(String, Box<Expression>)>),
     Identifier(String),
+    Null,
 
     BinaryOperation {
         operation: OpCode,

jexl-parser/src/lib.rs

@@ -136,6 +136,23 @@ mod tests {
         );
     }
 
+    #[test]
+    fn test_dot_op_equality_with_null() {
+        let exp = "foo.bar == null";
+        let parsed = Parser::parse(exp).unwrap();
+        assert_eq!(
+            parsed,
+            Expression::BinaryOperation {
+                operation: OpCode::Equal,
+                left: Box::new(Expression::DotOperation {
+                    subject: Box::new(Expression::Identifier("foo".to_string())),
+                    ident: "bar".to_string()
+                }),
+                right: Box::new(Expression::Null),
+            }
+        );
+    }
+
     #[test]
     fn test_dot_op_object_literal() {
         let exp = "{'foo': 1}.foo";
@@ -151,4 +168,9 @@ mod tests {
             }
         );
     }
+
+    #[test]
+    fn test_parsing_null() {
+        assert_eq!(Parser::parse("null"), Ok(Expression::Null));
+    }
 }

jexl-parser/src/parser.lalrpop

@@ -76,6 +76,7 @@ Expr80: Box<Expression> = {
     String => Box::new(Expression::String(<>)),
     Array => Box::new(Expression::Array(<>)),
     Object => Box::new(Expression::Object(<>)),
+    Null => Box::new(Expression::Null),
     Identifier => Box::new(Expression::Identifier(<>)),
     "(" <Expression> ")",
 };
@@ -126,6 +127,9 @@ String: String = {
     <s: r#"'([^'\\]*(\\')?)*'"#> => s[1..s.len() - 1].to_string().replace("\\'", "'"),
 };
 
+Null: Option<Box<Expression>> = {
+    "null" => None,
+}
 
 Identifier: String = {
     r#"[a-zA-Z_][a-zA-Z0-9_]*"# => <>.to_string()