2 files changed, 39 insertions, 4 deletions

diff --git a/rust/src/ast.rs b/rust/src/ast.rs
index 722e9e0..8729fa2 100644
--- a/rust/src/ast.rs
+++ b/rust/src/ast.rs
@@ -7,6 +7,7 @@ pub enum Value {
     Sym(String),
     App(Box<Value>, Box<Value>),
     Lam(String, Box<Value>),
+    Def(String, Box<Value>),
 }
 
 impl Display for Value {
@@ -17,6 +18,7 @@ impl Display for Value {
             Value::Sym(s) => write!(f, "{}", s),
             Value::App(l, r) => write!(f, "({} {})", l, r),
             Value::Lam(var, body) => write!(f, "(lam {} {})", var, body),
+            Value::Def(var, value) => write!(f, "(def {} {})", var, value),
         }
     }
 }
diff --git a/rust/src/parser.rs b/rust/src/parser.rs
index 48ef2e0..d733008 100644
--- a/rust/src/parser.rs
+++ b/rust/src/parser.rs
@@ -6,8 +6,10 @@ enum Token {
     RParen,
     Lambda,
     Word(String),
+    Define,
 }
 
+#[derive(Debug)]
 struct Parser {
     tokens: Vec<Token>,
     index: usize,
@@ -55,7 +57,7 @@ pub fn parse(arg: &str) -> Vec<Value> {
     let mut parser = Parser { tokens, index: 0 };
     let mut result = Vec::new();
     while parser.index < parser.tokens.len() {
-        let expr = parse_expression(&mut parser)
+        let expr = parse_toplevel(&mut parser)
             .map_err(|e| panic!("Syntax error: {}", e))
             .unwrap();
         result.push(expr);
@@ -63,6 +65,23 @@ pub fn parse(arg: &str) -> Vec<Value> {
     result
 }
 
+fn parse_toplevel(parser: &mut Parser) -> Result<Value, String> {
+    parse_definition(parser).or_else(|_| parse_expression(parser))
+}
+
+fn parse_definition(parser: &mut Parser) -> Result<Value, String> {
+    parser.expect(Token::LParen)?;
+    parser.expect(Token::Define).map_err(|e| {
+        parser.backtrack();
+        e.to_string()
+    })?;
+    println!("parser {:?}", parser);
+    let var = parse_variable(parser)?;
+    let body = parse_expression(parser)?;
+    parser.expect(Token::RParen)?;
+    Ok(Value::Def(var, Box::new(body)))
+}
+
 fn parse_expression(parser: &mut Parser) -> Result<Value, String> {
     parse_abstraction(parser)
         .or_else(|_| parse_application(parser))
@@ -130,6 +149,8 @@ fn terminate(result: &mut Vec<Token>, word: &mut String) {
         let w = word.clone();
         if w == "lam" {
             result.push(Token::Lambda);
+        } else if w == "def" {
+            result.push(Token::Define);
         } else {
             result.push(Token::Word(w));
         }
@@ -251,6 +272,14 @@ mod tests {
     }
 
     #[test]
+    fn parse_definition() {
+        assert_eq!(
+            vec![Def("x".to_string(), Box::new(Num(12)))],
+            parse("(def x 12)")
+        );
+    }
+
+    #[test]
     fn parse_multiple_values() {
         assert_eq!(vec![Sym("foo".to_string()), Num(42)], parse("foo 42"));
     }
@@ -267,12 +296,16 @@ mod tests {
                 // see https://unicode.org/reports/tr18/#General_Category_Property for one letter unicode categories
                 identifier.prop_map(Sym),
             ];
-            leaf.prop_recursive(4, 128, 5, move |inner| {
+            let expr = leaf.prop_recursive(4, 128, 5, move |inner| {
                 prop_oneof![
                     (inner.clone(), inner.clone()).prop_map(|(l, r)| App(Box::new(l), Box::new(r))),
-                    (identifier, inner).prop_map(|(var, body)| Lam(var, Box::new(body))),
+                    (identifier, inner.clone()).prop_map(|(var, body)| Lam(var, Box::new(body))),
                 ]
-            })
+            });
+            prop_oneof![
+                expr.clone(),
+                (identifier, expr).prop_map(|(var, body)| Def(var, Box::new(body)))
+            ]
             .boxed()
         }
     }