Parse binary application with parens

2 files changed, 30 insertions, 2 deletions

diff --git a/rust/src/ast.rs b/rust/src/ast.rs
index b179704..0493f59 100644
--- a/rust/src/ast.rs
+++ b/rust/src/ast.rs
@@ -5,6 +5,7 @@ pub enum Value {
     Num(i32),
     Bool(bool),
     Sym(String),
+    App(Box<Value>, Box<Value>),
 }
 
 impl Display for Value {
@@ -13,6 +14,7 @@ impl Display for Value {
             Value::Num(i) => write!(f, "{}", i),
             Value::Bool(b) => write!(f, "{}", b),
             Value::Sym(s) => write!(f, "{}", s),
+            Value::App(l, r) => write!(f, "({} {})", l, r),
         }
     }
 }
diff --git a/rust/src/parser.rs b/rust/src/parser.rs
index 50cf918..8f95ca9 100644
--- a/rust/src/parser.rs
+++ b/rust/src/parser.rs
@@ -8,8 +8,27 @@ enum Token {
 }
 
 pub fn parse(arg: &str) -> Value {
-    let tokens = tokenize(arg);
-    parse_value(&tokens[0]).unwrap()
+    let mut tokens = tokenize(arg);
+    let mut state = Vec::new();
+    parse_tokens(&mut tokens, &mut state).unwrap()
+}
+
+fn parse_tokens(tokens: &mut [Token], state: &mut Vec<Value>) -> Result<Value, String> {
+    match tokens {
+        [] => state.pop().ok_or("Empty expression".to_string()),
+        [Token::LParen, rest @ ..] => parse_tokens(rest, state),
+        [Token::RParen, rest @ ..] => {
+            let right = state.pop().ok_or("Unbalanced parentheses".to_string())?;
+            let left = state.pop().ok_or("Unbalanced parentheses".to_string())?;
+            state.push(Value::App(Box::new(left), Box::new(right)));
+            parse_tokens(rest, state)
+        }
+        [Token::Word(s), rest @ ..] => {
+            let value = parse_value(&Token::Word(s.clone()))?;
+            state.push(value);
+            parse_tokens(rest, state)
+        }
+    }
 }
 
 fn parse_value(token: &Token) -> Result<Value, String> {
@@ -133,6 +152,13 @@ mod tests {
             tokenize("(   \r() 42)  \ntrue(  ")
         );
     }
+
+    #[test]
+    fn parse_application_of_two_values() {
+        assert_eq!(
+            App(Box::new(Sym("foo".to_string())), Box::new(Num(42))),
+            parse("(foo 42)")
+        );
     }
 
     impl Arbitrary for Value {