Allow multiple variables in single lambda

1 files changed, 33 insertions, 2 deletions

diff --git a/rust/src/parser.rs b/rust/src/parser.rs
index 23a512b..874b893 100644
--- a/rust/src/parser.rs
+++ b/rust/src/parser.rs
@@ -95,10 +95,30 @@ fn parse_abstraction(parser: &mut Parser) -> Result<Value, String> {
         parser.backtrack();
         e.to_string()
     })?;
-    let var = parse_variable(parser)?;
+    let vars = parse_variables(parser)?;
     let body = parse_expression(parser)?;
     parser.expect(Token::RParen)?;
-    Ok(Value::Lam(var, Box::new(body)))
+    let result = vars
+        .iter()
+        .rev()
+        .fold(body, |acc, var| Value::Lam(var.clone(), Box::new(acc)));
+    Ok(result)
+}
+
+fn parse_variables(parser: &mut Parser) -> Result<Vec<String>, String> {
+    parse_variable(parser)
+        .map(|s| vec![s])
+        .or_else(|_| parse_variables_list(parser))
+}
+
+fn parse_variables_list(parser: &mut Parser) -> Result<Vec<String>, String> {
+    let mut vars = Vec::new();
+    parser.expect(Token::LParen)?;
+    while let Ok(var) = parse_variable(parser) {
+        vars.push(var);
+    }
+    parser.expect(Token::RParen)?;
+    Ok(vars)
 }
 
 fn parse_variable(parser: &mut Parser) -> Result<String, String> {
@@ -290,6 +310,17 @@ mod tests {
     }
 
     #[test]
+    fn desugar_abstraction_with_several_variables_into_nested_lambdas() {
+        assert_eq!(
+            vec![Lam(
+                "x".to_string(),
+                Box::new(Lam("y".to_string(), Box::new(Sym("y".to_string()))))
+            )],
+            parse("(lam (x y) y)")
+        );
+    }
+
+    #[test]
     fn parse_definition() {
         assert_eq!(
             vec![Def("x".to_string(), Box::new(Num(12)))],