Ensure application has at least two values

1 files changed, 23 insertions, 6 deletions

diff --git a/rust/src/parser.rs b/rust/src/parser.rs
index 3ac01ef..121a10f 100644
--- a/rust/src/parser.rs
+++ b/rust/src/parser.rs
@@ -54,16 +54,20 @@ impl Parser {
 }
 
 pub fn parse(arg: &str) -> Vec<Value> {
+    parse_total(arg)
+        .map_err(|e| panic!("Syntax error: {}", e))
+        .unwrap()
+}
+
+pub fn parse_total(arg: &str) -> Result<Vec<Value>, String> {
     let tokens = tokenize(arg);
     let mut parser = Parser { tokens, index: 0 };
     let mut result = Vec::new();
     while parser.index < parser.tokens.len() {
-        let expr = parse_toplevel(&mut parser)
-            .map_err(|e| panic!("Syntax error: {}", e))
-            .unwrap();
+        let expr = parse_toplevel(&mut parser)?;
         result.push(expr);
     }
-    result
+    Ok(result)
 }
 
 fn parse_toplevel(parser: &mut Parser) -> Result<Value, String> {
@@ -83,10 +87,10 @@ fn parse_definition(parser: &mut Parser) -> Result<Value, String> {
 }
 
 fn parse_expression(parser: &mut Parser) -> Result<Value, String> {
-    parse_abstraction(parser)
+    parse_value(parser)
+        .or_else(|_| parse_abstraction(parser))
         .or_else(|_| parse_let(parser))
         .or_else(|_| parse_application(parser))
-        .or_else(|_| parse_value(parser))
 }
 
 fn parse_abstraction(parser: &mut Parser) -> Result<Value, String> {
@@ -148,6 +152,9 @@ fn parse_application(parser: &mut Parser) -> Result<Value, String> {
     while let Ok(expr) = parse_expression(parser) {
         exprs.push(expr);
     }
+    if exprs.is_empty() {
+        return Err("Application needs two values".to_string());
+    }
     parser.expect(Token::RParen)?;
     let app: Value = exprs.iter().fold(init, |acc, expr| {
         Value::App(Box::new(acc.clone()), Box::new(expr.clone()))
@@ -228,6 +235,8 @@ fn parse_number(token: &Token) -> Result<Value, String> {
 
 #[cfg(test)]
 mod tests {
+    use crate::parse_total;
+
     use super::Token::*;
     use super::Value;
     use super::Value::*;
@@ -302,6 +311,14 @@ mod tests {
     }
 
     #[test]
+    fn reject_application_of_single_value() {
+        assert_eq!(
+            Err("Application needs two values".to_string()),
+            parse_total("(foo )")
+        );
+    }
+
+    #[test]
     fn desugar_application_of_more_than_two_values() {
         assert_eq!(
             vec![App(