Simpler syntax for single param in lambdas

This kind of implies that I might want -> for function types after all,
instead of :

schala-lang/language/src/parsing.rs

@@ -227,7 +227,8 @@ primary := literal | paren_expr | if_expr | for_expr | while_expr | identifier_e
 /* Primary Expressions */
 
 list_expr := '[' (expression, ',')* ']'
-lambda_expr := '\' formal_param_list type_anno+ nonempty_func_body 
+lambda_expr := '\' lambda_param_list type_anno+ nonempty_func_body
+lambda_param_list := formal_param_list |  formal_param
 paren_expr := LParen paren_inner RParen
 paren_inner := (expression ',')*
 identifier_expr := named_struct | IDENTIFIER
@@ -638,7 +639,7 @@ impl Parser {
   #[recursive_descent_method]
   fn lambda_expr(&mut self) -> ParseResult<Expression> {
     expect!(self, Backslash);
-    let params = self.formal_param_list()?; //TODO make this allow some more concise syntaxes
+    let params = self.lambda_param_list()?;
     let type_anno = match self.peek() {
       Colon => Some(self.type_anno()?),
       _ => None,
@@ -647,6 +648,16 @@ impl Parser {
     Ok(Expression(ExpressionType::Lambda { params, type_anno, body }, None)) //TODO need to handle types somehow
   }
 
+  #[recursive_descent_method]
+  fn lambda_param_list(&mut self) -> ParseResult<Vec<FormalParam>> {
+    if let LParen = self.peek() {
+      self.formal_param_list()
+    } else {
+      let single_param = self.formal_param()?;
+      Ok(vec![single_param])
+    }
+  }
+
   #[recursive_descent_method]
   fn paren_expr(&mut self) -> ParseResult<Expression> {
     use self::ExpressionType::*;
@@ -1511,6 +1522,27 @@ fn a(x) {
     }
   }
 
+  #[test]
+  fn single_param_lambda() {
+    parse_test_wrap_ast! {
+      r"\x { x + 10 }", 
+      exst!(Lambda {
+        params: vec![(rc!(x), None)],
+        type_anno: None,
+        body: vec![exst!(s r"x + 10")]
+      })
+    }
+
+    parse_test_wrap_ast! {
+      r"\x: Nat { x + 10 }", 
+      exst!(Lambda {
+        params: vec![(rc!(x), Some(ty!("Nat")))],
+        type_anno: None,
+        body: vec![exst!(s r"x + 10")]
+      })
+    }
+  }
+
   #[test]
   fn more_advanced_lambdas() {
     parse_test! {