start parse simple expr

src/parser/parser.rs

@@ -1,11 +1,28 @@
 use super::types::*;
 use crate::ast;
 
-fn parse_expr(tokens: &mut Vec<LocatedToken>) -> Result<ast::Expr, Error> {
+fn parse_simple_expr(tokens: &mut Vec<LocatedToken>) -> Result<ast::Expr, Error> {
-    todo!()
+    let start = tokens.last()?;
+    match start {
+        Token::OpenParen => {
+            tokens.pop();
+            let expr = parse_expr(tokens)?;
+            if Some(p) = tokens.pop_if(|t| t.token == Token::CloseParen) {
+                Ok(expr)
+            } else {
+                Err(Error::UnexpectedToken { expected: Token::CloseParen, got: start})
+            }
+
+        }
+        Token::Number(n) => {
+            tokens.pop();
+            Ok(ast::Expr::Value(ast::Value::Number(n)))
+        }
+    }
 }
 
 enum Error {
+    UnexpectedToken{expected: Token, got: LocatedToken},
     UnexpectedToken(LocatedToken),
     UnexpectedEndOfInput,
 }
@@ -26,10 +43,10 @@ fn parse_definition(tokens: &mut Vec<LocatedToken>) -> Result<ast::Definition, E
                     expr,
                 })
             } else {
-                Err(Error::UnexpectedToken(start))
+                Err(Error::UnexpectedToken{expected: Token::Equals, got: start})
             }
         } else {
-            Err(Error::UnexpectedToken(start))
+            Err(Error::UnexpectedToken{expected: Token::Equals, got: start})
         }
     } else {
         Err(Error::UnexpectedEndOfInput)

src/parser/types.rs

@@ -26,3 +26,14 @@ pub enum Token {
     String(String),
     Identifier(String),
 }
+
+pub struct Tokens(Vec<LocatedToken>);
+
+impl Tokens {
+    fn peek(&mut self) -> Option<LocatedToken> {
+        self.last()
+    }
+    fn next(&mut self) -> Option<LocatedToken> {
+        self.pop()
+    }
+}