diff --git a/src/parser/parser.rs b/src/parser/parser.rs
index be9135a..4f9a396 100644
--- a/src/parser/parser.rs
+++ b/src/parser/parser.rs
@@ -1,52 +1,76 @@
 use super::types::*;
 use crate::ast;
 
-fn parse_simple_expr(tokens: &mut Vec<LocatedToken>) -> Result<ast::Expr, Error> {
-    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})
-            }
+fn parse_expr(tokens: &mut Tokens) -> ParseResult<ast::Expr> {
+    todo!()
+}
 
-        }
-        Token::Number(n) => {
-            tokens.pop();
-            Ok(ast::Expr::Value(ast::Value::Number(n)))
+fn parse_simple_expr(tokens: &mut Tokens) -> ParseResult<ast::Expr> {
+    tokens.one_of(vec![
+        |t| t.between(Token::OpenParen, Token::CloseParen, parse_expr),
+        parse_number,
+        |_| Err(Error::UnexpectedEndOfInput),
+    ])
+}
+
+fn parse_string(tokens: &mut Tokens) -> ParseResult<ast::Expr> {
+    {
+        if let Some(LocatedToken {
+            token: Token::String(string),
+            ..
+        }) = tokens.next_if(Token::String("".into()))
+        {
+            Ok(Some(ast::Expr::Value(ast::Value::String(string))))
+        } else {
+            Ok(None)
         }
     }
 }
 
-enum Error {
-    UnexpectedToken{expected: Token, got: LocatedToken},
-    UnexpectedToken(LocatedToken),
-    UnexpectedEndOfInput,
+fn parse_number(tokens: &mut Tokens) -> ParseResult<ast::Expr> {
+    {
+        if let Some(LocatedToken {
+            token: Token::Number(number),
+            ..
+        }) = tokens.next_if(Token::Number(0))
+        {
+            Ok(Some(ast::Expr::Value(ast::Value::Int(number.into()))))
+        } else {
+            Ok(None)
+        }
+    }
 }
 
-fn parse_definition(tokens: &mut Vec<LocatedToken>) -> Result<ast::Definition, Error> {
-    if let Some(start) = tokens.pop_if(|tok| tok.token == Token::Let) {
+fn parse_definition(tokens: &mut Tokens) -> Result<ast::Definition, Error> {
+    if let Some(start) = tokens.next_if(Token::Let) {
         if let Some(LocatedToken {
             token: Token::Identifier(identifier),
             ..
-        }) = tokens.pop_if(|tok| match tok.token {
-            Token::Identifier(_) => true,
-            _ => false,
-        }) {
-            if let Some(_eq) = tokens.pop_if(|tok| tok.token == Token::Equals) {
-                let expr = parse_expr(tokens)?;
-                Ok(ast::Definition {
-                    name: ast::Name(identifier),
-                    expr,
-                })
+        }) = tokens.next_if(Token::Identifier("".into()))
+        {
+            if let Some(_eq) = tokens.next_if(Token::Equals) {
+                if let Some(expr) = parse_expr(tokens)? {
+                    Ok(ast::Definition {
+                        name: ast::Name(identifier),
+                        expr,
+                    })
+                } else {
+                    Err(Error::UnexpectedToken {
+                        expected: Token::Equals,
+                        got: start,
+                    })
+                }
             } else {
-                Err(Error::UnexpectedToken{expected: Token::Equals, got: start})
+                Err(Error::UnexpectedToken {
+                    expected: Token::Equals,
+                    got: start,
+                })
             }
         } else {
-            Err(Error::UnexpectedToken{expected: Token::Equals, got: start})
+            Err(Error::UnexpectedToken {
+                expected: Token::Equals,
+                got: start,
+            })
         }
     } else {
         Err(Error::UnexpectedEndOfInput)
diff --git a/src/parser/types.rs b/src/parser/types.rs
index 362c7c7..e722b13 100644
--- a/src/parser/types.rs
+++ b/src/parser/types.rs
@@ -21,7 +21,6 @@ pub enum Token {
     Comma,
     Semicolon,
     Colon,
-    Name(String),
     Number(u32),
     String(String),
     Identifier(String),
@@ -30,10 +29,45 @@ pub enum Token {
 pub struct Tokens(Vec<LocatedToken>);
 
 impl Tokens {
-    fn peek(&mut self) -> Option<LocatedToken> {
-        self.last()
+    pub fn peek(&mut self) -> Option<&LocatedToken> {
+        self.0.last()
     }
-    fn next(&mut self) -> Option<LocatedToken> {
-        self.pop()
+    pub fn next(&mut self) -> Option<LocatedToken> {
+        self.0.pop()
+    }
+    pub fn next_if(&mut self, token: Token) -> Option<LocatedToken> {
+        self.0.pop_if(|t| match (&t.token, &token) {
+            (a, b) if a == b => true,
+            (Token::Identifier(_), Token::Identifier(_)) => true,
+            (Token::Number(_), Token::Number(_)) => true,
+            (Token::String(_), Token::String(_)) => true,
+            _ => false,
+        })
+    }
+    pub fn between<T>(&mut self, start: Token, end: Token, parser: Parser<T>) -> ParseResult<T> {
+        let _start = self.next_if(start).ok_or(Error::UnexpectedEndOfInput)?;
+        if let Some(result) = parser(self)? {
+            let _end = self.next_if(end).ok_or(Error::UnexpectedEndOfInput)?;
+            Ok(Some(result))
+        } else {
+            Err(Error::UnexpectedTokenForParser)
+        }
+    }
+    pub fn one_of<T>(&mut self, parsers: Vec<Parser<T>>) -> ParseResult<T> {
+        for parser in parsers {
+            if let Some(result) = parser(self)? {
+                return Ok(Some(result));
+            }
+        }
+        Ok(None)
     }
 }
+
+pub type Parser<T> = fn(&mut Tokens) -> ParseResult<T>;
+pub type ParseResult<T> = Result<Option<T>, Error>;
+
+pub enum Error {
+    UnexpectedToken { expected: Token, got: LocatedToken },
+    UnexpectedEndOfInput,
+    UnexpectedTokenForParser,
+}