added ParseResult

src/error.rs

@@ -0,0 +1,20 @@
+use std::{error::Error, fmt::Display};
+
+#[derive(Debug)]
+pub struct ParseError {
+    message: String
+}
+
+impl ParseError {
+    pub fn new(message: impl Into<String>) -> Self {
+        Self { message: message.into() }
+    }
+}
+
+impl Display for ParseError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "parse error: {}", self.message)
+    }
+}
+
+impl Error for ParseError {}

src/main.rs

@@ -3,6 +3,7 @@ use std::{fs::File, io::Read};
 mod token;
 mod expr;
 mod lexer;
+mod error;
 use lexer::Lexer;
 mod parser;
 use parser::Parser;
@@ -29,7 +30,7 @@ fn main() -> std::io::Result<()> {
 
     let mut parser = Parser::new(tokens);
     loop {
-        let parsed = parser.parse();
+        let parsed = parser.parse().unwrap_or(expr::Expr::EOL);
         match parsed {
             expr::Expr::EOL => break,
             _ => {

src/parser.rs

@@ -1,5 +1,8 @@
 use crate::token::Token;
 use crate::expr::{Expr, UnaryOp, BinaryOp, infix_bp, prefix_bp};
+use crate::error::ParseError;
+
+type ParseResult<T> = Result<T, ParseError>;
 
 pub struct Parser {
     tokens: Vec<Token>,
@@ -28,39 +31,49 @@ impl Parser {
         tok
     }
 
-    fn parse_atom(&mut self) -> Expr {
+    fn expect(&mut self, expected: &Token) -> ParseResult<Token> {
         match self.next() {
-            Some(Token::Number(n)) => Expr::Number(n),
-            Some(Token::Ident(id)) => Expr::Ident(id),
-            Some(Token::LParen) => {
-                let expr = self.parse_expr(0);
-
-                match self.next() {
-                    Some(Token::RParen) => expr,
-                    _ => panic!("expected ')'")
-                }
-            },
-            tok => panic!("unknown token: {tok:?}")
+            Some(tok) if &tok == expected => Ok(tok),
+            other => Err(ParseError::new(format!(
+                "expected {expected:?}, got {other:?}"
+            )))
         }
     }
 
-    fn parse_prefix(&mut self) -> Expr {
+    fn parse_atom(&mut self) -> ParseResult<Expr> {
+        match self.next() {
+            Some(Token::Number(n)) => Ok(Expr::Number(n)),
+            Some(Token::Ident(id)) => Ok(Expr::Ident(id)),
+            Some(Token::LParen) => {
+                let expr = self.parse_expr(0)?;
+
+                self.expect(&Token::RParen)?;
+
+                Ok(expr)
+            },
+            tok => Err(ParseError::new(format!(
+                "unexpected token: {tok:?}"
+            )))
+        }
+    }
+
+    fn parse_prefix(&mut self) -> ParseResult<Expr> {
         match self.peek() {
             Some(Token::Minus) => {
                 self.advance();
                 let op = UnaryOp::Neg;
 
                 let bp = prefix_bp(&op);
-                let rhs = self.parse_expr(bp);
+                let rhs = self.parse_expr(bp)?;
 
-                Expr::Unary { op, right: Box::new(rhs) }
+                Ok(Expr::Unary { op, right: Box::new(rhs) })
             },
             _ => self.parse_atom()
         }
     }
 
-    fn parse_expr(&mut self, min_bp: u8) -> Expr {
-        let mut lhs = self.parse_prefix();
+    fn parse_expr(&mut self, min_bp: u8) -> ParseResult<Expr> {
+        let mut lhs = self.parse_prefix()?;
 
         loop {
             let op = match self.peek() {
@@ -81,16 +94,16 @@ impl Parser {
 
             self.advance();
 
-            let rhs = self.parse_expr(right_bp);
+            let rhs = self.parse_expr(right_bp)?;
 
             lhs = Expr::Binary { left: Box::new(lhs), op, right: Box::new(rhs) }
         }
 
-        lhs
+        Ok(lhs)
     }
 
-    pub fn parse(&mut self) -> Expr {
-        if self.peek().is_none() {return Expr::EOL}
+    pub fn parse(&mut self) -> ParseResult<Expr> {
+        if self.peek().is_none() {return Ok(Expr::EOL)}
         self.parse_expr(0)
     }
 }