add pratt parser

src/parser.rs

@@ -0,0 +1,139 @@
+use crate::token::Token;
+
+#[derive(Debug)]
+pub enum BinaryOp {
+    Add,
+    Sub,
+    Mul,
+    Div,
+    Mod
+}
+
+#[derive(Debug)]
+pub enum UnaryOp {
+    Neg
+}
+
+fn prefix_bp(op: &UnaryOp) -> u8 {
+    match op {
+        UnaryOp::Neg => 5
+    }
+}
+
+fn infix_bp(op: &BinaryOp) -> (u8, u8) {
+    match op {
+        BinaryOp::Add | BinaryOp::Sub => (1, 2),
+        BinaryOp::Mul | BinaryOp::Div | BinaryOp::Mod => (3, 4)
+    }
+}
+
+#[derive(Debug)]
+pub enum Expr {
+    Number(i64),
+    Ident(String),
+    EOL,
+
+    Binary {
+        op: BinaryOp,
+        left: Box<Expr>,
+        right: Box<Expr>
+    },
+
+    Unary {
+        op: UnaryOp,
+        right: Box<Expr>
+    }
+}
+
+pub struct Parser {
+    tokens: Vec<Token>,
+    pos: usize
+}
+
+impl Parser {
+    pub fn new(tokens: Vec<Token>) -> Self {
+        Self {
+            tokens,
+            pos: 0
+        }
+    }
+
+    fn advance(&mut self) {
+        self.pos += 1;
+    }
+
+    fn peek(&self) -> Option<&Token> {
+        self.tokens.get(self.pos)
+    }
+
+    fn next(&mut self) -> Option<Token> {
+        let tok = self.peek().cloned();
+        self.advance();
+        tok
+    }
+
+    fn parse_atom(&mut self) -> Expr {
+        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:?}")
+        }
+    }
+
+    fn parse_prefix(&mut self) -> Expr {
+        match self.peek() {
+            Some(Token::Sub) => {
+                self.advance();
+                let op = UnaryOp::Neg;
+
+                let bp = prefix_bp(&op);
+                let rhs = self.parse_expr(bp);
+
+                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();
+
+        loop {
+            let op = match self.peek() {
+                Some(Token::Add) => BinaryOp::Add,
+                Some(Token::Sub) => BinaryOp::Sub,
+                Some(Token::Mul) => BinaryOp::Mul,
+                Some(Token::Div) => BinaryOp::Div,
+                Some(Token::Mod) => BinaryOp::Mod,
+                _ => break
+            };
+
+            let (left_bp, right_bp) = infix_bp(&op);
+
+            if left_bp < min_bp {
+                break;
+            }
+
+            self.advance();
+
+            let rhs = self.parse_expr(right_bp);
+
+            lhs = Expr::Binary { left: Box::new(lhs), op, right: Box::new(rhs) }
+        }
+
+        lhs
+    }
+
+    pub fn parse(&mut self) -> Expr {
+        if self.peek() == None {return Expr::EOL}
+        self.parse_expr(0)
+    }
+}