parse prefix operators

src/ast.rs

@@ -1,4 +1,7 @@
-use std::{fmt::Debug, rc::Rc};
+use std::{
+    fmt::{Debug, Display},
+    rc::Rc,
+};
 
 use crate::token::{Token, TokenType};
 
@@ -79,6 +82,33 @@ impl Node for IntegerLiteral {}
 
 impl Expression for IntegerLiteral {}
 
+#[derive(Debug)]
+pub enum PrefixOperator {
+    Minus,
+    Bang,
+}
+
+impl Display for PrefixOperator {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        use PrefixOperator::*;
+        match self {
+            Bang => write!(f, "!")?,
+            Minus => write!(f, "-")?,
+        }
+        Ok(())
+    }
+}
+
+#[derive(Debug)]
+pub struct Prefix {
+    pub operator: PrefixOperator,
+    pub right: Rc<dyn Expression>,
+}
+
+impl Node for Prefix {}
+
+impl Expression for Prefix {}
+
 #[derive(Debug)]
 pub struct DummyExpression {}
 

src/parser.rs

@@ -2,8 +2,8 @@ use std::{fmt, rc::Rc};
 
 use crate::{
     ast::{
-        DummyExpression, Expression, ExpressionStatement, Identifier, IntegerLiteral, Let, Program,
+        DummyExpression, Expression, ExpressionStatement, Identifier, IntegerLiteral, Let, Prefix,
-        Return, Statement,
+        PrefixOperator, Program, Return, Statement,
     },
     lexer::Lexer,
     token::{Token, TokenType},
@@ -20,6 +20,8 @@ pub enum Error {
         expected: TokenType,
         actual: Option<TokenType>,
     },
+    NoPrefixParseFnFound,
+    EmptyExprFollowingPrefixOp(PrefixOperator),
 }
 
 #[derive(PartialEq, PartialOrd)]
@@ -40,10 +42,15 @@ impl fmt::Display for Error {
         match self {
             UnexpectedTokenType { expected, actual } => write!(
                 f,
-                "expected token `{:?}`, actual token: `{:?}`",
+                "expected token `{expected}`, actual token: `{}`",
-                expected, actual
+                match actual {
-            )?,
+                    Some(token) => format!("{token}"),
-        }
+                    None => "None".to_string(),
+                },
+            ),
+            NoPrefixParseFnFound => write!(f, "no prefix parse function found"),
+            EmptyExprFollowingPrefixOp(op) => write!(f, "empty expression following `{op}`"),
+        }?;
         Ok(())
     }
 }
@@ -153,7 +160,12 @@ impl Parser {
     }
 
     fn parse_expression(&mut self, precedence: Precedence) -> Option<Rc<dyn Expression>> {
-        let prefix = self.prefix_parse_fn()?;
+        let prefix = if let Some(prefix) = self.prefix_parse_fn() {
+            prefix
+        } else {
+            self.errors.push(Error::NoPrefixParseFnFound);
+            return None;
+        };
 
         let left_exp = prefix(self);
         Some(left_exp)
@@ -176,6 +188,20 @@ impl Parser {
         Rc::new(IntegerLiteral { value })
     }
 
+    fn parse_prefix_expression(&mut self, operator: PrefixOperator) -> Rc<dyn Expression> {
+        self.next();
+
+        let right = if let Some(expr) = self.parse_expression(Precedence::Prefix) {
+            expr
+        } else {
+            self.errors
+                .push(Error::EmptyExprFollowingPrefixOp(operator));
+            return Rc::new(DummyExpression {});
+        };
+
+        Rc::new(Prefix { operator, right })
+    }
+
     fn expect_peek(&mut self, token_type: TokenType) -> bool {
         let peek_token = if let Some(token) = &self.peek_token {
             token
@@ -262,6 +288,12 @@ impl Parser {
                     Self::parse_integer_literal(parser, value)
                 }))
             }
+            Minus => Some(Box::new(|parser| {
+                Self::parse_prefix_expression(parser, PrefixOperator::Minus)
+            })),
+            Bang => Some(Box::new(|parser| {
+                Self::parse_prefix_expression(parser, PrefixOperator::Bang)
+            })),
             _ => None,
         }
     }
@@ -294,7 +326,11 @@ impl Iterator for Parser {
 mod tests {
     use std::rc::Rc;
 
-    use crate::{ast::Statement, lexer::Lexer};
+    use crate::{
+        ast::{PrefixOperator, Statement},
+        lexer::{self, Lexer},
+        token::Token,
+    };
 
     use super::Parser;
 
@@ -386,6 +422,49 @@ mod tests {
         }
     }
 
+    #[test]
+    fn prefix_expressions() {
+        struct Test {
+            input: String,
+            token: Token,
+            operator: PrefixOperator,
+            value: i64,
+        }
+
+        let tests = vec![
+            Test {
+                input: "!15".to_owned(),
+                token: Token::Bang,
+                operator: PrefixOperator::Bang,
+                value: 15,
+            },
+            Test {
+                input: "-15".to_owned(),
+                token: Token::Minus,
+                operator: PrefixOperator::Minus,
+                value: 15,
+            },
+        ];
+
+        for test in tests {
+            let l = Lexer::new(test.input);
+
+            let mut parser = Parser::new(l);
+
+            let program = parser.parse().unwrap();
+            check_parser_errors(parser);
+
+            assert_eq!(program.statements.len(), 1);
+
+            test_prefix_expression(
+                program.statements.first().unwrap().clone(),
+                test.token,
+                test.operator,
+                test.value,
+            );
+        }
+    }
+
     fn test_let_statement(stmt: Rc<dyn Statement>, name: &str) {
         assert_eq!(
             format!("{stmt:?}"),
@@ -424,4 +503,16 @@ mod tests {
             ),
         );
     }
+
+    fn test_prefix_expression(
+        stmt: Rc<dyn Statement>,
+        token: Token,
+        operator: PrefixOperator,
+        num: i64,
+    ) {
+        assert_eq!(
+            format!("{stmt:?}"),
+            format!("ExpressionStatement {{ token: {token:?}, expression: Prefix {{ operator: {operator:?}, right: IntegerLiteral {{ value: {num} }} }} }}"),
+        );
+    }
 }

src/token.rs

@@ -1,3 +1,5 @@
+use std::fmt::Display;
+
 #[derive(Debug, PartialEq, Clone)]
 pub enum Token {
     Illegal,
@@ -72,6 +74,43 @@ pub enum TokenType {
     Return,
 }
 
+impl Display for TokenType {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        use TokenType::*;
+        let name = match self {
+            Illegal => "Illegal",
+            EOF => "EOF",
+            Ident => "Ident",
+            Int => "Int",
+            Assign => "=",
+            Plus => "+",
+            Minus => "-",
+            Bang => "!",
+            Asterisk => "*",
+            Slash => "/",
+            Lt => "<",
+            Gt => ">",
+            Eq => "==",
+            NotEq => "!=",
+            Comma => ",",
+            Semicolon => ";",
+            Lparen => "(",
+            Rparen => ")",
+            Lbrace => "{",
+            Rbrace => "}",
+            Function => "fn",
+            Let => "let",
+            True => "true",
+            False => "false",
+            If => "if",
+            Else => "else",
+            Return => "return",
+        };
+        write!(f, "{name}")?;
+        Ok(())
+    }
+}
+
 impl Token {
     pub fn token_type(&self) -> TokenType {
         match self {