1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
|
// This implements the grammar of Lox as described starting in the
// Crafting Interpreters chapter "Representing Code". Note that the
// upstream Java implementation works around Java being bad at value
// classes by writing a code generator for Java.
//
// My Rust implementation skips this step because it's unnecessary, we
// have real types.
use crate::errors::{Error, ErrorKind};
use crate::scanner::{Token, TokenKind};
// AST
#[derive(Debug)]
struct Binary<'a> {
left: Box<Expr<'a>>,
operator: Token<'a>,
right: Box<Expr<'a>>,
}
#[derive(Debug)]
struct Grouping<'a>(Box<Expr<'a>>);
#[derive(Debug)]
enum Literal {
Boolean(bool),
Number(f64),
String(String),
Nil,
}
#[derive(Debug)]
struct Unary<'a> {
operator: Token<'a>,
right: Box<Expr<'a>>,
}
#[derive(Debug)]
enum Expr<'a> {
Binary(Binary<'a>),
Grouping(Grouping<'a>),
Literal(Literal),
Unary(Unary<'a>),
}
// Parser
/*
expression → equality ;
equality → comparison ( ( "!=" | "==" ) comparison )* ;
comparison → term ( ( ">" | ">=" | "<" | "<=" ) term )* ;
term → factor ( ( "-" | "+" ) factor )* ;
factor → unary ( ( "/" | "*" ) unary )* ;
unary → ( "!" | "-" ) unary
| primary ;
primary → NUMBER | STRING | "true" | "false" | "nil"
| "(" expression ")" ;
*/
struct Parser<'a> {
tokens: Vec<Token<'a>>,
current: usize,
}
type ExprResult<'a> = Result<Expr<'a>, Error>;
impl<'a> Parser<'a> {
// recursive-descent parser functions
fn expression(&mut self) -> ExprResult<'a> {
self.equality()
}
fn equality(&mut self) -> ExprResult<'a> {
self.binary_operator(
&[TokenKind::BangEqual, TokenKind::EqualEqual],
Self::comparison,
)
}
fn comparison(&mut self) -> ExprResult<'a> {
self.binary_operator(
&[
TokenKind::Greater,
TokenKind::GreaterEqual,
TokenKind::Less,
TokenKind::LessEqual,
],
Self::term,
)
}
fn term(&mut self) -> ExprResult<'a> {
self.binary_operator(&[TokenKind::Minus, TokenKind::Plus], Self::factor)
}
fn factor(&mut self) -> ExprResult<'a> {
self.binary_operator(&[TokenKind::Slash, TokenKind::Star], Self::unary)
}
fn unary(&mut self) -> ExprResult<'a> {
if self.match_token(&[TokenKind::Bang, TokenKind::Minus]) {
return Ok(Expr::Unary(Unary {
operator: self.previous(),
right: Box::new(self.unary()?),
}));
}
return self.primary();
}
fn primary(&mut self) -> ExprResult<'a> {
let next = self.advance();
let literal = match next.kind {
TokenKind::True => Literal::Boolean(true),
TokenKind::False => Literal::Boolean(false),
TokenKind::Nil => Literal::Nil,
TokenKind::Number(num) => Literal::Number(num),
TokenKind::String(string) => Literal::String(string),
TokenKind::LeftParen => {
let expr = self.expression()?;
self.consume(&TokenKind::RightParen, ErrorKind::UnmatchedParens)?;
return Ok(Expr::Grouping(Grouping(Box::new(expr))));
}
// This branch indicates a parser bug, not invalid input.
unexpected => panic!("Parser encountered unexpected token '{:?}'", unexpected),
};
Ok(Expr::Literal(literal))
}
// internal helpers
/// Check if the next token is in `oneof`, and advance if it is.
fn match_token(&mut self, oneof: &[TokenKind]) -> bool {
for token in oneof {
if self.check_token(token) {
self.advance();
return true;
}
}
return false;
}
/// Return the next token and advance parser state.
fn advance(&mut self) -> Token<'a> {
if !self.is_at_end() {
self.current += 1;
}
return self.previous();
}
fn is_at_end(&self) -> bool {
self.check_token(&TokenKind::Eof)
}
/// Is the next token `token`?
fn check_token(&self, token: &TokenKind) -> bool {
self.peek().kind == *token
}
fn peek(&self) -> &Token<'a> {
&self.tokens[self.current]
}
fn previous(&self) -> Token<'a> {
self.tokens[self.current - 1].clone()
}
fn consume(&mut self, kind: &TokenKind, err: ErrorKind) -> Result<(), Error> {
if self.check_token(kind) {
self.advance();
return Ok(());
}
Err(Error {
line: self.peek().line,
kind: err,
})
}
fn binary_operator(
&mut self,
oneof: &[TokenKind],
each: fn(&mut Parser<'a>) -> ExprResult<'a>,
) -> ExprResult<'a> {
let mut expr = each(self)?;
while self.match_token(oneof) {
expr = Expr::Binary(Binary {
left: Box::new(expr),
operator: self.previous(),
right: Box::new(each(self)?),
})
}
return Ok(expr);
}
}
|
