about summary refs log tree commit diff
path: root/users/tazjin/rlox/src/interpreter.rs
blob: 0b936e89d2e1addf5e6c0a52d091cff2c99720e6 (plain) (blame)
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
use crate::errors::{Error, ErrorKind};
use crate::parser::{self, Declaration, Expr, Literal, Program, Statement};
use crate::scanner::TokenKind;
use std::collections::HashMap;

// Tree-walk interpreter

#[derive(Debug, Default)]
struct Environment {
    values: HashMap<String, Literal>,
}

impl Environment {
    fn define(&mut self, name: &str, value: Literal) {
        self.values.insert(name.into(), value);
    }

    fn get(&self, name: &parser::Variable) -> Result<Literal, Error> {
        if let TokenKind::Identifier(ident) = &name.0.kind {
            return self
                .values
                .get(ident)
                .map(Clone::clone)
                .ok_or_else(|| Error {
                    line: name.0.line,
                    kind: ErrorKind::UndefinedVariable(ident.into()),
                });
        }

        Err(Error {
            line: name.0.line,
            kind: ErrorKind::InternalError("unexpected identifier kind".into()),
        })
    }
}

#[derive(Debug, Default)]
pub struct Interpreter {
    globals: Environment,
}

impl Interpreter {
    fn interpret_stmt<'a>(&self, stmt: &Statement<'a>) -> Result<(), Error> {
        match stmt {
            Statement::Expr(expr) => {
                self.eval(expr)?;
            }
            Statement::Print(expr) => {
                let result = self.eval(expr)?;
                println!("{:?}", result)
            }
        }

        Ok(())
    }

    fn interpret_var<'a>(&mut self, var: &parser::Var<'a>) -> Result<(), Error> {
        if let TokenKind::Identifier(ident) = &var.name.kind {
            let init = var.initialiser.as_ref().ok_or_else(|| Error {
                line: var.name.line,
                kind: ErrorKind::InternalError("missing variable initialiser".into()),
            })?;

            self.globals.define(ident, self.eval(init)?);
            return Ok(());
        }

        Err(Error {
            line: var.name.line,
            kind: ErrorKind::InternalError("unexpected identifier kind".into()),
        })
    }

    pub fn interpret<'a>(&mut self, program: &Program<'a>) -> Result<(), Error> {
        for decl in program {
            match decl {
                Declaration::Stmt(stmt) => self.interpret_stmt(stmt)?,
                Declaration::Var(var) => self.interpret_var(var)?,
            }
        }

        Ok(())
    }

    fn eval<'a>(&self, expr: &Expr<'a>) -> Result<Literal, Error> {
        match expr {
            Expr::Literal(lit) => Ok(lit.clone()),
            Expr::Grouping(grouping) => self.eval(&*grouping.0),
            Expr::Unary(unary) => self.eval_unary(unary),
            Expr::Binary(binary) => self.eval_binary(binary),
            Expr::Variable(var) => self.globals.get(var),
        }
    }

    fn eval_unary<'a>(&self, expr: &parser::Unary<'a>) -> Result<Literal, Error> {
        let right = self.eval(&*expr.right)?;

        match (&expr.operator.kind, right) {
            (TokenKind::Minus, Literal::Number(num)) => Ok(Literal::Number(-num)),
            (TokenKind::Bang, right) => Ok(Literal::Boolean(!eval_truthy(&right))),

            (op, right) => Err(Error {
                line: expr.operator.line,
                kind: ErrorKind::TypeError(format!(
                    "Operator '{:?}' can not be called with argument '{:?}'",
                    op, right
                )),
            }),
        }
    }

    fn eval_binary<'a>(&self, expr: &parser::Binary<'a>) -> Result<Literal, Error> {
        let left = self.eval(&*expr.left)?;
        let right = self.eval(&*expr.right)?;

        let result = match (&expr.operator.kind, left, right) {
            // Numeric
            (TokenKind::Minus, Literal::Number(l), Literal::Number(r)) => Literal::Number(l - r),
            (TokenKind::Slash, Literal::Number(l), Literal::Number(r)) => Literal::Number(l / r),
            (TokenKind::Star, Literal::Number(l), Literal::Number(r)) => Literal::Number(l * r),
            (TokenKind::Plus, Literal::Number(l), Literal::Number(r)) => Literal::Number(l + r),

            // Strings
            (TokenKind::Plus, Literal::String(l), Literal::String(r)) => {
                Literal::String(format!("{}{}", l, r))
            }

            // Comparators (on numbers only?)
            (TokenKind::Greater, Literal::Number(l), Literal::Number(r)) => Literal::Boolean(l > r),
            (TokenKind::GreaterEqual, Literal::Number(l), Literal::Number(r)) => {
                Literal::Boolean(l >= r)
            }
            (TokenKind::Less, Literal::Number(l), Literal::Number(r)) => Literal::Boolean(l < r),
            (TokenKind::LessEqual, Literal::Number(l), Literal::Number(r)) => {
                Literal::Boolean(l <= r)
            }

            // Equality
            (TokenKind::Equal, l, r) => Literal::Boolean(l == r),
            (TokenKind::BangEqual, l, r) => Literal::Boolean(l != r),

            (op, left, right) => {
                return Err(Error {
                    line: expr.operator.line,
                    kind: ErrorKind::TypeError(format!(
                        "Operator '{:?}' can not be called with arguments '({:?}, {:?})'",
                        op, left, right
                    )),
                })
            }
        };

        Ok(result)
    }
}

// Interpreter functions not dependent on interpreter-state.

fn eval_truthy(lit: &Literal) -> bool {
    match lit {
        Literal::Nil => false,
        Literal::Boolean(b) => *b,
        _ => true,
    }
}