67 lines
2.2 KiB
Rust
67 lines
2.2 KiB
Rust
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use std::collections::HashMap;
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use thiserror::Error;
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use crate::{token::Token, value::Value};
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#[derive(Error, Debug)]
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pub enum EnvironmentError {
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#[error("line {0}: undefined variable: {1}")]
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UndefinedVariable(usize, String),
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}
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/// Environment mapping variable names to their respective values. Can have an optional enclosing
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/// environment. The outermost (global) environment has no enclosing one.
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#[derive(Default, Debug, Clone)]
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pub struct Environment {
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values: HashMap<String, Value>,
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enclosing: Option<Box<Environment>>,
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}
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impl Environment {
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pub fn with_enclosing(enclosing: Environment) -> Self {
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Self {
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values: HashMap::default(),
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enclosing: Some(Box::new(enclosing)),
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}
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}
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/// Define a new variable binding with a value.
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///
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/// Overwrite any binding of the same name.
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pub fn define(&mut self, name: String, value: Value) {
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self.values.insert(name, value);
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}
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/// Assign a new value to an existing (defined) variable. Error if there is no such variable in
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/// this environment or any of the enclosing ones.
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pub fn assign(&mut self, token: &Token, value: Value) -> Result<(), EnvironmentError> {
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if self.values.contains_key(token.lexeme.as_str()) {
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self.values.insert(token.lexeme.clone(), value);
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Ok(())
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} else if let Some(enclosing) = &mut self.enclosing {
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enclosing.assign(token, value)
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} else {
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Err(EnvironmentError::UndefinedVariable(
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token.line,
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token.lexeme.clone(),
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))
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}
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}
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/// Get the value of an existing (defined) variable. Error if there is no such variable in
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/// this environment or any of the enclosing ones.
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pub fn get(&self, token: &Token) -> Result<Value, EnvironmentError> {
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if let Some(v) = self.values.get(token.lexeme.as_str()) {
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Ok(v.clone())
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} else if let Some(enclosing) = &self.enclosing {
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enclosing.get(token)
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} else {
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Err(EnvironmentError::UndefinedVariable(
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token.line,
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token.lexeme.clone(),
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))
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}
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}
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}
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