statements in rust

rust/rox/src/environment.rs

@@ -0,0 +1,66 @@
+use std::collections::HashMap;
+
+use thiserror::Error;
+
+use crate::{token::Token, value::Value};
+
+#[derive(Error, Debug)]
+pub enum EnvironmentError {
+    #[error("line {0}: undefined variable: {1}")]
+    UndefinedVariable(usize, String),
+}
+
+/// Environment mapping variable names to their respective values. Can have an optional enclosing
+/// environment. The outermost (global) environment has no enclosing one.
+#[derive(Default, Debug, Clone)]
+pub struct Environment {
+    values: HashMap<String, Value>,
+    enclosing: Option<Box<Environment>>,
+}
+
+impl Environment {
+    pub fn with_enclosing(enclosing: Environment) -> Self {
+        Self {
+            values: HashMap::default(),
+            enclosing: Some(Box::new(enclosing)),
+        }
+    }
+
+    /// Define a new variable binding with a value.
+    ///
+    /// Overwrite any binding of the same name.
+    pub fn define(&mut self, name: String, value: Value) {
+        self.values.insert(name, value);
+    }
+
+    /// Assign a new value to an existing (defined) variable. Error if there is no such variable in
+    /// this environment or any of the enclosing ones.
+    pub fn assign(&mut self, token: &Token, value: Value) -> Result<(), EnvironmentError> {
+        if self.values.contains_key(token.lexeme.as_str()) {
+            self.values.insert(token.lexeme.clone(), value);
+            Ok(())
+        } else if let Some(enclosing) = &mut self.enclosing {
+            enclosing.assign(token, value)
+        } else {
+            Err(EnvironmentError::UndefinedVariable(
+                token.line,
+                token.lexeme.clone(),
+            ))
+        }
+    }
+
+    /// Get the value of an existing (defined) variable. Error if there is no such variable in
+    /// this environment or any of the enclosing ones.
+    pub fn get(&self, token: &Token) -> Result<Value, EnvironmentError> {
+        if let Some(v) = self.values.get(token.lexeme.as_str()) {
+            Ok(v.clone())
+        } else if let Some(enclosing) = &self.enclosing {
+            enclosing.get(token)
+        } else {
+            Err(EnvironmentError::UndefinedVariable(
+                token.line,
+                token.lexeme.clone(),
+            ))
+        }
+    }
+}