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-rw-r--r--users/tazjin/rlox/src/treewalk/interpreter.rs482
1 files changed, 482 insertions, 0 deletions
diff --git a/users/tazjin/rlox/src/treewalk/interpreter.rs b/users/tazjin/rlox/src/treewalk/interpreter.rs
new file mode 100644
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+++ b/users/tazjin/rlox/src/treewalk/interpreter.rs
@@ -0,0 +1,482 @@
+use crate::errors::{Error, ErrorKind};
+use crate::parser::{self, Block, Expr, Literal, Statement};
+use crate::treewalk::resolver;
+use crate::scanner::{self, TokenKind};
+use std::collections::HashMap;
+use std::rc::Rc;
+use std::sync::RwLock;
+
+// Implementation of built-in functions.
+mod builtins;
+
+#[cfg(test)]
+mod tests;
+
+// Tree-walk interpreter
+
+// Representation of all callables, including builtins & user-defined
+// functions.
+#[derive(Clone, Debug)]
+pub enum Callable {
+    Builtin(&'static dyn builtins::Builtin),
+    Function {
+        func: Rc<parser::Function>,
+        closure: Rc<RwLock<Environment>>,
+    },
+}
+
+impl Callable {
+    fn arity(&self) -> usize {
+        match self {
+            Callable::Builtin(builtin) => builtin.arity(),
+            Callable::Function { func, .. } => func.params.len(),
+        }
+    }
+
+    fn call(&self, lox: &mut Interpreter, args: Vec<Value>) -> Result<Value, Error> {
+        match self {
+            Callable::Builtin(builtin) => builtin.call(args),
+
+            Callable::Function { func, closure } => {
+                let mut fn_env: Environment = Default::default();
+                fn_env.enclosing = Some(closure.clone());
+
+                for (param, value) in func.params.iter().zip(args.into_iter()) {
+                    fn_env.define(param, value)?;
+                }
+
+                let result =
+                    lox.interpret_block_with_env(Some(Rc::new(RwLock::new(fn_env))), &func.body);
+
+                match result {
+                    // extract returned values if applicable
+                    Err(Error {
+                        kind: ErrorKind::FunctionReturn(value),
+                        ..
+                    }) => Ok(value),
+
+                    // otherwise just return the result itself
+                    _ => result,
+                }
+            }
+        }
+    }
+}
+
+// Representation of an in-language value.
+#[derive(Clone, Debug)]
+pub enum Value {
+    Literal(Literal),
+    Callable(Callable),
+}
+
+impl PartialEq for Value {
+    fn eq(&self, other: &Self) -> bool {
+        match (self, other) {
+            (Value::Literal(lhs), Value::Literal(rhs)) => lhs == rhs,
+            // functions do not have equality
+            _ => false,
+        }
+    }
+}
+
+impl From<Literal> for Value {
+    fn from(lit: Literal) -> Value {
+        Value::Literal(lit)
+    }
+}
+
+impl Value {
+    fn expect_literal(self) -> Result<Literal, Error> {
+        match self {
+            Value::Literal(lit) => Ok(lit),
+            _ => unimplemented!(), // which error? which line?
+        }
+    }
+}
+
+#[derive(Debug, Default)]
+pub struct Environment {
+    enclosing: Option<Rc<RwLock<Environment>>>,
+    values: HashMap<String, Value>,
+}
+
+impl Environment {
+    fn define(&mut self, name: &scanner::Token, value: Value) -> Result<(), Error> {
+        let ident = identifier_str(name)?;
+        self.values.insert(ident.into(), value);
+        Ok(())
+    }
+
+    fn get(&self, ident: &str, line: usize, depth: usize) -> Result<Value, Error> {
+        if depth > 0 {
+            match &self.enclosing {
+                None => {
+                    return Err(Error {
+                        line,
+                        kind: ErrorKind::InternalError(format!(
+                            "invalid depth {} for {}",
+                            depth, ident
+                        )),
+                    })
+                }
+                Some(parent) => {
+                    let env = parent.read().expect("fatal: environment lock poisoned");
+                    return env.get(ident, line, depth - 1);
+                }
+            }
+        }
+
+        self.values
+            .get(ident)
+            .map(Clone::clone)
+            .ok_or_else(|| Error {
+                line,
+                kind: ErrorKind::UndefinedVariable(ident.into()),
+            })
+    }
+
+    fn assign(&mut self, name: &scanner::Token, value: Value) -> Result<(), Error> {
+        let ident = identifier_str(name)?;
+
+        match self.values.get_mut(ident) {
+            Some(target) => {
+                *target = value;
+                Ok(())
+            }
+            None => {
+                if let Some(parent) = &self.enclosing {
+                    return parent.write().unwrap().assign(name, value);
+                }
+
+                Err(Error {
+                    line: name.line,
+                    kind: ErrorKind::UndefinedVariable(ident.into()),
+                })
+            }
+        }
+    }
+}
+
+fn identifier_str(name: &scanner::Token) -> Result<&str, Error> {
+    if let TokenKind::Identifier(ident) = &name.kind {
+        Ok(ident)
+    } else {
+        Err(Error {
+            line: name.line,
+            kind: ErrorKind::InternalError("unexpected identifier kind".into()),
+        })
+    }
+}
+
+#[derive(Debug)]
+pub struct Interpreter {
+    env: Rc<RwLock<Environment>>,
+}
+
+impl Interpreter {
+    /// Create a new interpreter and configure the initial global
+    /// variable set.
+    pub fn create() -> Self {
+        let mut globals = HashMap::new();
+
+        globals.insert(
+            "clock".into(),
+            Value::Callable(Callable::Builtin(&builtins::Clock {})),
+        );
+
+        Interpreter {
+            env: Rc::new(RwLock::new(Environment {
+                enclosing: None,
+                values: globals,
+            })),
+        }
+    }
+
+    // Environment modification helpers
+    fn define_var(&mut self, name: &scanner::Token, value: Value) -> Result<(), Error> {
+        self.env
+            .write()
+            .expect("environment lock is poisoned")
+            .define(name, value)
+    }
+
+    fn assign_var(&mut self, name: &scanner::Token, value: Value) -> Result<(), Error> {
+        self.env
+            .write()
+            .expect("environment lock is poisoned")
+            .assign(name, value)
+    }
+
+    fn get_var(&mut self, var: &parser::Variable) -> Result<Value, Error> {
+        let ident = identifier_str(&var.name)?;
+        let depth = var.depth.ok_or_else(|| Error {
+            line: var.name.line,
+            kind: ErrorKind::UndefinedVariable(ident.into()),
+        })?;
+
+        self.env
+            .read()
+            .expect("environment lock is poisoned")
+            .get(ident, var.name.line, depth)
+    }
+
+    // Interpreter itself
+    pub fn interpret(&mut self, mut program: Block) -> Result<Value, Error> {
+        let globals = self.env.read()
+            .expect("static globals lock poisoned")
+            .values.keys().map(Clone::clone)
+            .collect::<Vec<String>>();
+
+        resolver::resolve(&globals, &mut program)?;
+        self.interpret_block_with_env(None, &program)
+    }
+
+    /// Interpret the block in the supplied environment. If no
+    /// environment is supplied, a new one is created using the
+    /// current one as its parent.
+    fn interpret_block_with_env(
+        &mut self,
+        env: Option<Rc<RwLock<Environment>>>,
+        block: &parser::Block,
+    ) -> Result<Value, Error> {
+        let env = match env {
+            Some(env) => env,
+            None => {
+                let env: Rc<RwLock<Environment>> = Default::default();
+                set_enclosing_env(&env, self.env.clone());
+                env
+            }
+        };
+
+        let previous = std::mem::replace(&mut self.env, env);
+        let result = self.interpret_block(block);
+
+        // Swap it back, discarding the child env.
+        self.env = previous;
+
+        return result;
+    }
+
+    fn interpret_block(&mut self, program: &Block) -> Result<Value, Error> {
+        let mut value = Value::Literal(Literal::Nil);
+
+        for stmt in program {
+            value = self.interpret_stmt(stmt)?;
+        }
+
+        Ok(value)
+    }
+
+    fn interpret_stmt(&mut self, stmt: &Statement) -> Result<Value, Error> {
+        let value = match stmt {
+            Statement::Expr(expr) => self.eval(expr)?,
+            Statement::Print(expr) => {
+                let result = self.eval(expr)?;
+                let output = format!("{:?}", result);
+                println!("{}", output);
+                Value::Literal(Literal::String(output))
+            }
+            Statement::Var(var) => return self.interpret_var(var),
+            Statement::Block(block) => return self.interpret_block_with_env(None, block),
+            Statement::If(if_stmt) => return self.interpret_if(if_stmt),
+            Statement::While(while_stmt) => return self.interpret_while(while_stmt),
+            Statement::Function(func) => return self.interpret_function(func.clone()),
+            Statement::Return(ret) => {
+                return Err(Error {
+                    line: 0,
+                    kind: ErrorKind::FunctionReturn(self.eval(&ret.value)?),
+                })
+            }
+        };
+
+        Ok(value)
+    }
+
+    fn interpret_var(&mut self, var: &parser::Var) -> Result<Value, Error> {
+        let init = var.initialiser.as_ref().ok_or_else(|| Error {
+            line: var.name.line,
+            kind: ErrorKind::InternalError("missing variable initialiser".into()),
+        })?;
+        let value = self.eval(init)?;
+        self.define_var(&var.name, value.clone())?;
+        Ok(value)
+    }
+
+    fn interpret_if(&mut self, if_stmt: &parser::If) -> Result<Value, Error> {
+        let condition = self.eval(&if_stmt.condition)?;
+
+        if eval_truthy(&condition) {
+            self.interpret_stmt(&if_stmt.then_branch)
+        } else if let Some(else_branch) = &if_stmt.else_branch {
+            self.interpret_stmt(else_branch)
+        } else {
+            Ok(Value::Literal(Literal::Nil))
+        }
+    }
+
+    fn interpret_while(&mut self, stmt: &parser::While) -> Result<Value, Error> {
+        let mut value = Value::Literal(Literal::Nil);
+        while eval_truthy(&self.eval(&stmt.condition)?) {
+            value = self.interpret_stmt(&stmt.body)?;
+        }
+
+        Ok(value)
+    }
+
+    fn interpret_function(&mut self, func: Rc<parser::Function>) -> Result<Value, Error> {
+        let name = func.name.clone();
+        let value = Value::Callable(Callable::Function {
+            func,
+            closure: self.env.clone(),
+        });
+        self.define_var(&name, value.clone())?;
+        Ok(value)
+    }
+
+    fn eval(&mut self, expr: &Expr) -> Result<Value, Error> {
+        match expr {
+            Expr::Assign(assign) => self.eval_assign(assign),
+            Expr::Literal(lit) => Ok(lit.clone().into()),
+            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.get_var(var),
+            Expr::Logical(log) => self.eval_logical(log),
+            Expr::Call(call) => self.eval_call(call),
+        }
+    }
+
+    fn eval_unary(&mut self, expr: &parser::Unary) -> Result<Value, Error> {
+        let right = self.eval(&*expr.right)?;
+
+        match (&expr.operator.kind, right) {
+            (TokenKind::Minus, Value::Literal(Literal::Number(num))) => {
+                Ok(Literal::Number(-num).into())
+            }
+            (TokenKind::Bang, right) => Ok(Literal::Boolean(!eval_truthy(&right)).into()),
+
+            (op, right) => Err(Error {
+                line: expr.operator.line,
+                kind: ErrorKind::TypeError(format!(
+                    "Operator '{:?}' can not be called with argument '{:?}'",
+                    op, right
+                )),
+            }),
+        }
+    }
+
+    fn eval_binary(&mut self, expr: &parser::Binary) -> Result<Value, Error> {
+        let left = self.eval(&*expr.left)?.expect_literal()?;
+        let right = self.eval(&*expr.right)?.expect_literal()?;
+
+        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.into())
+    }
+
+    fn eval_assign(&mut self, assign: &parser::Assign) -> Result<Value, Error> {
+        let value = self.eval(&assign.value)?;
+        self.assign_var(&assign.name, value.clone())?;
+        Ok(value)
+    }
+
+    fn eval_logical(&mut self, logical: &parser::Logical) -> Result<Value, Error> {
+        let left = eval_truthy(&self.eval(&logical.left)?);
+        let right = eval_truthy(&self.eval(&logical.right)?);
+
+        match &logical.operator.kind {
+            TokenKind::And => Ok(Literal::Boolean(left && right).into()),
+            TokenKind::Or => Ok(Literal::Boolean(left || right).into()),
+            kind => Err(Error {
+                line: logical.operator.line,
+                kind: ErrorKind::InternalError(format!("Invalid logical operator: {:?}", kind)),
+            }),
+        }
+    }
+
+    fn eval_call(&mut self, call: &parser::Call) -> Result<Value, Error> {
+        let callable = match self.eval(&call.callee)? {
+            Value::Callable(c) => c,
+            Value::Literal(v) => {
+                return Err(Error {
+                    line: call.paren.line,
+                    kind: ErrorKind::RuntimeError(format!("not callable: {:?}", v)),
+                })
+            }
+        };
+
+        let mut args = vec![];
+        for arg in &call.args {
+            args.push(self.eval(arg)?);
+        }
+
+        if callable.arity() != args.len() {
+            return Err(Error {
+                line: call.paren.line,
+                kind: ErrorKind::RuntimeError(format!(
+                    "Expected {} arguments, but got {}",
+                    callable.arity(),
+                    args.len(),
+                )),
+            });
+        }
+
+        callable.call(self, args)
+    }
+}
+
+// Interpreter functions not dependent on interpreter-state.
+
+fn eval_truthy(lit: &Value) -> bool {
+    if let Value::Literal(lit) = lit {
+        match lit {
+            Literal::Nil => false,
+            Literal::Boolean(b) => *b,
+            _ => true,
+        }
+    } else {
+        false
+    }
+}
+
+fn set_enclosing_env(this: &RwLock<Environment>, parent: Rc<RwLock<Environment>>) {
+    this.write()
+        .expect("environment lock is poisoned")
+        .enclosing = Some(parent);
+}