1 files changed, 182 insertions, 0 deletions
diff --git a/users/grfn/achilles/src/interpreter/mod.rs b/users/grfn/achilles/src/interpreter/mod.rs
new file mode 100644
index 000000000000..a8ba2dd3acdc
--- /dev/null
+++ b/users/grfn/achilles/src/interpreter/mod.rs
@@ -0,0 +1,182 @@
+mod error;
+mod value;
+
+pub use self::error::{Error, Result};
+pub use self::value::{Function, Value};
+use crate::ast::hir::{Binding, Expr};
+use crate::ast::{BinaryOperator, FunctionType, Ident, Literal, Type, UnaryOperator};
+use crate::common::env::Env;
+
+#[derive(Debug, Default)]
+pub struct Interpreter<'a> {
+    env: Env<&'a Ident<'a>, Value<'a>>,
+}
+
+impl<'a> Interpreter<'a> {
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    fn resolve(&self, var: &'a Ident<'a>) -> Result<Value<'a>> {
+        self.env
+            .resolve(var)
+            .cloned()
+            .ok_or_else(|| Error::UndefinedVariable(var.to_owned()))
+    }
+
+    pub fn eval(&mut self, expr: &'a Expr<'a, Type>) -> Result<Value<'a>> {
+        let res = match expr {
+            Expr::Ident(id, _) => self.resolve(id),
+            Expr::Literal(Literal::Int(i), _) => Ok((*i).into()),
+            Expr::Literal(Literal::Bool(b), _) => Ok((*b).into()),
+            Expr::Literal(Literal::String(s), _) => Ok(s.clone().into()),
+            Expr::Literal(Literal::Unit, _) => unreachable!(),
+            Expr::UnaryOp { op, rhs, .. } => {
+                let rhs = self.eval(rhs)?;
+                match op {
+                    UnaryOperator::Neg => -rhs,
+                    _ => unimplemented!(),
+                }
+            }
+            Expr::BinaryOp { lhs, op, rhs, .. } => {
+                let lhs = self.eval(lhs)?;
+                let rhs = self.eval(rhs)?;
+                match op {
+                    BinaryOperator::Add => lhs + rhs,
+                    BinaryOperator::Sub => lhs - rhs,
+                    BinaryOperator::Mul => lhs * rhs,
+                    BinaryOperator::Div => lhs / rhs,
+                    BinaryOperator::Pow => todo!(),
+                    BinaryOperator::Equ => Ok(lhs.eq(&rhs).into()),
+                    BinaryOperator::Neq => todo!(),
+                }
+            }
+            Expr::Let { bindings, body, .. } => {
+                self.env.push();
+                for Binding { ident, body, .. } in bindings {
+                    let val = self.eval(body)?;
+                    self.env.set(ident, val);
+                }
+                let res = self.eval(body)?;
+                self.env.pop();
+                Ok(res)
+            }
+            Expr::If {
+                condition,
+                then,
+                else_,
+                ..
+            } => {
+                let condition = self.eval(condition)?;
+                if *(condition.as_type::<bool>()?) {
+                    self.eval(then)
+                } else {
+                    self.eval(else_)
+                }
+            }
+            Expr::Call { ref fun, args, .. } => {
+                let fun = self.eval(fun)?;
+                let expected_type = FunctionType {
+                    args: args.iter().map(|_| Type::Int).collect(),
+                    ret: Box::new(Type::Int),
+                };
+
+                let Function {
+                    args: function_args,
+                    body,
+                    ..
+                } = fun.as_function(expected_type)?;
+                let arg_values = function_args.iter().zip(
+                    args.iter()
+                        .map(|v| self.eval(v))
+                        .collect::<Result<Vec<_>>>()?,
+                );
+                let mut interpreter = Interpreter::new();
+                for (arg_name, arg_value) in arg_values {
+                    interpreter.env.set(arg_name, arg_value);
+                }
+                Ok(Value::from(*interpreter.eval(body)?.as_type::<i64>()?))
+            }
+            Expr::Fun {
+                type_args: _,
+                args,
+                body,
+                type_,
+            } => {
+                let type_ = match type_ {
+                    Type::Function(ft) => ft.clone(),
+                    _ => unreachable!("Function expression without function type"),
+                };
+
+                Ok(Value::from(value::Function {
+                    // TODO
+                    type_,
+                    args: args.iter().map(|(arg, _)| arg.to_owned()).collect(),
+                    body: (**body).to_owned(),
+                }))
+            }
+        }?;
+        debug_assert_eq!(&res.type_(), expr.type_());
+        Ok(res)
+    }
+}
+
+pub fn eval<'a>(expr: &'a Expr<'a, Type>) -> Result<Value<'a>> {
+    let mut interpreter = Interpreter::new();
+    interpreter.eval(expr)
+}
+
+#[cfg(test)]
+mod tests {
+    use std::convert::TryFrom;
+
+    use super::value::{TypeOf, Val};
+    use super::*;
+    use BinaryOperator::*;
+
+    fn int_lit(i: u64) -> Box<Expr<'static, Type<'static>>> {
+        Box::new(Expr::Literal(Literal::Int(i), Type::Int))
+    }
+
+    fn do_eval<T>(src: &str) -> T
+    where
+        for<'a> &'a T: TryFrom<&'a Val<'a>>,
+        T: Clone + TypeOf,
+    {
+        let expr = crate::parser::expr(src).unwrap().1;
+        let hir = crate::tc::typecheck_expr(expr).unwrap();
+        let res = eval(&hir).unwrap();
+        res.as_type::<T>().unwrap().clone()
+    }
+
+    #[test]
+    fn simple_addition() {
+        let expr = Expr::BinaryOp {
+            lhs: int_lit(1),
+            op: Mul,
+            rhs: int_lit(2),
+            type_: Type::Int,
+        };
+        let res = eval(&expr).unwrap();
+        assert_eq!(*res.as_type::<i64>().unwrap(), 2);
+    }
+
+    #[test]
+    fn variable_shadowing() {
+        let res = do_eval::<i64>("let x = 1 in (let x = 2 in x) + x");
+        assert_eq!(res, 3);
+    }
+
+    #[test]
+    fn conditional_with_equals() {
+        let res = do_eval::<i64>("let x = 1 in if x == 1 then 2 else 4");
+        assert_eq!(res, 2);
+    }
+
+    #[test]
+    #[ignore]
+    fn function_call() {
+        let res = do_eval::<i64>("let id = fn x = x in id 1");
+        assert_eq!(res, 1);
+    }
+}