Implement functions, both top-level and anonymous

Implement both top-level and anonymous functions, but not closures in
either case.

10 files changed, 501 insertions, 125 deletions

diff --git a/Cargo.lock b/Cargo.lock
index 485e9f4cdf..d8eaedeca1 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -8,6 +8,7 @@ dependencies = [
  "clap",
  "derive_more",
  "inkwell",
+ "itertools",
  "llvm-sys",
  "nom",
  "nom-trace",
@@ -246,6 +247,15 @@ dependencies = [
 ]
 
 [[package]]
+name = "itertools"
+version = "0.10.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "37d572918e350e82412fe766d24b15e6682fb2ed2bbe018280caa810397cb319"
+dependencies = [
+ "either",
+]
+
+[[package]]
 name = "lazy_static"
 version = "1.4.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
diff --git a/Cargo.toml b/Cargo.toml
index eda15e5546..c9796a8215 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -9,6 +9,7 @@ anyhow = "1.0.38"
 clap = "3.0.0-beta.2"
 derive_more = "0.99.11"
 inkwell = { git = "https://github.com/TheDan64/inkwell", branch = "master", features = ["llvm11-0"] }
+itertools = "0.10.0"
 llvm-sys = "110.0.1"
 nom = "6.1.2"
 nom-trace = { git = "https://github.com/glittershark/nom-trace", branch = "nom-6" }
diff --git a/src/ast/mod.rs b/src/ast/mod.rs
index 2dcf955fe6..7f3543271d 100644
--- a/src/ast/mod.rs
+++ b/src/ast/mod.rs
@@ -2,10 +2,12 @@ use std::borrow::Cow;
 use std::convert::TryFrom;
 use std::fmt::{self, Display, Formatter};
 
+use itertools::Itertools;
+
 #[derive(Debug, PartialEq, Eq)]
 pub struct InvalidIdentifier<'a>(Cow<'a, str>);
 
-#[derive(Debug, PartialEq, Eq, Hash)]
+#[derive(Debug, PartialEq, Eq, Hash, Clone)]
 pub struct Ident<'a>(pub Cow<'a, str>);
 
 impl<'a> From<&'a Ident<'a>> for &'a str {
@@ -69,7 +71,7 @@ impl<'a> TryFrom<String> for Ident<'a> {
     }
 }
 
-#[derive(Debug, PartialEq, Eq)]
+#[derive(Debug, PartialEq, Eq, Copy, Clone)]
 pub enum BinaryOperator {
     /// `+`
     Add,
@@ -93,7 +95,7 @@ pub enum BinaryOperator {
     Neq,
 }
 
-#[derive(Debug, PartialEq, Eq)]
+#[derive(Debug, PartialEq, Eq, Copy, Clone)]
 pub enum UnaryOperator {
     /// !
     Not,
@@ -102,12 +104,12 @@ pub enum UnaryOperator {
     Neg,
 }
 
-#[derive(Debug, PartialEq, Eq)]
+#[derive(Debug, PartialEq, Eq, Clone)]
 pub enum Literal {
     Int(u64),
 }
 
-#[derive(Debug, PartialEq, Eq)]
+#[derive(Debug, PartialEq, Eq, Clone)]
 pub enum Expr<'a> {
     Ident(Ident<'a>),
 
@@ -134,33 +136,101 @@ pub enum Expr<'a> {
         then: Box<Expr<'a>>,
         else_: Box<Expr<'a>>,
     },
+
+    Fun(Box<Fun<'a>>),
+
+    Call {
+        fun: Box<Expr<'a>>,
+        args: Vec<Expr<'a>>,
+    },
 }
 
-#[derive(Debug, PartialEq, Eq)]
+impl<'a> Expr<'a> {
+    pub fn to_owned(&self) -> Expr<'static> {
+        match self {
+            Expr::Ident(ref id) => Expr::Ident(id.to_owned()),
+            Expr::Literal(ref lit) => Expr::Literal(lit.clone()),
+            Expr::UnaryOp { op, rhs } => Expr::UnaryOp {
+                op: *op,
+                rhs: Box::new((**rhs).to_owned()),
+            },
+            Expr::BinaryOp { lhs, op, rhs } => Expr::BinaryOp {
+                lhs: Box::new((**lhs).to_owned()),
+                op: *op,
+                rhs: Box::new((**rhs).to_owned()),
+            },
+            Expr::Let { bindings, body } => Expr::Let {
+                bindings: bindings
+                    .iter()
+                    .map(|(id, expr)| (id.to_owned(), expr.to_owned()))
+                    .collect(),
+                body: Box::new((**body).to_owned()),
+            },
+            Expr::If {
+                condition,
+                then,
+                else_,
+            } => Expr::If {
+                condition: Box::new((**condition).to_owned()),
+                then: Box::new((**then).to_owned()),
+                else_: Box::new((**else_).to_owned()),
+            },
+            Expr::Fun(fun) => Expr::Fun(Box::new((**fun).to_owned())),
+            Expr::Call { fun, args } => Expr::Call {
+                fun: Box::new((**fun).to_owned()),
+                args: args.iter().map(|arg| arg.to_owned()).collect(),
+            },
+        }
+    }
+}
+
+#[derive(Debug, PartialEq, Eq, Clone)]
 pub struct Fun<'a> {
-    pub name: Ident<'a>,
     pub args: Vec<Ident<'a>>,
     pub body: Expr<'a>,
 }
 
+impl<'a> Fun<'a> {
+    fn to_owned(&self) -> Fun<'static> {
+        Fun {
+            args: self.args.iter().map(|arg| arg.to_owned()).collect(),
+            body: self.body.to_owned(),
+        }
+    }
+}
+
 #[derive(Debug, PartialEq, Eq)]
 pub enum Decl<'a> {
-    Fun(Fun<'a>),
+    Fun { name: Ident<'a>, body: Fun<'a> },
+}
+
+#[derive(Debug, PartialEq, Eq, Clone)]
+pub struct FunctionType {
+    pub args: Vec<Type>,
+    pub ret: Box<Type>,
+}
+
+impl Display for FunctionType {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        write!(f, "fn {} -> {}", self.args.iter().join(", "), self.ret)
+    }
 }
 
-#[derive(Debug, PartialEq, Eq, Clone, Copy)]
+#[derive(Debug, PartialEq, Eq, Clone)]
 pub enum Type {
     Int,
     Float,
     Bool,
+    Function(FunctionType),
 }
 
 impl Display for Type {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
-            Self::Int => f.write_str("int"),
-            Self::Float => f.write_str("float"),
-            Self::Bool => f.write_str("bool"),
+            Type::Int => f.write_str("int"),
+            Type::Float => f.write_str("float"),
+            Type::Bool => f.write_str("bool"),
+            Type::Function(ft) => ft.fmt(f),
         }
     }
 }
diff --git a/src/codegen/llvm.rs b/src/codegen/llvm.rs
index ff92c33731..1d1c742a94 100644
--- a/src/codegen/llvm.rs
+++ b/src/codegen/llvm.rs
@@ -1,3 +1,4 @@
+use std::convert::{TryFrom, TryInto};
 use std::path::Path;
 use std::result;
 
@@ -7,7 +8,7 @@ pub use inkwell::context::Context;
 use inkwell::module::Module;
 use inkwell::support::LLVMString;
 use inkwell::types::FunctionType;
-use inkwell::values::{BasicValueEnum, FunctionValue};
+use inkwell::values::{AnyValueEnum, BasicValueEnum, FunctionValue};
 use inkwell::IntPredicate;
 use thiserror::Error;
 
@@ -35,8 +36,9 @@ pub struct Codegen<'ctx, 'ast> {
     context: &'ctx Context,
     pub module: Module<'ctx>,
     builder: Builder<'ctx>,
-    env: Env<'ast, BasicValueEnum<'ctx>>,
-    function: Option<FunctionValue<'ctx>>,
+    env: Env<'ast, AnyValueEnum<'ctx>>,
+    function_stack: Vec<FunctionValue<'ctx>>,
+    identifier_counter: u32,
 }
 
 impl<'ctx, 'ast> Codegen<'ctx, 'ast> {
@@ -48,7 +50,8 @@ impl<'ctx, 'ast> Codegen<'ctx, 'ast> {
             module,
             builder,
             env: Default::default(),
-            function: None,
+            function_stack: Default::default(),
+            identifier_counter: 0,
         }
     }
 
@@ -57,22 +60,24 @@ impl<'ctx, 'ast> Codegen<'ctx, 'ast> {
         name: &str,
         ty: FunctionType<'ctx>,
     ) -> &'a FunctionValue<'ctx> {
-        self.function = Some(self.module.add_function(name, ty, None));
+        self.function_stack
+            .push(self.module.add_function(name, ty, None));
         let basic_block = self.append_basic_block("entry");
         self.builder.position_at_end(basic_block);
-        self.function.as_ref().unwrap()
+        self.function_stack.last().unwrap()
     }
 
-    pub fn finish_function(&self, res: &BasicValueEnum<'ctx>) {
+    pub fn finish_function(&mut self, res: &BasicValueEnum<'ctx>) -> FunctionValue<'ctx> {
         self.builder.build_return(Some(res));
+        self.function_stack.pop().unwrap()
     }
 
     pub fn append_basic_block(&self, name: &str) -> BasicBlock<'ctx> {
         self.context
-            .append_basic_block(self.function.unwrap(), name)
+            .append_basic_block(*self.function_stack.last().unwrap(), name)
     }
 
-    pub fn codegen_expr(&mut self, expr: &'ast Expr<'ast>) -> Result<BasicValueEnum<'ctx>> {
+    pub fn codegen_expr(&mut self, expr: &'ast Expr<'ast>) -> Result<AnyValueEnum<'ctx>> {
         match expr {
             Expr::Ident(id) => self
                 .env
@@ -81,13 +86,13 @@ impl<'ctx, 'ast> Codegen<'ctx, 'ast> {
                 .ok_or_else(|| Error::UndefinedVariable(id.to_owned())),
             Expr::Literal(Literal::Int(i)) => {
                 let ty = self.context.i64_type();
-                Ok(BasicValueEnum::IntValue(ty.const_int(*i, false)))
+                Ok(AnyValueEnum::IntValue(ty.const_int(*i, false)))
             }
             Expr::UnaryOp { op, rhs } => {
                 let rhs = self.codegen_expr(rhs)?;
                 match op {
                     UnaryOperator::Not => unimplemented!(),
-                    UnaryOperator::Neg => Ok(BasicValueEnum::IntValue(
+                    UnaryOperator::Neg => Ok(AnyValueEnum::IntValue(
                         self.builder.build_int_neg(rhs.into_int_value(), "neg"),
                     )),
                 }
@@ -96,29 +101,23 @@ impl<'ctx, 'ast> Codegen<'ctx, 'ast> {
                 let lhs = self.codegen_expr(lhs)?;
                 let rhs = self.codegen_expr(rhs)?;
                 match op {
-                    BinaryOperator::Add => {
-                        Ok(BasicValueEnum::IntValue(self.builder.build_int_add(
-                            lhs.into_int_value(),
-                            rhs.into_int_value(),
-                            "add",
-                        )))
-                    }
-                    BinaryOperator::Sub => {
-                        Ok(BasicValueEnum::IntValue(self.builder.build_int_sub(
-                            lhs.into_int_value(),
-                            rhs.into_int_value(),
-                            "add",
-                        )))
-                    }
-                    BinaryOperator::Mul => {
-                        Ok(BasicValueEnum::IntValue(self.builder.build_int_sub(
-                            lhs.into_int_value(),
-                            rhs.into_int_value(),
-                            "add",
-                        )))
-                    }
+                    BinaryOperator::Add => Ok(AnyValueEnum::IntValue(self.builder.build_int_add(
+                        lhs.into_int_value(),
+                        rhs.into_int_value(),
+                        "add",
+                    ))),
+                    BinaryOperator::Sub => Ok(AnyValueEnum::IntValue(self.builder.build_int_sub(
+                        lhs.into_int_value(),
+                        rhs.into_int_value(),
+                        "add",
+                    ))),
+                    BinaryOperator::Mul => Ok(AnyValueEnum::IntValue(self.builder.build_int_sub(
+                        lhs.into_int_value(),
+                        rhs.into_int_value(),
+                        "add",
+                    ))),
                     BinaryOperator::Div => {
-                        Ok(BasicValueEnum::IntValue(self.builder.build_int_signed_div(
+                        Ok(AnyValueEnum::IntValue(self.builder.build_int_signed_div(
                             lhs.into_int_value(),
                             rhs.into_int_value(),
                             "add",
@@ -126,7 +125,7 @@ impl<'ctx, 'ast> Codegen<'ctx, 'ast> {
                     }
                     BinaryOperator::Pow => unimplemented!(),
                     BinaryOperator::Equ => {
-                        Ok(BasicValueEnum::IntValue(self.builder.build_int_compare(
+                        Ok(AnyValueEnum::IntValue(self.builder.build_int_compare(
                             IntPredicate::EQ,
                             lhs.into_int_value(),
                             rhs.into_int_value(),
@@ -170,34 +169,83 @@ impl<'ctx, 'ast> Codegen<'ctx, 'ast> {
 
                 self.builder.position_at_end(join_block);
                 let phi = self.builder.build_phi(self.context.i64_type(), "join");
-                phi.add_incoming(&[(&then_res, then_block), (&else_res, else_block)]);
-                Ok(phi.as_basic_value())
+                phi.add_incoming(&[
+                    (&BasicValueEnum::try_from(then_res).unwrap(), then_block),
+                    (&BasicValueEnum::try_from(else_res).unwrap(), else_block),
+                ]);
+                Ok(phi.as_basic_value().into())
+            }
+            Expr::Call { fun, args } => {
+                if let Expr::Ident(id) = &**fun {
+                    let function = self
+                        .module
+                        .get_function(id.into())
+                        .or_else(|| self.env.resolve(id)?.clone().try_into().ok())
+                        .ok_or_else(|| Error::UndefinedVariable(id.to_owned()))?;
+                    let args = args
+                        .iter()
+                        .map(|arg| Ok(self.codegen_expr(arg)?.try_into().unwrap()))
+                        .collect::<Result<Vec<_>>>()?;
+                    Ok(self
+                        .builder
+                        .build_call(function, &args, "call")
+                        .try_as_basic_value()
+                        .left()
+                        .unwrap()
+                        .into())
+                } else {
+                    todo!()
+                }
+            }
+            Expr::Fun(fun) => {
+                let Fun { args, body } = &**fun;
+                let fname = self.fresh_ident("f");
+                let cur_block = self.builder.get_insert_block().unwrap();
+                let env = self.env.save(); // TODO: closures
+                let function = self.codegen_function(&fname, args, body)?;
+                self.builder.position_at_end(cur_block);
+                self.env.restore(env);
+                Ok(function.into())
             }
         }
     }
 
+    pub fn codegen_function(
+        &mut self,
+        name: &str,
+        args: &'ast [Ident<'ast>],
+        body: &'ast Expr<'ast>,
+    ) -> Result<FunctionValue<'ctx>> {
+        let i64_type = self.context.i64_type();
+        self.new_function(
+            name,
+            i64_type.fn_type(
+                args.iter()
+                    .map(|_| i64_type.into())
+                    .collect::<Vec<_>>()
+                    .as_slice(),
+                false,
+            ),
+        );
+        self.env.push();
+        for (i, arg) in args.iter().enumerate() {
+            self.env.set(
+                arg,
+                self.cur_function().get_nth_param(i as u32).unwrap().into(),
+            );
+        }
+        let res = self.codegen_expr(body)?.try_into().unwrap();
+        self.env.pop();
+        Ok(self.finish_function(&res))
+    }
+
     pub fn codegen_decl(&mut self, decl: &'ast Decl<'ast>) -> Result<()> {
         match decl {
-            Decl::Fun(Fun { name, args, body }) => {
-                let i64_type = self.context.i64_type();
-                self.new_function(
-                    name.into(),
-                    i64_type.fn_type(
-                        args.iter()
-                            .map(|_| i64_type.into())
-                            .collect::<Vec<_>>()
-                            .as_slice(),
-                        false,
-                    ),
-                );
-                self.env.push();
-                for (i, arg) in args.iter().enumerate() {
-                    self.env
-                        .set(arg, self.function.unwrap().get_nth_param(i as u32).unwrap());
-                }
-                let res = self.codegen_expr(body)?;
-                self.env.pop();
-                self.finish_function(&res);
+            Decl::Fun {
+                name,
+                body: Fun { args, body },
+            } => {
+                self.codegen_function(name.into(), args, body)?;
                 Ok(())
             }
         }
@@ -205,7 +253,7 @@ impl<'ctx, 'ast> Codegen<'ctx, 'ast> {
 
     pub fn codegen_main(&mut self, expr: &'ast Expr<'ast>) -> Result<()> {
         self.new_function("main", self.context.i64_type().fn_type(&[], false));
-        let res = self.codegen_expr(expr)?;
+        let res = self.codegen_expr(expr)?.try_into().unwrap();
         self.finish_function(&res);
         Ok(())
     }
@@ -229,6 +277,15 @@ impl<'ctx, 'ast> Codegen<'ctx, 'ast> {
             ))
         }
     }
+
+    fn fresh_ident(&mut self, prefix: &str) -> String {
+        self.identifier_counter += 1;
+        format!("{}{}", prefix, self.identifier_counter)
+    }
+
+    fn cur_function(&self) -> &FunctionValue<'ctx> {
+        self.function_stack.last().unwrap()
+    }
 }
 
 #[cfg(test)]
@@ -248,9 +305,7 @@ mod tests {
             .create_jit_execution_engine(OptimizationLevel::None)
             .unwrap();
 
-        codegen.new_function("test", context.i64_type().fn_type(&[], false));
-        let res = codegen.codegen_expr(&expr)?;
-        codegen.finish_function(&res);
+        codegen.codegen_function("test", &[], &expr)?;
 
         unsafe {
             let fun: JitFunction<unsafe extern "C" fn() -> T> =
@@ -279,4 +334,10 @@ mod tests {
             2
         );
     }
+
+    #[test]
+    fn function_call() {
+        let res = jit_eval::<i64>("let id = fn x = x in id 1").unwrap();
+        assert_eq!(res, 1);
+    }
 }
diff --git a/src/codegen/mod.rs b/src/codegen/mod.rs
index 4620b6b48e..6f95d90b45 100644
--- a/src/codegen/mod.rs
+++ b/src/codegen/mod.rs
@@ -14,9 +14,7 @@ pub fn jit_eval<T>(expr: &Expr) -> Result<T> {
         .module
         .create_jit_execution_engine(OptimizationLevel::None)
         .map_err(Error::from)?;
-    codegen.new_function("eval", context.i64_type().fn_type(&[], false));
-    let res = codegen.codegen_expr(&expr)?;
-    codegen.finish_function(&res);
+    codegen.codegen_function("test", &[], &expr)?;
 
     unsafe {
         let fun: JitFunction<unsafe extern "C" fn() -> T> =
diff --git a/src/commands/compile.rs b/src/commands/compile.rs
index e16b8c87a6..be8767575a 100644
--- a/src/commands/compile.rs
+++ b/src/commands/compile.rs
@@ -5,10 +5,13 @@ use clap::Clap;
 use crate::common::Result;
 use crate::compiler::{self, CompilerOptions};
 
+/// Compile a source file
 #[derive(Clap)]
 pub struct Compile {
+    /// File to compile
     file: PathBuf,
 
+    /// Output file
     #[clap(short = 'o')]
     out_file: PathBuf,
 
diff --git a/src/common/env.rs b/src/common/env.rs
index 8b5cde49e9..f499323639 100644
--- a/src/common/env.rs
+++ b/src/common/env.rs
@@ -1,4 +1,5 @@
 use std::collections::HashMap;
+use std::mem;
 
 use crate::ast::Ident;
 
@@ -25,6 +26,14 @@ impl<'ast, V> Env<'ast, V> {
         self.0.pop();
     }
 
+    pub fn save(&mut self) -> Self {
+        mem::take(self)
+    }
+
+    pub fn restore(&mut self, saved: Self) {
+        *self = saved;
+    }
+
     pub fn set(&mut self, k: &'ast Ident<'ast>, v: V) {
         self.0.last_mut().unwrap().insert(k, v);
     }
diff --git a/src/interpreter/mod.rs b/src/interpreter/mod.rs
index adff3568c2..fc1556d1c2 100644
--- a/src/interpreter/mod.rs
+++ b/src/interpreter/mod.rs
@@ -2,13 +2,13 @@ mod error;
 mod value;
 
 pub use self::error::{Error, Result};
-pub use self::value::Value;
-use crate::ast::{BinaryOperator, Expr, Ident, Literal, UnaryOperator};
+pub use self::value::{Function, Value};
+use crate::ast::{BinaryOperator, Expr, FunctionType, Ident, Literal, Type, UnaryOperator};
 use crate::common::env::Env;
 
 #[derive(Debug, Default)]
 pub struct Interpreter<'a> {
-    env: Env<'a, Value>,
+    env: Env<'a, Value<'a>>,
 }
 
 impl<'a> Interpreter<'a> {
@@ -16,14 +16,14 @@ impl<'a> Interpreter<'a> {
         Self::default()
     }
 
-    fn resolve(&self, var: &'a Ident<'a>) -> Result<Value> {
+    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>) -> Result<Value> {
+    pub fn eval(&mut self, expr: &'a Expr<'a>) -> Result<Value<'a>> {
         match expr {
             Expr::Ident(id) => self.resolve(id),
             Expr::Literal(Literal::Int(i)) => Ok((*i).into()),
@@ -63,12 +63,44 @@ impl<'a> Interpreter<'a> {
                 else_,
             } => {
                 let condition = self.eval(condition)?;
-                if *(condition.into_type::<bool>()?) {
+                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(fun) => Ok(Value::from(value::Function {
+                // TODO
+                type_: FunctionType {
+                    args: fun.args.iter().map(|_| Type::Int).collect(),
+                    ret: Box::new(Type::Int),
+                },
+                args: fun.args.iter().map(|arg| arg.to_owned()).collect(),
+                body: fun.body.to_owned(),
+            })),
         }
     }
 }
@@ -92,12 +124,12 @@ mod tests {
 
     fn parse_eval<T>(src: &str) -> T
     where
-        for<'a> &'a T: TryFrom<&'a Val>,
+        for<'a> &'a T: TryFrom<&'a Val<'a>>,
         T: Clone + TypeOf,
     {
         let expr = crate::parser::expr(src).unwrap().1;
         let res = eval(&expr).unwrap();
-        res.into_type::<T>().unwrap().clone()
+        res.as_type::<T>().unwrap().clone()
     }
 
     #[test]
@@ -108,7 +140,7 @@ mod tests {
             rhs: int_lit(2),
         };
         let res = eval(&expr).unwrap();
-        assert_eq!(*res.into_type::<i64>().unwrap(), 2);
+        assert_eq!(*res.as_type::<i64>().unwrap(), 2);
     }
 
     #[test]
@@ -122,4 +154,10 @@ mod tests {
         let res = parse_eval::<i64>("let x = 1 in if x == 1 then 2 else 4");
         assert_eq!(res, 2);
     }
+
+    #[test]
+    fn function_call() {
+        let res = parse_eval::<i64>("let id = fn x = x in id 1");
+        assert_eq!(res, 1);
+    }
 }
diff --git a/src/interpreter/value.rs b/src/interpreter/value.rs
index 69e4d4ffeb..496e9c4230 100644
--- a/src/interpreter/value.rs
+++ b/src/interpreter/value.rs
@@ -6,108 +6,153 @@ use std::rc::Rc;
 use derive_more::{Deref, From, TryInto};
 
 use super::{Error, Result};
-use crate::ast::Type;
+use crate::ast::{Expr, FunctionType, Ident, Type};
 
-#[derive(Debug, PartialEq, From, TryInto)]
+#[derive(Debug, Clone)]
+pub struct Function<'a> {
+    pub type_: FunctionType,
+    pub args: Vec<Ident<'a>>,
+    pub body: Expr<'a>,
+}
+
+#[derive(From, TryInto)]
 #[try_into(owned, ref)]
-pub enum Val {
+pub enum Val<'a> {
     Int(i64),
     Float(f64),
     Bool(bool),
+    Function(Function<'a>),
+}
+
+impl<'a> fmt::Debug for Val<'a> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Val::Int(x) => f.debug_tuple("Int").field(x).finish(),
+            Val::Float(x) => f.debug_tuple("Float").field(x).finish(),
+            Val::Bool(x) => f.debug_tuple("Bool").field(x).finish(),
+            Val::Function(Function { type_, .. }) => {
+                f.debug_struct("Function").field("type_", type_).finish()
+            }
+        }
+    }
 }
 
-impl From<u64> for Val {
+impl<'a> PartialEq for Val<'a> {
+    fn eq(&self, other: &Self) -> bool {
+        match (self, other) {
+            (Val::Int(x), Val::Int(y)) => x == y,
+            (Val::Float(x), Val::Float(y)) => x == y,
+            (Val::Bool(x), Val::Bool(y)) => x == y,
+            (Val::Function(_), Val::Function(_)) => false,
+            (_, _) => false,
+        }
+    }
+}
+
+impl<'a> From<u64> for Val<'a> {
     fn from(i: u64) -> Self {
         Self::from(i as i64)
     }
 }
 
-impl Display for Val {
+impl<'a> Display for Val<'a> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
             Val::Int(x) => x.fmt(f),
             Val::Float(x) => x.fmt(f),
             Val::Bool(x) => x.fmt(f),
+            Val::Function(Function { type_, .. }) => write!(f, "<{}>", type_),
         }
     }
 }
 
-impl Val {
+impl<'a> Val<'a> {
     pub fn type_(&self) -> Type {
         match self {
             Val::Int(_) => Type::Int,
             Val::Float(_) => Type::Float,
             Val::Bool(_) => Type::Bool,
+            Val::Function(Function { type_, .. }) => Type::Function(type_.clone()),
         }
     }
 
-    pub fn into_type<'a, T>(&'a self) -> Result<&'a T>
+    pub fn as_type<'b, T>(&'b self) -> Result<&'b T>
     where
-        T: TypeOf + 'a + Clone,
-        &'a T: TryFrom<&'a Self>,
+        T: TypeOf + 'b + Clone,
+        &'b T: TryFrom<&'b Self>,
     {
         <&T>::try_from(self).map_err(|_| Error::InvalidType {
             actual: self.type_(),
             expected: <T as TypeOf>::type_of(),
         })
     }
+
+    pub fn as_function<'b>(&'b self, function_type: FunctionType) -> Result<&'b Function<'a>> {
+        match self {
+            Val::Function(f) if f.type_ == function_type => Ok(&f),
+            _ => Err(Error::InvalidType {
+                actual: self.type_(),
+                expected: Type::Function(function_type),
+            }),
+        }
+    }
 }
 
 #[derive(Debug, PartialEq, Clone, Deref)]
-pub struct Value(Rc<Val>);
+pub struct Value<'a>(Rc<Val<'a>>);
 
-impl Display for Value {
+impl<'a> Display for Value<'a> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         self.0.fmt(f)
     }
 }
 
-impl<T> From<T> for Value
+impl<'a, T> From<T> for Value<'a>
 where
-    Val: From<T>,
+    Val<'a>: From<T>,
 {
     fn from(x: T) -> Self {
         Self(Rc::new(x.into()))
     }
 }
 
-impl Neg for Value {
-    type Output = Result<Value>;
+impl<'a> Neg for Value<'a> {
+    type Output = Result<Value<'a>>;
 
     fn neg(self) -> Self::Output {
-        Ok((-self.into_type::<i64>()?).into())
+        Ok((-self.as_type::<i64>()?).into())
     }
 }
 
-impl Add for Value {
-    type Output = Result<Value>;
+impl<'a> Add for Value<'a> {
+    type Output = Result<Value<'a>>;
 
     fn add(self, rhs: Self) -> Self::Output {
-        Ok((self.into_type::<i64>()? + rhs.into_type::<i64>()?).into())
+        Ok((self.as_type::<i64>()? + rhs.as_type::<i64>()?).into())
     }
 }
 
-impl Sub for Value {
-    type Output = Result<Value>;
+impl<'a> Sub for Value<'a> {
+    type Output = Result<Value<'a>>;
 
     fn sub(self, rhs: Self) -> Self::Output {
-        Ok((self.into_type::<i64>()? - rhs.into_type::<i64>()?).into())
+        Ok((self.as_type::<i64>()? - rhs.as_type::<i64>()?).into())
     }
 }
 
-impl Mul for Value {
-    type Output = Result<Value>;
+impl<'a> Mul for Value<'a> {
+    type Output = Result<Value<'a>>;
 
     fn mul(self, rhs: Self) -> Self::Output {
-        Ok((self.into_type::<i64>()? * rhs.into_type::<i64>()?).into())
+        Ok((self.as_type::<i64>()? * rhs.as_type::<i64>()?).into())
     }
 }
 
-impl Div for Value {
-    type Output = Result<Value>;
+impl<'a> Div for Value<'a> {
+    type Output = Result<Value<'a>>;
 
     fn div(self, rhs: Self) -> Self::Output {
-        Ok((self.into_type::<f64>()? / rhs.into_type::<f64>()?).into())
+        Ok((self.as_type::<f64>()? / rhs.as_type::<f64>()?).into())
     }
 }
 
diff --git a/src/parser/mod.rs b/src/parser/mod.rs
index 811450da0d..be432b8adf 100644
--- a/src/parser/mod.rs
+++ b/src/parser/mod.rs
@@ -156,6 +156,10 @@ where
     }
 }
 
+fn is_reserved(s: &str) -> bool {
+    matches!(s, "if" | "then" | "else" | "let" | "in" | "fn")
+}
+
 fn ident<'a, E>(i: &'a str) -> nom::IResult<&'a str, Ident, E>
 where
     E: ParseError<&'a str>,
@@ -170,7 +174,12 @@ where
                 }
                 idx += 1;
             }
-            Ok((&i[idx..], Ident::from_str_unchecked(&i[..idx])))
+            let id = &i[..idx];
+            if is_reserved(id) {
+                Err(nom::Err::Error(E::from_error_kind(i, ErrorKind::Satisfy)))
+            } else {
+                Ok((&i[idx..], Ident::from_str_unchecked(id)))
+            }
         } else {
             Err(nom::Err::Error(E::from_error_kind(i, ErrorKind::Satisfy)))
         }
@@ -228,14 +237,65 @@ named!(if_(&str) -> Expr, do_parse! (
 
 named!(ident_expr(&str) -> Expr, map!(ident, Expr::Ident));
 
+named!(paren_expr(&str) -> Expr,
+       delimited!(complete!(tag!("(")), expr, complete!(tag!(")"))));
+
+named!(funcref(&str) -> Expr, alt!(
+    ident_expr |
+    paren_expr
+));
+
+named!(no_arg_call(&str) -> Expr, do_parse!(
+    fun: funcref
+        >> multispace0
+        >> complete!(tag!("()"))
+        >> (Expr::Call {
+            fun: Box::new(fun),
+            args: vec![],
+        })
+));
+
+named!(fun_expr(&str) -> Expr, do_parse!(
+    tag!("fn")
+        >> multispace1
+        >> args: separated_list0!(multispace1, ident)
+        >> multispace0
+        >> char!('=')
+        >> multispace0
+        >> body: expr
+        >> (Expr::Fun(Box::new(Fun {
+            args,
+            body
+        })))
+));
+
+named!(arg(&str) -> Expr, alt!(
+    ident_expr |
+    literal |
+    paren_expr
+));
+
+named!(call_with_args(&str) -> Expr, do_parse!(
+    fun: funcref
+        >> multispace1
+        >> args: separated_list1!(multispace1, arg)
+        >> (Expr::Call {
+            fun: Box::new(fun),
+            args
+        })
+));
+
 named!(simple_expr(&str) -> Expr, alt!(
     let_ |
     if_ |
+    fun_expr |
     literal |
     ident_expr
 ));
 
 named!(pub expr(&str) -> Expr, alt!(
+    no_arg_call |
+    call_with_args |
     map!(token_tree, |tt| {
         ExprParser.parse(&mut tt.into_iter()).unwrap()
     }) |
@@ -243,8 +303,8 @@ named!(pub expr(&str) -> Expr, alt!(
 
 //////
 
-named!(fun(&str) -> Fun, do_parse!(
-    tag!("fn")
+named!(fun_decl(&str) -> Decl, do_parse!(
+    complete!(tag!("fn"))
         >> multispace0
         >> name: ident
         >> multispace1
@@ -253,21 +313,24 @@ named!(fun(&str) -> Fun, do_parse!(
         >> char!('=')
         >> multispace0
         >> body: expr
-        >> (Fun {
+        >> (Decl::Fun {
             name,
-            args,
-            body
+            body: Fun {
+                args,
+                body
+            }
         })
 ));
 
 named!(pub decl(&str) -> Decl, alt!(
-    fun => { |f| Decl::Fun(f) }
+    fun_decl
 ));
 
-named!(pub toplevel(&str) -> Vec<Decl>, separated_list0!(multispace1, decl));
+named!(pub toplevel(&str) -> Vec<Decl>, many0!(decl));
 
 #[cfg(test)]
 mod tests {
+    use nom_trace::print_trace;
     use std::convert::{TryFrom, TryInto};
 
     use super::*;
@@ -281,7 +344,9 @@ mod tests {
 
     macro_rules! test_parse {
         ($parser: ident, $src: expr) => {{
-            let (rem, res) = $parser($src).unwrap();
+            let res = $parser($src);
+            print_trace!();
+            let (rem, res) = res.unwrap();
             assert!(
                 rem.is_empty(),
                 "non-empty remainder: \"{}\", parsed: {:?}",
@@ -435,11 +500,87 @@ mod tests {
         let res = test_parse!(decl, "fn id x = x");
         assert_eq!(
             res,
-            Decl::Fun(Fun {
+            Decl::Fun {
                 name: "id".try_into().unwrap(),
-                args: vec!["x".try_into().unwrap()],
-                body: *ident_expr("x"),
-            })
+                body: Fun {
+                    args: vec!["x".try_into().unwrap()],
+                    body: *ident_expr("x"),
+                }
+            }
         )
     }
+
+    #[test]
+    fn no_arg_call() {
+        let res = test_parse!(expr, "f()");
+        assert_eq!(
+            res,
+            Expr::Call {
+                fun: ident_expr("f"),
+                args: vec![]
+            }
+        );
+    }
+
+    #[test]
+    fn call_with_args() {
+        let res = test_parse!(expr, "f x 1");
+        assert_eq!(
+            res,
+            Expr::Call {
+                fun: ident_expr("f"),
+                args: vec![*ident_expr("x"), Expr::Literal(Literal::Int(1))]
+            }
+        )
+    }
+
+    #[test]
+    fn call_funcref() {
+        let res = test_parse!(expr, "(let x = 1 in x) 2");
+        assert_eq!(
+            res,
+            Expr::Call {
+                fun: Box::new(Expr::Let {
+                    bindings: vec![(
+                        Ident::try_from("x").unwrap(),
+                        Expr::Literal(Literal::Int(1))
+                    )],
+                    body: ident_expr("x")
+                }),
+                args: vec![Expr::Literal(Literal::Int(2))]
+            }
+        )
+    }
+
+    #[test]
+    fn anon_function() {
+        let res = test_parse!(expr, "let id = fn x = x in id 1");
+        assert_eq!(
+            res,
+            Expr::Let {
+                bindings: vec![(
+                    Ident::try_from("id").unwrap(),
+                    Expr::Fun(Box::new(Fun {
+                        args: vec![Ident::try_from("x").unwrap()],
+                        body: *ident_expr("x")
+                    }))
+                )],
+                body: Box::new(Expr::Call {
+                    fun: ident_expr("id"),
+                    args: vec![Expr::Literal(Literal::Int(1))],
+                })
+            }
+        );
+    }
+
+    #[test]
+    fn multiple_decls() {
+        let res = test_parse!(
+            toplevel,
+            "fn id x = x
+             fn plus x y = x + y
+             fn main = plus (id 2) 7"
+        );
+        assert_eq!(res.len(), 3);
+    }
 }