RFC: Functions as first-class types

[wegendt-bosch]

Introduction

In many languages, functions can be passed to functions. In some, this is even easy. Eclipse Mita should be one of those :)

Current Situation

No support OOTB, only through generated types, but not implemented.

Proposal

Add function references to the language. Closures and currying could follow, but let's not go overboard.

Code Example

fn id(x: int32): int32 {
  return x;
}
fn add(x: int32, y: int32): int32 {
  return x + y;
}

fn foo(x: int32, f: int32 -> int32) {
  print(f(x));
}

fn fooUnicode(x: int32, f: int32 → int32) {
  print(f(x));
}

// Parameters could be named?
fn binaryFoo(x: int32, y: int32, f: (a: int32, b: int32) -> int32) {
  print(f(b = x, a = y));
}

Implementation

Translation to C is straight forward:

f: (int32, int32) -> int32 = add;

z = f(x, y);

corresponds to

Retcode_T (*f)(int *result, int x, int y) = &add;

(*f)(&z, x, y);

[wegendt-bosch]

While we are on it, we could also unify syntax:

instead of writing

fn id(x: int32): int32 {
  return x;
}

we could write

let id: (x: int32) -> int32 = {
  return x;
}

[wegendt-bosch]

Currently, both of these references of foo compile to the same code:

fn foo(): int32 {
  return 1;
}
fn bar() {
  let x1 = foo();
  let x2 = foo;
}

If we were a referentially transparent language, this wouldn't interfere: passing a function () -> int32 or a value int32 would be the same (and indeed both would have isomorph types), since all such functions would have to be constant.

However, since our functions have side effects not represented by a type system, calling foo or passing it as a function reference is different, as are their types.

If we always interpret function names as calls without arguments, we can't formulate references to those functions. Options are:

1. Disallow "Supplier"-functions as arguments
2. Make every function call explicit
3. Introduce special syntax for function references
4. Detect whether a referenced function should be a call and when it should be a reference, choose one of the above options for undetectable cases