[swift-evolution] [Pitch] Flattening the function type of unapplied instance methods

[Dave Abrahams]

on Mon Feb 22 2016, Joe Groff <swift-evolution at swift.org> wrote:

> Today, you can reference an instance property as a member of its type
> to access it as a fully unbound function, which is currently curried:
>
> struct Foo {
>   var x: Int
>   func foo(y y: Int) { return x + y }
> }
>
> let foo = Foo.foo
> foo(Foo(x: 1))(y: 2) // returns 3
>
> However, this is problematic for `mutating` methods. Since the first
> argument is `inout`, the mutation window for the parameter formally
> ends before the second argument can be applied to complete the
> call. Currently we miscompile this, and form a closure over a dangling
> pointer, leading to undefined behavior:
>
> struct Bar {
>   var x = 0
>   mutating func bar() { x += 1 }
> }
>
> let bar = Bar.bar
> var a = Bar()
> bar(&a)() // This might appear to work, if we don't optimize too hard
>
> let closure: () -> ()
> do {
>   var b = Bar()
>   closure = bar(&b)
> }
> closure() // This scribbles dead stack space
>
> var c = Bar() {
>   didSet { print("c was set") }
> }
>
> bar(&c)() // This will scribble over c after didSet is called, if not worse
>
> We can close this hole by disallowing a reference to Bar.bar, like we
> already disallow partial applications. However, I think it would be in
> line with our other simplifications of the function type system to
> change the type of `Bar.bar` and other unapplied instance method
> references to no longer be curried. 
> In addition to providing a model
> for unapplied instance methods that works with mutating methods, this
> would also eliminate a type difference between free functions and
> methods of the same arity, allowing for easier code reuse. For
> instance, `reduce` takes a closure of type (State, Element) ->
> State. Flattening the formal type of instance methods would allow
> binary methods to be used as-is with `reduce`, like binary free
> functions can:
>
> func sumOfInts(ints: [Int]) -> Int { 
>   return ints.reduce(0, combine: +)
> }
> func unionOfSets<T>(sets: [Set<T>]) -> Set<T> { 
>   return ints.reduce([], combine: Set.union)
> }
>
> What do you all think?

+1; it's about time.


-- 
-Dave