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

[Thorsten Seitz]

+1

-Thorsten 

> Am 22.02.2016 um 23:52 schrieb Joe Groff via swift-evolution <swift-evolution at swift.org>:
> 
> 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?
> 
> -Joe
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20160223/577261a4/attachment.html>