[swift-evolution] Splat

Alex Hoppen alex at ateamer.de
Thu Feb 11 08:00:49 CST 2016

I really like your suggestion of functions conforming to a protocol. I thought about this a little while and how an extendable FunctionType (or Applicable as you called it) protocol may impact existing code. For that I would even go one step further and add another step of protocol indirection such that the function’s signature is just a specialised form of a protocol SignatureType (this allows even better syntax when extending FunctionType as you can see in my last example).

So this protocol would look like

protocol FunctionSignatureType {
  associatedtype Parameters
  associatedtype ReturnType

Parameters will be set to the functions parameters in tuple notation

(((Int, String), secondParameter: Int) -> String).Input == ((Int, String), secondParameter: Int)

FunctionType would then only have one associated type:

protocol FunctionType {
  associatedtype Signature: FunctionSignatureType

Signature could for example be ((Int, String), secondParameter: Int) -> String.

We could then declare the apply function as a simple extension to FunctionType just like you suggested

extension FunctionType {
  func apply(tuple: Signature.Parameters) -> Signature.ReturnType {
    // Add some compiler magic here

This would make apply another normal Swift function with a special implementation just like print, + and so on. I think that providing the ability to extend FunctionTypes would be a huge win, because several functions that used to be global could now just be methods on FunctionType. For example to execute a function asynchronously via GCD could now be declared as:

extension FunctionType where Signature == (() -> Void) {
  func dispatchAsync(queue: dispatch_queue_t) {
    dispatch_async(queue, self)

I don’t know how this fits into the compiler and if functions can be made to conform to a protocol anyway but from the outside this looks like a solution to me that fits very well in the current style of Swift.

- Alex

> On 11 Feb 2016, at 08:54, Brent Royal-Gordon via swift-evolution <swift-evolution at swift.org> wrote:
>> Still find it quite confusing, because I expected x.methodName to be a bound method and here it's a special syntactic form.  What happens if a protocol defines "func apply(to:)"?  Is that legal?  Would function types automatically conform to the protocol?
> For `apply(to:)`, it really would just be a method available on function types. You could put an `apply(to:)` method on any other type, and it wouldn't have any effect on things. You could declare a protocol with `apply(to:)`, but it wouldn't do anything to any function types. Conceptually, there would only be a few special things about them:
> 1. The compiler generates the `apply(to:)` methods automatically. We could, perhaps, have it generate a conformance to an `Applicable` protocol like this one, but that's probably overkill:
> 	protocol Applicable {
> 		typealias ReturnValue
> 		typealias ArgumentTuple
> 		func apply(to: ArgumentTuple) -> ReturnValue
> 	}
> (Actually, as I think about this, I wonder if `Applicable` could give us the `@splatting` property for free: take a generic parameter on Applicable and someone can specify a bare function, but you can't call it directly, only through its `apply(to:)` method.)
> 2. If `fn` is overloaded, `fn.apply(x)` will end up selecting an `fn` overload based on the type of `x`. Concrete example: `(+).apply(tupleOfInts)` would give you the `Int, Int` implementation of the `+` operator.
> 3. There's no way to add your own methods to a function type. (At least, I'm not proposing there would be. There's no particular reason we couldn't have other methods on functions, particularly if there's an `Applicable` protocol to extend.)
> But `apply` is not a keyword, `apply(to:)` does not receive any special parsing, and you can still splatter `apply`s all around your code with no consequences whatsoever. Honestly, that's the main virtue of the `apply(to:)` suggestion: that there's really very little to it.
> -- 
> Brent Royal-Gordon
> Architechies
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> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution

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