[swift-evolution] Splat

[Matthew Johnson]

> On Feb 11, 2016, at 8:00 AM, Alex Hoppen via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 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
>   }
> }

I really like the idea of a FuncionType protocol.  However, `apply` should be a requirement, not just in an extension.  This would allow other types to conform to the protocol.  

> 
> 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 <mailto: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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> 
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