Are there any possible explicit uses of instances (values) of empty tuples (), i.e., of instances of typealias 'Void'?

问题

Question:

Are there any possible explicit uses for the empty tuple (), as a value (and not as a type) in Swift 2.x?

I know that these empty tuples can be used in the standard sense to define void functions. When I mistakenly defined a variable with a empty tuple value var a = () (of type ()), I started wondering if these empty tuple values can be used in some context. Does anyone know of such an application?

Example: possible application with array and optionals?

As an example, we can create an optional array of empty tuples that, naturally, can only hold either nil or ():

/* Optionals */
var foo: ()? = ()
print(foo.dynamicType) // Optional<()>

var arr : [()?] = [foo]
for i in 2...8 {
    if i%2 == 0 {
        arr.append(nil)
    }
    else {
        arr.append(foo)
    }
}
print(arr) // [Optional(()), nil, Optional(()), ... ]

With the small memory footprint of empty tuple, this could seem neat for micro-memory-management for a "boolean nil/not nil", but since type Bool have the same small footprint, I can't really see any direct use here, even in the (unlike) scenario that we really need to go bit-low optimization on our operations.

Perhaps I'm just chasing my own tail with some narrow unusable applications, but anyway: are there any possible explicit uses for these void () beings (as instances, not types)?

回答1:

There are lots of places that () can be useful when playing around with "CS" problems, which often have the form "implement X using Y even though you really already have X." So for instance, I might say, implement Set using Dictionary. Well, a Dictionary is a Key/Value pair. What should the type of the Value be? I've actually seen this done in languages that have Dictionaries but not Sets, and people often use 1 or true as the value. But that's not really what you mean. That opens up ambiguity. What if the value is false? Is it in the set or not? The right way to implement Set in terms of Dictionary is as [Key: ()], and then you wind up with lines of code like:

set[key] = ()

There are other, equivalent versions, like your Optional<()>. I could also implement integers as [()] or Set<()>. It's a bit silly, but I've done things like that to explore number theory before.

That said, these are all almost intentionally impractical solutions. How about a practical one? Those usually show up when in generic programming. For example, imagine a function with this kind of form:

func doThingAndReturn<T>(retval: T, f: () -> Void) -> T {
    f()
    return retval
}

This isn't as silly as it sounds. Something along these lines could easily show up in a Command pattern. But what if there's no retval; I don't care about the return? Well, that's fine, just pass a () value.

func doThing(f: () -> Void) {
    doThingAndReturn((), f: f)
}

Similarly, you might want a function like zipMap:

func zipMap<T, U>(funcs: [(T) -> U], vals: [T]) -> [U] {
    return zip(funcs, vals).map { $0($1) }
}

This applies a series of functions that take T to values of type T. We could use that even if T happens to (), but we'd have to generate a bunch of () values to make that work. For example:

func gen<T>(funcs: [() -> T]) -> [T] {
    return zipMap(funcs, vals: Array(count: funcs.count, repeatedValue: ()))
}

I wouldn't expect this to come up very often in Swift because Swift is mostly an imperative language and hides its Void in almost all cases. But you really do see things like this show up in functional languages like Scala when they bridge over into imperative programming.

回答2:

Suppose you have two functions overloading the same name:

func foo()
func foo() -> Int

The first doesn't return anything, the second returns some kind of value. Attempting to call either of these will in most cases get you a compiler error about ambiguity.

foo()
let a = foo()

You'd think that the compiler would know that the first call unambiguously refers to the first function, because it assumes no return value. But in actuality, the return type of a function with no declared return type is Void, or (). So the first call is actually more like this:

let _ = foo()

And absent a type annotation for the discarded lvalue, the compiler can't infer which foo to call. You can use explicit type annotations to disambiguate:

let b: Void = foo()
let c: Int = foo()

Okay, it's not a very great or common use case for Void, because it's a situation you'd tend to avoid getting into in the first place. But you asked for a use... and it is a use of () as a value and not just a type, because you can retrieve it from b after assignment (for all the good that does you).

Just beware, when you look deeply into the Void, the Void also looks into you. Or something like that.

来源：https://stackoverflow.com/questions/34561452/are-there-any-possible-explicit-uses-of-instances-values-of-empty-tuples-i

标签

swift

void