Sorry for mentioning this issue again, as it seems to have been already
much discussed, but i've had the unfortunate experience of dealing with the
consequences of this proposal in my code since xcode beta 6, which i really
can't get my head around.
Could someone explain what is the rational behind the choice of having
parameter names prohibited for closures but compulsory for functions ?
As a software developper (and not a language expert), I would have hoped
to get functions behave as close to closures as possible.
aka :
func myAdd(a : Int, b: Int) -> Int
myAdd(a: 1 , b :2 ) -- OK
vs
let myAdd = (_ a: Int, _ b: Int) -> Int
myAdd (a:1, b: 2) -- not ok still ?
This is a topic for swift-evolution; adding swift-evolution, and BCC’ing
swift-dev.
After having read the argument that "parameter names are part of the
function names, and not its type", i'm convinced that the whole proposal
makes sense. However i can't get my head around that specific line of the
proposal :
"If the invocation refers to a value, property, or variable of function
type, the argument labels do not need to be supplied. *It will be an
error to supply argument labels in this situation*."
Why make it an error in case of closures ?
A closure is an expression that creates an anonymous function, hence there
is no place to put the argument labels.
If we agree that parameter are part of the name, then it should behave
just like a name. Specifying names shouldn't matter more than the name of
the variable storing the closure. It seems to me, humbly, that the fact
that part of the name is split and written closer to the parameters could
be considered just as syntactic sugar.
We could invent a language extension there. The point of requiring the
underscores in:
let myAdd: (_ a: Int, _ b: Int) -> Int
Is to allow for some future evolution here. IIRC, it was discussed in the
review thread, that we could imagine ‘let’s with compound names, e.g.,
let myAdd(a:b:): (Int, Int) -> Int
Or perhaps allow syntactic sugar such as
let myAdd: (a: Int, b: Int) -> Int
To be the same thing. Again, this is future language extensions.
Another hint that something's wrong : the proposal still lets the
possibility to specify names in type declarations for documentation
purpose, using "_" . But then why not let us specify those names at call
site too ?
Because they are parameter names, not argument labels. If you declare a
function with parameter names but not argument labels:
func f(_ a: Int) { }
You *cannot* specify argument labels at the call site:
f(a: 1) // error: first argument is unlabeled
callback( nil, nil, nil, request) isn't really pleasant to read compared
to callback(data:nil, error:nil, info:nil, request: request)
This was a known issue with the Swift 3 change, and there are (known)
possible future language directions to bring back some of this. We focused
on fixing the type system oddities first in Swift 3 (that’s the breaking
part) and can consider improvements in the future.
