Hi all,
I’ve been working on implementing SE-0111: Remove type system significance of
function argument labels
<https://github.com/apple/swift-evolution/blob/master/proposals/0111-remove-arg-label-type-significance.md>.
Actually implementing this feature completely requires major surgery to the
type system, ASTs, and so on. While
<https://github.com/apple/swift/commit/847b78245a473cc7160e45289311c85b6d6365b5>
I
<https://github.com/apple/swift/commit/604adff1bdb9c6f515accd284156af4fe8153cde>
am
<https://github.com/apple/swift/commit/5cce5c4d1d6121cc812d63f74441a612c33f5226>
making
<https://github.com/apple/swift/commit/b40c89d4c6777bb69c3f7efe0e8ab8748c476994>
progress
<https://github.com/apple/swift/commit/e4d8f486a81021a2f66d8796dd3acaff3e02e4db>
on this feature, it’s become clear that I cannot finish this in the Swift 3
time frame.
On the other hand, this is a source-breaking change that we’ve accepted for
Swift 3, and it’s one that is hard to implement for Swift 4 in a way that also
allows us to implement the Swift 3 model in the same compiler. Rock, meet hard
place.
So, I’m going to try a new implementation strategy, which (I think) will allow
us to get the behavior of SE-0111 but without refactoring the whole world.
Essentially, we need the argument labels to persist in the type system when
we’re performing a call to a function. In all other cases, the argument labels
should be stripped from the type. Note that we can determine *syntactically*
whether a particular declaration reference is a direct callee, which means we
can indicate when a particular reference should have argument labels (for call
matching) or nor (for unapplied references to a function). For example, given:
func f(a: Int, b: Int) -> Int { … }
let f1 = f(a: 1, b: 1) // treat ‘f’ as having type (a: Int, b: Int)
-> Int, because we need argument labels for type checking
let f2 = f // treat “f’ as having type (Int, Int) ->
Int, because argument labels aren’t part of the type of ‘f’ (conceptually)
let f3 = f(a:b:)(1, 2) // treat ‘f’ as having type (Int, Int) -> Int,
because references that include the argument labels don’t need further matching
of the argument labels
References to instances methods on a metatype are a bit more interesting,
because of the curried ’Self’, but can still be handled syntactically:
struct X {
func g(a: Int, b: Int) -> Int { … }
}
let x = X()
let g1 = X.g // treat “X.g” as having type (X) ->
(Int, Int) -> Int, because argument labels aren’t part of the type of ‘g’
(conceptually)
let g2 = X.g(x) // treat “X.g” as having type (X) ->
(Int, Int) -> Int, because argument labels aren’t part of the type of ‘g’
(conceptually)
let g3 = X.g(x)(a: 1, b: 2) // treat “X.g” has having type (X) -> (a:
Int, b: Int) -> Int, because the argument labels are needed to match up the
arguments at the call site
let g3 = X.g(a:b:)(x)(1, 2) // treat “X.g” has having type (X) ->
(Int, Int) -> Int, because references that include the argument labels don’t
need further matching of those argument labels
We can deal with this by, basically, computing the number of direct
applications to a given declaration and recording that in the AST. Then we use
that information when forming a reference to the given function declaration.
Along with this, we eliminate the ability to write a function type with
argument labels in the type system, so we get the “var f: (_ a: Int, _ b: Int)
-> Int” syntactic change as well.
- Doug
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