Just +100. One more wise decision from the core team. Thank you for all of
your work.
On 14.07.2016 7:47, Chris Lattner via swift-evolution wrote:
Proposal:
https://github.com/apple/swift-evolution/blob/master/proposals/0111-remove-arg-label-type-significance.md
Shortly after SE-0111 was accepted last week, several people newly noticed
the proposal and started a discussion about how it appears to be a
regression for closure parameters (e.g. callbacks) that could formerly
carry labels, but are now not allowed to. These folks observed that it
would be more expressive (and consistent with the rest of Swift) to allow
parameter labels in function types, because the invocation site of a
closure “should" be required to provide those labels. The core team has
been following the discussion, agrees that this is a concern, and wants to
update the community with a path forward.
The reality of the situation is that the current implementation of
parameter labels in function types is inherently broken. Specifically, as
one example, there is an implicit conversion from "(a: Int) -> Int” to
“(Int) -> Int”. However, there is also an implicit conversion from "(Int)
-> Int” to “(b : Int) -> Int”. This means that the compiler currently
allows converting from “(a: Int) -> Int” to “(b: Int) -> Int”, which
doesn’t make sense, introduces surprising behavior, introduces complexity
into the compiler implementation, and is generally a problem. We do have
one specific hack to prevent conversion of (e.g.) “(a : Int, b : Int) ->
Void” to “(b : Int, a : Int) -> Void”, but this only triggers in specific
cases. There are other more complex cases as well, e.g. when using
generics "T<(a : Int)->Int>” cannot be considered compatible with "T<(b :
Int)->Int>”.
These problems are what initially motivated SE-0111. However, given the
feedback, the core team went back to the drawing board to determine
whether: a) SE-0111 by itself is the right long term answer, b) whether
there were alternate models that could solve the same problems in a
different way, or c) whether SE-0111 was the right first step to "ultimate
glory" in the field of closure parameter labels. After a long discussion,
and many alternatives considered, the core team believes in c), that
SE-0111 (with a minor modification) is the right step for Swift 3, because
it paves the way for the right model over the long term.
----8<----
The specific revision requested by the core team to SE-0111 is that all
“cosmetic” labels should be required to include an API name of _. For
example, this would not be allowed:
var op : (lhs : Int, rhs : Int) -> Int
instead, it should be spelled as:
var op : (_ lhs : Int, _ rhs : Int) -> Int
With this change, we believe that we have paved the way for a purely
additive proposal (and thus, post-Swift 3) that will restore the expressive
capability of closures with parameter labels.
----8<----
Here is a sketch of how that would work, in two steps:
First, we extend declaration names for variables, properties, and
parameters to allow *parameter names* as part of their declaration name.
For example:
var op(lhs:,rhs:) : (Int, Int) -> Int // variable or property.
x = op(lhs: 1, rhs: 2) // use of the variable or property.
// API name of parameter is “opToUse”, internal name is "op(lhs:,rhs:)”.
func foo(opToUse op(lhs:,rhs:) : (Int, Int) -> Int) {
x = op(lhs: 1, rhs: 2) // use of the parameter
}
foo(opToUse: +) // call of the function
This will restore the ability to express the idea of a closure parameter
that carries labels as part of its declaration, without requiring parameter
labels to be part of the type system (allowing, e.g. the operator + to be
passed into something that requires parameter labels).
Second, extend the rules for function types to allow parameter API labels
*if and only if* they are used as the type of a declaration that allows
parameter labels, and interpret them as a sugar form for providing those
labels on the underlying declaration. This means that the example above
could be spelled as:
var op : (lhs: Int, rhs: Int) -> Int // Nice declaration syntax
x = op(lhs: 1, rhs: 2) // Same as above
// API name of parameter is “opToUse”, internal name is "op(lhs:,rhs:)”.
func foo(opToUse op : (lhs: Int, rhs: Int) -> Int) {
x = op(lhs: 1, rhs: 2) // Same as above.
}
foo(opToUse: +) // Same as above.
These two steps will provide the simple and expressive design approach that
we have now, without all of the problems that representing parameter labels
in the type system introduces. The core team believes that the temporary
regression in expressiveness is an acceptable loss for Swift 3,
particularly given that this will have no impact on Cocoa or the standard
library. In the case of Cocoa, recall that C and Objective-C don’t have
parameter labels on their corresponding concepts (Blocks and C function
pointers), and the higher order functions in the standard library should
not require parameter labels either.
-Chris & the Core Team
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