[swift-evolution] [Accepted] SE-0111: Remove type system significance of function argument labels

[Diego Sánchez]

This is great news! Thank you!!

On Thursday, 14 July 2016, Chris Lattner via swift-evolution <
swift-evolution at swift.org> 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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