[swift-evolution] SE-0111 and Curried argument labels: Unintended Consequences

[Erica Sadun]

SE-0111 established that Swift's type system would not allow function argument labels to be expressed as part of a function type. As I've been working with curried functions, I'm discovering an unintended consequence of this proposal in that it strips curried functions of their external labels and the resulting calls of their readability.

```
public func projected(
    function f: @escaping (CGFloat) -> CGFloat) ->
    (_ p0: CGPoint, _ p1: CGPoint) -> 
    (_ percent: CGFloat) -> CGPoint
{
```

Calling the first level of currying still reads acceptably:

```
let projectedFunction = projected(function: fToApply)
```

But after that, the enforced label-less arguments mean all further semantics have to stay within the name of the assigned partially applied function symbol and all arguments must be supplied without meaning, which is not in the spirit of API guidelines or under the umbrella of Swiftiness:

```
let fixedFunction = projectedFunction(p0, p1)
let value = fixedFunction(0.2)
```

There's no way to give either the line segment start and end points or the percent-of-progress arguments any labels.  Further, Xcode/QuickHelp does not provide any support for documenting the roles of each curried return type.

Could this be addressed specifically for currying or has the boat sailed forever on this topic?

-- E

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