[swift-evolution] pattern matching on variable-sized data type

[Jean-Denis Muys]

Hello,

I would like to suggest an additive evolution to Swift that might be in scope of phase 1 of Swift 4 (because it might have an impact on the ABI).

The idea is to extend the pattern matching abilities of Swift to enable a recursive programming style that’s very common in languages such as Lisp, ML, or Prolog on a collection that is processed as a list. By analogy to ML, Swift could do that on tuples, or on arrays, or on any similar, perhaps new, data type. This would allow the following for example:

func listOfDifferenceOfListElements (list: List<Int>) -> Int {
    
    switch list {
    case 〘〙: {
        return 〘〙
        }
    case 〘 let a 〙: {
        return 〘 a 〙
        }
    case 〘 let a, let b ⫸ let tail 〙: {
        return 〘 a-b 〙 ⋙ sumDifferenceOfListElements(tail)
        }
    }
}


Where I deliberately used unusual Unicode characters to denote syntax that would need to be invented:

- 〘〙 to denote the list-like data structure. It would be old style parenthesis if we wanted that to be Swift’s tuple, or the usual bracket if it was arrays
- ⫸ to pattern-match the tail of the list, i.e. the list composed of any and all elements following whatever has been pattern-matched so far
- ⋙ to denote a list append operator.

If we wanted the list data-type to be tuples, this would require the ability to build longer tuples from existing ones, i.e. build (a, b, c) from a and (b, c), or from (a) and (b, c) (appending tuples). Array seems more suitable, however.

So this post is to assess the interest in such a feature. Also note that while I have tried to have an occasional look at this mailing list in the past, due to its overwhelming volume, I may very well have missed a similar discussion in the past. In that case, I would appreciate a pointer.

As someone who developed Lisp and Prolog software professionally in a rather distant past, with an ever renewed sense of wonder, I would very much love to be able to use that programming style again when it makes sense.

If there is interest, I would be willing to write up an evolution proposal.

Jean-Denis Muys

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