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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