[swift-evolution] Pattern matching with Arrays

Robert Widmann devteam.codafi at gmail.com
Thu Dec 22 21:43:47 CST 2016

Do you think there’s room for a more general Pattern Synonyms-like <https://ghc.haskell.org/trac/ghc/wiki/PatternSynonyms> feature that could extend this to things that look tuple-y?  We had a short conversation on Twitter 'round about the release of Swift 1.2 about Swiftz’s HList <https://github.com/typelift/Swiftz/blob/master/Sources/HList.swift#L185> implementation and my desire to be able to destructure them into tuples for native pattern matching.

~Robert Widmann

> On Dec 22, 2016, at 9:46 AM, Joe Groff via swift-evolution <swift-evolution at swift.org> wrote:
>> On Dec 17, 2016, at 9:43 PM, Mathew Sanders via swift-evolution <swift-evolution at swift.org> wrote:
>> I've just joined the list (hi!) so not sure if this has been discussed in the past.
>> Curious to hear if a future version of Swift might ever include some sort of pattern-matching extended to Arrays. Maybe something like the following:
>> let numbers: [Int]
>> switch numbers {
>> case []:
>>    // to match an empty array
>> case [1, 2, 3]:
>>    // to match array with specific values
>> case [_, _, _]:
>>    // to match array with 3 Ints
>> case let [first, last]:
>>    // match a 2 element array and bind first and last elements
>> case let [first, next...]:
>>    // to match array with at least one element
>>    // bind first element, and bind other elements (could be empty)
>>    // first: Int, next: [Int]            
>> }
>> I love the pattern matching on tuples, and would love to see if extend to Arrays as well, but not sure if it fits with future goals for the language.
> Yeah, this is something I've wanted to add for a while, along with pattern matching for dictionaries, which together with array matching I think would make a lot of JSON/plist wrangling much more pleasant. Since it's an additive feature, it's taken a backseat to more pressing source- and ABI-breaking changes we've wanted to get out of the way first.
> -Joe
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