[swift-evolution] Pattern matching with Arrays

[Erica Sadun]

On Dec 22, 2016, at 8:43 PM, Robert Widmann via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 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 <mailto:swift-evolution at swift.org>> wrote:
>> 
>>> 
>>> On Dec 17, 2016, at 9:43 PM, Mathew Sanders via swift-evolution <swift-evolution at swift.org <mailto: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.

I'd be excited to see a feature like this for arrays, dictionaries, and array-like items such as strings. I know that this cannot be addressed for a while, but I'd love to see the idea formalized a bit and placed on a list somewhere so it doesn't just disappear into the aether. 

It's a pity we don't have a formal registry for "Some Day" items like this that get mentioned on-list, but don't properly fit into Jira or the SE pull-request queue.

-- E

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