[swift-evolution] Proposal: Pattern Matching Partial Function (#111)

[Craig Cruden]

cases was added instead of using `cases` because of the potential problems foreseen where the parser would have trouble identifying a list of values.  

cases would be restricted to key: value, key: value, key: value and would be considered syntactic sugar that would expand out to 
    case key1: value1
    case key2: value2
    case key3: value3 
as far as the compilation was concerned.


> On 2016-02-05, at 1:31:09, Maximilian Hünenberger via swift-evolution <swift-evolution at swift.org> wrote:
> 
> I definitely see the point against dictionaries but I’m afraid of the actual pattern matching in "cases".
> 
> match(1) {
>     case 1, 3: 10
>     case 2, 4: 20
>     default: 30
> }
> 
> // with "cases"
> 
> match(1) { cases
>     1, 3: 10,
>     2, 4: 20,
>     default: 30
> }
> 
> // it is very hard to disambiguate between pattern and return value even though for the compiler it could be doable
> match(1) { cases
>     1, 3: 10, 2, 4: 20, default: 30
> }
> 
> 
> There should be a more visible distinction between two "case-item-map" like "|". But we have to consider "expression-patterns" where operators (like "|") are ambiguous.
> 
> - Maximilian
> 
>> Am 04.02.2016 um 18:48 schrieb Paul Ossenbruggen <possen at gmail.com <mailto:possen at gmail.com>>:
>> 
>> That is a good point and I forgot about that. There are frequent cases where what I am dealing with is not hashable and it is generally a lot of work to make it hashable, including adding a heading function which if done wrong, can lead to errors.  
>> 
>> Sent from my iPhone
>> 
>> On Feb 4, 2016, at 8:38 AM, Charles Constant via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>>> I still vote for keeping "cases"
>>> 
>>> I see it as a replacement for dictionary literal "pattern matching":
>>> [1 : "one", 2 : "two", 3 : "three"][1] ?? "undefined"
>>> 
>>> A dictionary needs keys that are Hashable. A dictionary produces an optional. We've discussed this, and more, earlier in the thread.
>>> 
>>> Even though it would be nice to have but I don’t think that I would use it frequently.
>>> 
>>> Granted, it's a bit ugly, but given the choice, I would pick "cases" over "case case case case ..." every time.
>>> 
>>> In addition, to be more consistent with "case", "cases" would introduce pattern matching which doesn’t seem right with this concise syntax.
>>> 
>>> I haven't thought this through. It just bums me out a little to replace the switch with something that still has imho unnecessary verbosity.
>>> 
>>> 
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> 
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