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

[Maximilian Hünenberger]

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>:
> 
> 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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