[swift-evolution] Draft Proposal: Declare variables in 'case' labels with multiple patterns

[Jed Lewison]

Also +1. I've run into this a couple of times in the past week and the proposal would allow shorter and easier to read code.

Sent from my iPhone

> On Jan 23, 2016, at 10:09 AM, Joe Groff via swift-evolution <swift-evolution at swift.org> wrote:
> 
> This would be great. Other pattern matching languages with ML heritage can do this, the only reason Swift couldn't was time.
> 
> -Joe
> 
>> On Jan 22, 2016, at 7:39 PM, Andrew Bennett via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> Hi,
>> 
>> I'd like to discuss declaring variables in case labels with multiple patterns. I've written a draft proposal, but I'd like to discuss it first before formally proposing anything.
>> 
>> https://github.com/therealbnut/swift-evolution/blob/a137202e41588b71d3c0511cff85f82ec5f65629/proposals/0023-declare-variables-in-case-labels-with-multiple-patterns.md
>> 
>> In short:
>> 
>> switch value {
>> case let .Case1(x, 2), .Case2(2, x):
>>     print(x)
>> }
>> 
>> The original proposal is here, it may need to adapt after discussion, so I'll try to keep the proposal at that link up-to-date.
>> 
>> Thanks,
>> Andrew
>> 
>> Declare variables in 'case' labels with multiple patterns
>> Proposal: SE-0022
>> Author(s): Andrew Bennett
>> Status: In Discussion
>> Review manager: Not In Review
>> Introduction
>> 
>> In Swift 2, it is possible to match multiple patterns in cases. However cases cannot contain multiple patterns if the case declares variables.
>> 
>> The following code currently produces an error:
>> 
>> enum MyEnum {
>>     case Case1(Int,Float)
>>     case Case2(Float,Int)
>> }
>> switch value {
>> case let .Case1(x, 2), .Case2(2, x):
>>     print(x)
>> case .Case1, .Case2:
>>     break
>> }
>> The error is:
>> 
>> `case` labels with multiple patterns cannot declare variables. 
>> This proposal aims to remove this error when each pattern declares the same variables with the same types.
>> 
>> Motivation
>> 
>> This change reduces repeditive code, and therefore reduces mistakes. It's consistent with multi-pattern matching when variables aren't defined.
>> 
>> Proposed solution
>> 
>> Allow case labels with multiple patterns to declare patterns by matching variable names in each pattern.
>> 
>> Using the following enum:
>> 
>> enum MyEnum {
>>     case Case1(Int,Float)
>>     case Case2(Float,Int)
>> }
>> These cases should be possible:
>> 
>> case let .Case1(x, _), .Case2(_, x):
>> case let .Case1(y, x), .Case2(x, y):
>> case let .Case1(x), .Case2(x):
>> case .Case1(let x, _), .Case2(_, let x):
>> Detailed design
>> 
>> Allow case labels with multiple patterns if the case labels match the following constraints:
>> 
>> All patterns declare exactly the same variables.
>> The same variable has the same type in each pattern.
>> Therefore each pattern is able to produce the same variables for the case label.
>> 
>> Impact on existing code
>> 
>> This should have no impact on existing code, although it should offer many opportunities for existing code to be simplified.
>> 
>> Alternatives considered
>> 
>> Using a closure or inline function
>> 
>> Code repitition can be reduced with one pattern per 'case' and handling the result with an inline function.
>> 
>> func handleCases(value: MyEnum, apply: Int -> Int) -> Int {
>>     func handleX(x: Int) -> Int {
>>         return apply(x) + 1
>>     }
>>     let out: Int
>>     switch value {
>>     case .Case1(let x, 2):
>>         out = handleX(x)
>>     case .Case2(2, let x):
>>         out = handleX(x)
>>     case .Case1, .Case2:
>>         out = -1
>>     }
>>     return out
>> }
>> This syntax is much more verbose, makes control flow more confusing, and has the limitations of the what the inline function may capture.
>> 
>> In the above example apply cannot be @noescape because handleX captures it.
>> 
>> Also in the above example if out is captured and assigned by handleX then it must be var, not let. This can produce shorter syntax, but is not as safe; out may accidentally be assigned more than once, additionally out also needs to initialized (which may not be possible or desirable).
>> 
>> Extending the fallthrough syntax
>> 
>> A similar reduction in code repetition can be achieved if fallthrough allowed variables to be mapped onto the next case, for example:
>> 
>> switch test {
>>     case .Case1(let x, 2): 
>>         fallthrough .Case2(_, x)
>>     case .Case2(3, .let x):
>>         print("x: \(x)")
>> }
>> This is not as intuitive, is a hack, and fallthrough should probably be discouraged. It is much more flexible, a programmer could adjust the value of x before fallthrough. Flexibility increases the chances of programmer error, perhaps not as much as code-repitition though.
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> 
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