[swift-evolution] Haskell-like as-patterns in switch statements

Ionuț G. Stan ionut.g.stan at gmail.com
Mon Apr 24 06:40:51 CDT 2017

On 24/04/2017 10:42, Jonathan Hull wrote:
> You can also do:
> switch self {
>    case .Int: return "int"
>    case .Fun(.Fun(let p), let r): return "(\(p)) -> \(r)"
>    case .Fun(let p, let r): return "(\(p) -> \(r))"
> }

I didn't know you can do that and it seems consistent with other 
features of the language. However, it doesn't do what I was aiming for. 
In your example `p` will be bound to a `(Type, Type)`, not a `Type`. So 
the string result, instead of being `(int -> int) -> int` is actually 
`((int, int)) -> int`.

Anyway, I've learned something new, so thanks!

>> On Apr 23, 2017, at 6:55 AM, Ionuț G. Stan via swift-evolution <swift-evolution at swift.org> wrote:
>> Hi all,
>> I'm unsure if this is the appropriate venue to discuss this, so apologies if it's not.
>> I was wondering if the issue of supporting as-patterns in switch statements has been brought up before or not, and what was the outcome of that.
>> By as-patterns I mean the ability to bind sub-patterns to identifiers. See this question of mine on StackOverflow for an example: http://stackoverflow.com/questions/43566117/haskell-like-as-patterns-in-swift/43567041#43567041
>> Maybe an equal sign could be used to denote that, so my example above would become:
>>    switch self {
>>    case .Int: return "int"
>>    case .Fun(let p = .Fun, let r): return "(\(p)) -> \(r)"
>>    case .Fun(let p, let r): return "(\(p) -> \(r))"
>>    }
>> Note the `let p = .Fun` part.
>> Alternatively, enhancing the `where` clause of a `case` to support the same features as an `if case` expression, would be an acceptable choice, too?
>>    case .Fun(let p, let r) where case .Fun = p:
>>        return "(\(p)) -> \(r)"
>> Thanks for reading.
>> --
>> Ionuț G. Stan  |  http://igstan.ro  |  http://bucharestfp.ro
>> _______________________________________________
>> swift-evolution mailing list
>> swift-evolution at swift.org
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Ionuț G. Stan  |  http://igstan.ro  |  http://bucharestfp.ro

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