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

[Karl Wagner]

> On 23 Apr 2017, at 15:55, 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
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I think the second (using where clauses to provide a more specific constraint) is probably more consistent with the rest of the language.

   switch self {
   case .Int:
     ...
   case .Fun(let outer_p, let r) where case .Fun(let inner_p,_) = outer_p, case .Int = inner_p: 
     ...
   case .Fun(let p, let r) where case .Fun(_,_) = p: 
     ...
   case .Fun(let p, let r):
     ...
   }

Looks pretty good IMO, although I do wonder if we couldn’t drop the second “case”.

- Karl

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