[swift-evolution] Proposal: pattern match based on associated value(s) only

[Andrew Bennett]

Hi Amir,

I put forward a proposal a short while ago that may allow you to achieve
similar to what you want, it is here:

*Declare variables in 'case' labels with multiple patterns*
https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160118/007431.html

https://github.com/apple/swift-evolution/pull/119

It would allow you to express your logic like this:

  switch vehicle {

  case .Car(let color), .Motorcycle(let color), .Helicopter(let color):

      switch color {

      case .Red:   ...

      case .Green: ...

      case .Blue:  ...

      }

  }

That proposal seemed to be well received with no votes against, and after a
few tweaks seemed to have support from the Swift team.

What do you think about that approach? It's not quite as concise as what
you proposed but it can be generalised more easily, and handles pattern
matching of cases where `color` may not be the only associated value.


On Wed, Feb 24, 2016 at 1:13 AM, Amir Michail via swift-evolution <
swift-evolution at swift.org> wrote:

>
> On Feb 23, 2016, at 7:27 AM, Taras Zakharko <taras.zakharko at uzh.ch> wrote:
>
> I think that other designs (e.g. using classes/structs/protocols) offer a
> much better solution for this particular problem
>
>
> That might be more work than just listing all the cases...
>
> On 23 Feb 2016, at 13:17, Amir Michail via swift-evolution <
> swift-evolution at swift.org> wrote:
>
>
> On Feb 22, 2016, at 10:11 PM, Brent Royal-Gordon <brent at architechies.com>
> wrote:
>
> For example:
>
> enum Color {
> ...
> }
>
> enum Vehicle {
> case Car(Color)
> case Motorcycle(Color)
> ...
> }
>
> switch vehicle {
> case _(.Red): … // pattern match based on associated value only
> }
>
>
> As usual, Amir:
>
> 1. Why do you want this feature?
>
>
> In this example, if there are many kinds of vehicles, it would be annoying
> to have to write a case for each one to match associated value .Red.
>
>
> 2. What's actually wrong with the status quo?
>
>
> What would that be for this example? Many cases?
>
> 3. Can you talk about a non-toy example you've encountered where it would
> have made your code better?
>
> 4. Can you talk about the proposed design in a little more detail? Are
> there weird edge cases, and how would you handle them? (In the case of this
> feature, here's one I see: Suppose you have two different associated value
> types but they both have a .Red case. What happens then?)
>
> A half-dozen-word subject line and a code example simply aren't enough to
> evaluate a proposal. Without knowing exactly what you're proposing and why
> you think it's a good idea, we can't really evaluate it properly.
>
> --
> Brent Royal-Gordon
> Architechies
>
>
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