[swift-evolution] Proposal: label-only constructors

[Dave Abrahams]

> On Dec 13, 2015, at 4:49 AM, Drew Crawford via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Oftentimes I want to have multiple parameterless constructors. In the immediate case, I want a Key for various different cryptographic algorithms:
> 
>    let a = Key(forRSA: true) //2048-bits
>    let b = Key(forCurve25519: true) //256-bits
>    let c = Key(forAES: true) //128-bits
> 
> There is no "parameter" to pass here; in particular the user should not be expected to know off the top of their head what the key size for Curve25519 is.
> 
> With the current limits of the language, I am generally forced to vend a parameter and associated label (I typically use type Bool) and pass a "dontcare" value in, e.g. `true` in the above.
> 
> I propose to eliminate this, so I can write
> 
>    Key {
>        init(forRSA) { /* */ }
>    }
>    let a = Key(forRSA)
> 
> This eliminates the parameter, and the associated mystery about what happens if you pass `false` to one of these constructors.
> 
> FAQ:
> 
> Q: Can't you have one constructor that takes an enum parameter?
> A: Enum parameters cannot have cases with distinct access modifiers, as constructors often are.  Also, combining unrelated code into one constructor makes me sad.

Doesn't this work?

struct Key {
  enum RSA_ { case RSA }
  init(_: RSA_) { ... }

  enum AES_ { case AES }
  init(_: AES_) { ... }

  // etc.
}

It's admittedly awkward at the declaration site but it still looks good at the use-site:

  Key(.RSA)
  Key(.AES)

etc.

> 
> Q: Can't you create subclasses RSAKey, Curve25519Key etc. with distinct constructors?
> A: Well first of all, Key is probably a `struct`, so no, you can't create subclasses of it.  Also, the only thing differentiating them is the constructor, not e.g. other overridden methods or variables, so subclassing feels an unnecessarily heavy abstraction.
> 
> Q: Can we extend this to support arbitrary labels e.g. `init(random length: 2000)` vs `init(zeroed length: 2000)`, `mkdir(path, creatingIntermediateDirectoriesIfRequired)` and many more?
> A: Yes, although these are probably more controversial proposals than simply supporting multiple parameterless constructors.
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-Dave