Placeholder types

[xAlien95 (Stefano De Carolis)]

@rintaro

• Initializing placeholder types

  func foo() -> Result { _() }
  

  Assuming [_](repeating: "foo", count: 12) is supported (as per "Generic parameter inference"), initializing _ alone probably should be supported too. But we have alternative spelling .init(), so...

While discussing about an ExpressibleByTupleLiteral protocol last year, it has been suggested to directly initialize with tuples instead of .init in environments in which the compiler can correctly infer the right initializer:

@Joe_Groff

[...] I think it'd be better to make tuple syntax a shorthand for initializing a contextual type (like .init(...) but without the .init ) than to introduce a protocol for making certain types able to use tuple syntax. [...]

• It would make expressing large, well-typed literals generally more streamlined, since you wouldn't need .init(...) or T(...) prefixes that can be deduced from context.

_() fits well from both an explicit and ergonomic standpoint in that regard, in my opinion.

---

@dabrahams

In the example you gave, Key is not a part of the structure of the type being declared, and putting it at the head of the declaration gives it too much weight.

Some off-topic considerations

It's true that Key isn't needed before the structure name, but if you look in the Generics Manifesto section cited before, it has also been proposed an alternative and more compact syntax which involves the direct placement of the auxiliary parameter types, i.e. Key, in the structure name:

Note that when one is extending nominal types, we could simplify the syntax somewhat to make the same-type constraint implicit in the syntax:

extension<T> Array<T?> {
  var someValues: [T] {
    var result = [T]()
    for opt in self {
      if let value = opt { result.append(value) }
    }
    return result
  }
}

If we allow parameterized extensions without a where clause, we cannot annotate auxiliary parameter types there. They should be put after extension and, generalizing, after struct (and let and var for variables? Take a look at the Generic constants subsection).