[swift-evolution] ternary operator ?: suggestion

[Thorsten Seitz]

> Am 19.01.2016 um 06:28 schrieb Chris Lattner via swift-evolution <swift-evolution at swift.org>:
> 
> If you extend the same analogy to switch, then the most important cases are when the pattern being matched and the values being processed are lexically small, and have few cases.  We have a lot of syntactic sugar for processing optionals (e.g. if/let, the ?? operator, etc), but ?? for example doesn’t apply to general pattern matching.  With the expression above, for example, you could match on an enum analogously to the ?? operator like this:
> 
> result = someEnum ? case .SomeCase(let x): x, default: y
> 
> If you compare that to a switch statement, I can see how that could be compelling.  OTOH, the larger the expression (the more cases) and the more complex the patterns, the better a switch statement starts to look.

For me the killer feature is not being able to write it inline, it is making it clear that an assignment happens, i.e.

let result = someEnum ?
    case .SomeCase(let x): x
    default: y

is IMO much more readable (= allowing to instantly see what’s going on) than

let result
switch someEnum {
case .SomeCase(let x): result = x
default: result = y
}

The first one makes the variable which is assigned stand out because the rest is indented and the equals sign is prominently visible.
The latter version is just a large heap of tokens hiding some assignments :-)

For the same reason

let result = match(someEnum) {
    case .SomeCase(let x): x
    default: y
}

would be just as appealing to me. For using this as a switch-expression I’m strongly against introducing a special map method for basic types, though and rather propose to use a global function „match“ (more on that further below).

The inline version (if desired) is not too different from the ?-based switch-expression:

let result = match(someEnum) { case .SomeCase(let x): x; default: y }



The big advantage of the partial function proposal would be that it is much more general as it is usable everywhere a unary function argument is used, i.e. for map etc.
With the ?-based switch-expression I would have to write

let result: [Int] = collection.map { element in element ?
    case .SomeCase(let x): x
    default: y
}

That is not too bad but requires the boilerplate of having to introduce a name which is instantly consumed and not used anywhere else.
Admittedly there might be cases (no pun intended) where this might be useful, though, like

let result: [SomeEnum] = collection.map { element in element ?
    case .SomeCase(let x) where x > 5: element
    default: .OtherCase
}

Hmmm. Would this use case be common enough to make this version using a ?-based switch-expression be preferable over the partial function syntax?



As I already said above I’m strongly against introducing a map function for basic types. This brings confusion to the notion of a partial function and to what a map function is and is not needed IMO.

Just define the following global function:

func match<T,U>(x: T, @noescape mapping: T -> U) -> U {
    return mapping(x)
}

That’s all that is needed. Except for the case-expression shorthand, of course :-)



Without that proposed shorthand („partial function“) I currently have to write

let result = match(someEnum) { (arg: Foo) -> Int in
    switch arg {
    case .SomeCase(let x): return x
    default: return 42
    }
}

instead of the equivalent

let result = match(someEnum) {
    case .SomeCase(let x): return x
    default: return 42
}

as proposed (with the added change of using match instead of map).


-Thorsten


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