[swift-evolution] [pitch] Eliminate the "T1 -> T2" syntax, require "(T1) -> T2"

Radosław Pietruszewski radexpl at gmail.com
Tue Apr 19 02:46:20 CDT 2016

Noooooo :(

I understand and appreciate the rationale, uniformity between declaration and use site being a good thing, but IMHO the proposal just brings unnecessary noise, far outweighing the small benefit of having the symmetry.

1. What I’m worried the most is the “parentheses blindness”. In higher-order functions, or just when I take a simple callback closure, there are just a lot of parentheses (add to that generics, and there’s a lot of angled brackets too). And it just becomes hard to instantly decipher. To me, `func blah(f: Int -> Float) -> String` is easier to read that `func blah(f: (Int) -> Float) -> String`. Or just notice how noisy `(f: () -> ())` is. This is why I like the convention of using `Void` for void-returning functions. There’s less noise in `(f: () -> Void)`, and even better in `(f: Int -> Void)`. I don’t have to mentally match parentheses, because whenever possible, there’s just one set of parens around the main function declaration. When punctuation like parentheses is used sparingly, it carries a lot of weight. Requiring parentheses around T in T -> U doesn’t seem to have a significant reason aside from style/taste.

2. I’m not convinced at all that `(Foo) -> Bar` is immediately more obvious to people. I don’t have data to back it up, but my intuition is that `Foo -> Bar` is simple and understandable. “A function from Foo to Bar”, I’m thinking. I don’t have to mentally parse the vacuous parentheses, just to conclude that there’s, in fact, just one parameter. And when there is more than one parameter, the parentheses in `(Foo, Bar) -> Baz` instantly carry more weight.

3. Swift has been really good at removing unnecessary punctuation. Parentheses in if statements, semicolons, shortcut forms of closures, etc. This is a good thing. As I said before, using punctuation only when it matters makes it stand out, and in places where it doesn’t, by removing it we’re increasing the signal-to-noise ratio. To me, parentheses in `(Foo) -> Bar` don’t matter. I can see why one could argue for them, or prefer them, but it seems like a merely stylistic choice. Let’s keep them where it matters, and leave this to personal preference.

— Radek

> On 15 Apr 2016, at 06:57, Chris Lattner via swift-evolution <swift-evolution at swift.org> wrote:
> We currently accept function type syntax without parentheses, like:
>  Int -> Float
>  String -> ()
> etc.  The original rationale aligned with the fact that we wanted to treat all functions as taking a single parameter (which was often of tuple type) and producing a tuple value (which was sometimes a tuple, in the case of void and multiple return values).  However, we’ve long since moved on from that early design point: there are a number of things that you can only do in a parameter list now (varargs, default args, etc), implicit tuple splat has been removed, and  the compiler has long ago stopped modeling function parameters this way.  Beyond that, it eliminates one potential style war.
> Given all this, I think it makes sense to go for syntactic uniformity between parameter list and function types, and just require parenthesis on the argument list.  The types above can be trivially written as:
>  (Int) -> Float
>  (String) -> ()
> Thoughts?
> -Chris
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