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

[Chris Lattner]

On Apr 14, 2016, at 10:40 PM, John McCall <rjmccall at apple.com> wrote:
>>> To me, the unparenthesized style suggests that the input and output are peers, which feels more natural for the sort of value-to-value transform/predicate where this most commonly occurs.  Parenthesizing the input feels fussier, which contributes to a sense that the argument is just one component to producing the result.
>>> The parentheses are grammatically unnecessary in most cases (by frequency of use in higher-use programming, not by feature count).  
>> 
>> I agree with your point that many simple higher order programming examples (e.g. map, filter, etc) take a single argument.  That said, I don’t agree that this means that we should syntactically privilege this special case.
> 
> "Special case" is a loaded phrase.  Why is it a special case as a parameter if it isn't a special case as a result?

Because, as I tried to explain in my original post, parameters *are* a special case.  The result type of a function is just a type.  The parameter list allows things that types do not: default arguments and variadics.

As a concrete example, surely you aren’t arguing that we should support:

	let x : Int… -> Int

are you?

>>> I guess the flip side is that call and declaration syntax both require parentheses (unless the only argument is a trailing closure), but again, we had strong justifications for that: declarations would always be ambiguous without parens, and calls would have serious problems (and the style-wars factor would be much larger, especially now with mandatory keyword arguments by default).
>> 
>> Right, but regardless of *why* we always require parens on Decls and ApplyExprs, we really do (and that isn’t going to change).  Being consistent between func decls and function types is quite important IMO.
> 
> So we should require function argument labels in function types?

Uhm, yes, we already do.  In:

	let x : (a : Int) -> Float
	let y : (Int) -> Float
	let z : Int -> Float

x and y have different (but compatible) types. y and z have identical types (sugared differently).

-Chris