> On Apr 15, 2016, at 2:47 PM, Joe Groff <jgr...@apple.com> wrote:
>> On Apr 15, 2016, at 11:43 AM, John McCall <rjmcc...@apple.com> wrote:
>>> On Apr 15, 2016, at 10:41 AM, Joe Groff <jgr...@apple.com> wrote:
>>>> On Apr 15, 2016, at 8:29 AM, John McCall via swift-evolution
>>>> <swift-evolution@swift.org> wrote:
>>>>
>>>>> On Apr 14, 2016, at 10:50 PM, Chris Lattner <clatt...@apple.com> wrote:
>>>>> On Apr 14, 2016, at 10:40 PM, John McCall <rjmcc...@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.
>>>>
>>>> Default arguments are not allowed in the type grammar. Nor are different
>>>> internal vs. external labels.
>>>>
>>>>> As a concrete example, surely you aren’t arguing that we should support:
>>>>>
>>>>> let x : Int… -> Int
>>>>>
>>>>> are you?
>>>>
>>>> No, but that's because the ... is a reference to the rest of the tuple and
>>>> doesn't read correctly outside of one.
>>>>
>>>>>>>> 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).
>>>>
>>>> When I said "function type", I was referring to this production in the
>>>> type grammar, not the type signature component of function declarations.
>>>> I'm not sure how I could've been clearer on that without actually using
>>>> the names of grammatical productions.
>>>>
>>>> My point was that allowing a function type to be written as "(Int) ->
>>>> Float" is already inconsistent with function declarations, because that is
>>>> not legal function declaration syntax; you would have to write "(_ : Int)
>>>> -> Float".
>>>>
>>>> The current language composes naturally here, and your proposal feels like
>>>> an odd extra rule.
>>>
>>> I feel like the current language no longer represents our reality, though
>>> (or at least, our current ideal vision for reality). We've pretty
>>> thoroughly broken the "functions have one argument" model.
>>
>> I don't see this syntax as an offshoot of the "functions always have one
>> argument" model. I agree that that model is dead.
>>
>> However, I don't think users require its death to be underlined and written
>> in bold; it only ever surfaced to them in bugs anyway. But many functions
>> do, nonetheless, have only one argument; and because of another change to
>> the model, where argument labels are becoming part of the function's name
>> and not its type, that argument can be written as just a type.
>>
>> So to me, this question is whether we add a weird special-case rule that
>> mandates the use of parentheses because they're required in a bunch of more
>> complex but less common situations.
>>
>>> Changing the type grammar to reflect this seems good to me. I would think
>>> of it as changing the function type grammar to:
>>>
>>> function-type ::= '(' (type (',' type)*)? ')' '->' type
>>>
>>> which, since the argument list can containing 0, 1, or many individual
>>> arguments, makes the parens more grammatically necessary.
>>
>> This is tautological.
>
> I don't think it is just a tautology. Without encoding it in the grammar,
> there's an ambiguity between tuples and multiple arguments; () -> T could
> mean either "takes a single () argument" or "takes no arguments". We could
> "obviously" disambiguate in favor of the latter interpretation, but then
> you're introducing special cases in the other direction to keep the U -> T
> syntax working.
This is a fair point, and one which would come up with (Int,Int) -> Float as
well — maybe the user really does mean to pass a single value that's an
(Int,Int) pair. I'm not sure this really works out to being a special case at
the user level, though, since in either model the user trying to write a
function that takes a single tuple-typed parameter simply has to introduce the
extra parens: (()) -> Float or ((Int, Int)) -> Float. (Assuming that the type
system preserves that structure in any way, of course.)
John.
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