[swift-evolution] [Review] SE-0066: Standardize function type argument syntax to require parentheses

Thu Apr 28 11:23:36 CDT 2016

> Am 28.04.2016 um 00:38 schrieb David Owens II <david at owensd.io>:
> 
> You forgot that parentheses are required for labeled closures with type information:
> 
> x.sorted { (x: Int, y: Int) in x > y }
> 
> You'd have to handle that case as well.

That’s a single tuple argument.

> x.sorted { x: Int, y: Int in x > y }

And that’s an argument list with two arguments.

> I think the above leads to potentially ambiguous parsing constructs. 

Why? The `in` should be unambiguously ending the argument list without parsing issues, shouldn’t it?

-Thorsten

> 
> Sent from my iPhone
> 
> On Apr 27, 2016, at 1:18 PM, Thorsten Seitz via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> 
>> 
>>> Am 27.04.2016 um 14:16 schrieb Vladimir.S via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>>:
>>> 
>>> > But keep away from closure expressions, please! There is nothing ambiguous
>>> > there.
>>> 
>>> Really?
>> 
>> Ok, you got me there :-)
>> I have to clarify: no ambiguity if parentheses would be prohibited around parameter lists in closure expressions like I suggested. 
>> Furthermore the current implementation seems to do some auto-(un)splatting which should go away.
>> 
>>> 
>>> func z1(block: (Int,Int) -> Void) {
>>>    block(1,2)
>>> }
>>> 
>>> z1 { x, y in print(x,y)} //
>> 
>> (Int, Int) is a parameter list, so this is ok
>> 
>>> z1 { x in print(x.0, x.1)} // ???
>> 
>> This should not work IMO as the type is (Int, Int) -> Void where (Int, Int) is a parameter list and not a tuple.
>> Seems to be tuple unsplatting at work here.
>> 
>>> z1 { (x, y) in print(x, y)} //
>> 
>> This should not work for the given definition of z1 as (x, y) is a tuple.
>> 
>>> func z2(block: ((Int,Int)) -> Void) {
>>>    block((1,2))
>>> }
>>> 
>>> z2 { x, y in print(x,y)} // ???
>> 
>> This should not work IMO (tuple splatting at work here)
>> 
>>> z2 { x in print(x.0, x.1)}
>> 
>> Fine, as x is a tuple.
>> 
>>> z2 { (x, y) in print(x, y)} // ???
>> 
>> Fine, as (x, y) is a tuple. This raises another issue, though: this is using pattern matching without having to write `let` or `case let`. That’s probably a good thing and I’d rather like to get rid of `let` for bindings in pattern matching in other places.
>> 
>>> //z2 { ((x, y)) in print(x, y)}  // compilation error
>> 
>> This should not work IMO as parentheses should not be allowed around argument lists in closure expressions.
>> 
>>> 
>>> // this will compile, but
>>> runtime error
>>> let ft : (Int,Int) -> Void = { x in print(x)} // hm..
>>> ft(1, 2)
>> 
>> There is no runtime error in my playground.
>> The result printed is (1, 2)
>> 
>> This should not work IMO as the type is (Int, Int) -> Void where (Int, Int) is a parameter list and not a tuple.
>> You would have to write
>> let ft : (Int,Int) -> Void = { x, y in print(x, y) } // 1 2
>> or
>> let ft : ((Int,Int)) -> Void = { x in print(x) } // (1, 2)
>> 
>> 
>> To rehash:
>> 
>> Rules for function type definitions:
>> - parentheses are required around argument lists of more than one argument, i.e. (Int, Int) -> Void (same as in SE-0066)
>> - parentheses are required around argument lists with a single tuple argument, i.e. ((Int, Int)) -> Void (same as in SE-0066)
>> - parentheses are prohibited around single non-tuple arguments, i.e. Int -> Void (different from SE-0066)
>> 
>> Rule for argument lists in closure expressions:
>> - parentheses are prohibited around the argument list (as it is clearly enclosed by `{ … in`, therefore parentheses can only be used for tuples (different from current state)
>> 
>> This would result in nice unambiguous code without unnecessary parentheses.
>> 
>> -Thorsten
>> 
>> 
>>> 
>>> On 27.04.2016 11:53, Thorsten Seitz via swift-evolution wrote:
>>>> I am strictly against requiring parentheses in closure expressions.
>>>> Parentheses are visual clutter if not really needed and for a closure
>>>> expression there is no need for parentheses as the parameter list is
>>>> already nicely bracketed by `{ ... in`.
>>>> Actually I would argue that parentheses around parameter lists in closure
>>>> expressions should be prohibited for that reason.
>>>> 
>>>> I'm not fond of requiring parentheses around single non-tuple parameters in
>>>> type declarations either but I could probably grudgingly live with that change.
>>>> But keep away from closure expressions, please! There is nothing ambiguous
>>>> there.
>>>> 
>>>> -Thorsten
>>>> 
>>>> 
>>>> Am 27. April 2016 um 00:07 schrieb David Owens II via swift-evolution
>>>> <swift-evolution at swift.org <mailto:swift-evolution at swift.org>>:
>>>> 
>>>>> 
>>>>>> On Apr 26, 2016, at 1:31 PM, Chris Lattner <clattner at apple.com <mailto:clattner at apple.com>
>>>>>> <mailto:clattner at apple.com <mailto:clattner at apple.com>>> wrote:
>>>>>> 
>>>>>> 
>>>>>>> On Apr 25, 2016, at 11:28 PM, David Owens II via swift-evolution
>>>>>>> <swift-evolution at swift.org <mailto:swift-evolution at swift.org> <mailto:swift-evolution at swift.org <mailto:swift-evolution at swift.org>>> wrote:
>>>>>>> 
>>>>>>> *What is your evaluation of the proposal?
>>>>>>> *
>>>>>>> I reluctantly agree with the proposal with the following caveat: I do
>>>>>>> not agree with the rationale to support being able to choose to
>>>>>>> omit the () for the parameter list of the closure declaration.
>>>>>>> 
>>>>>>> I see no cohesive argument that says that the parens should be required
>>>>>>> in some cases but not in others when talking about parameter lists.
>>>>>>> 
>>>>>>> I believe the proposal should be amended that the following should be
>>>>>>> the only allowable forms:
>>>>>> 
>>>>>> Hi David,
>>>>>> 
>>>>>> To be clear, this proposal is not about changing closure expressions, it
>>>>>> was just a FAQ, and the section at the end is simply my personal
>>>>>> opinion.  Changing closure expression syntax would be a separate proposal.
>>>>> 
>>>>> My argument is changing the parameter list in one context but not the
>>>>> other is only solving one of the potentially ambiguous use cases instead
>>>>> of the general case. My opinion is they should be changed as the same
>>>>> time if they are going to be changed at all.
>>>>> 
>>>>> -David
>>>>> 
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>>>> 
>>>> 
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