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

[Thorsten Seitz]

> Am 27.04.2016 um 14:16 schrieb Vladimir.S via swift-evolution <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>:
>> 
>>> 
>>>> On Apr 26, 2016, at 1:31 PM, Chris Lattner <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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