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Date: Tue, 30 May 2017 12:24:15 -0600
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To: =?UTF-8?Q?Bj=C3=B6rn_Larsson?= <bjorn.x.larsson@telia.com>
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Subject: Re: [PHP-DEV] [RFC]Discuss] Syntax for Arrow Functions
From: levim@php.net (Levi Morrison)

On Tue, May 30, 2017 at 12:16 PM, Bj=C3=B6rn Larsson
<bjorn.x.larsson@telia.com> wrote:
> Den 2017-05-30 kl. 19:58, skrev Levi Morrison:
>>
>> Internals,
>>
>> The previous discussion thread has died down significantly and so I'd
>> like to start a new one to refocus. This message has some redundant
>> information by design so people don't have to reference the other
>> thread so much.
>>
>> Based on the discussion there are a few different syntax choices
>> people liked. Overall it's a feature that people seem to want but
>> everyone seems to prefer a different syntax choice.
>>
>>    1. fn(params) =3D> expr
>>    2. function(params) =3D> expr
>>
>>    3. (params) =3D=3D> expr
>>    4. (params) =3D> expr
>>
>> Note that 3 and 4 require a more powerful grammar and parser and that
>> 4 has ambiguities. I think we can work around them by rules -- only
>> mentioning it because its popular because of JavaScript and do not
>> prefer this at all.
>>
>> Note that 1 requires a new keyword.
>>
>> Option 2 looks the best from that perspective but is by far the
>> longest; remember people are partially interested in this feature
>> because they want shorter closures which this doesn't really help.
>>
>> This is why everyone is so divisive. All options have drawbacks.
>> Additionally some people don't like binding by value and would prefer
>> ref, and others really would be against by-ref.
>>
>> Which brings me to an option I don't think was ever discussed on list:
>>
>>    5.
>>       [](params) =3D> expr     // binds no values
>>       [=3D](params) =3D> expr    // binds by value
>>       [&](params) =3D> expr    // binds by reference
>>
>> It has quite a few good qualities:
>>
>>    - No new keywords
>>    - Can choose between reference and value
>>    - Concise
>>    - Has precedence in C++, a major language
>>    - Can be done in our existing grammar and parser[1]
>>    - Can be extended to allow explicit binding of variables:
>>        // all equivalent
>>        // y is bound by value, array by reference
>>        [&, $y]($x) =3D> $array[] =3D $x + $y
>>        [=3D, &$array]($x) =3D> $array[] =3D $x + $y
>>
>> And of course it does have downsides:
>>
>>    - Symbol soup (it uses a lot of symbols)
>>    - A minor BC break. Empty arrays which are invoked as functions are
>> currently guaranteed to be errors at runtime and would have a new
>> valid meaning. Here's an example from inside an array literal:
>>
>>        // error at runtime previously
>>        [ []($x) =3D> $x ]
>>        // now an array with one item which is a closure that returns
>> its parameter
>>
>> Sara pointed out that we'd need to keep a leading `=3D` or `&` in the
>> array to disambiguate from our array closure form.
>>
>> Overall I'd prefer 1 or 5. What do you guys think?
>>
>>
>>    [1]: I'm pretty sure it can be done but until it's done I can't say
>> so confidently because sometimes there are things lurking in our
>> grammar I forget about.
>>
> As I said in the old thread, option 5 with =3D=3D> instead of =3D> might
> be an option. I think that would mitigate the minor BC break.
>
> r//Bj=C3=B6rn

The compatibility issue is with `[](params)` is that it is currently
an empty array literal that will be invoked; this is guaranteed to be
an error at runtime so it is unlikely to cause much trouble. A
trailing `=3D=3D>` would not help here.