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To: Levi Morrison <levim@php.net>
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Date: Tue, 30 May 2017 20:16:51 +0200
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Subject: Re: [PHP-DEV] [RFC]Discuss] Syntax for Arrow Functions
From: bjorn.x.larsson@telia.com (=?UTF-8?Q?Bj=c3=b6rn_Larsson?=)

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) => expr
>    2. function(params) => expr
>
>    3. (params) ==> expr
>    4. (params) => 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) => expr     // binds no values
>       [=](params) => expr    // binds by value
>       [&](params) => 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) => $array[] = $x + $y
>        [=, &$array]($x) => $array[] = $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) => $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 `=` 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 ==> instead of => might
be an option. I think that would mitigate the minor BC break.

r//Björn