Re: Re: Arrow function expressions in PHP

On Wed, Sep 30, 2015 at 11:59 PM, Stephen Coakley <me@stephencoakley.com> wrote: > On 09/26/2015 11:17 AM, Levi Morrison wrote: >> >> (Email in gist format: >> https://gist.github.com/morrisonlevi/fa7984c04ff176b5a87c) >> >> In EcmaScript 2015 (ES6) the expression (x) => x * 2 >> means to create >> an anonymous function with one parameter x that will return >> x * 2. >> For example: >> >> (x) => x * 2 >> // is equivalent to: >> function(x) { return x * 2; } >> >> A modified example from [documentation by Mozilla Developer >> Network][1] page demonstrates how they are useful: >> >> var a = [ >> "Hydrogen", >> "Helium", >> "Lithium", >> "Beryl­lium" >> ]; >> >> var a2 = a.map(function(s){ return s.length }); // pre-ES6 >> >> var a3 = a.map((s) => s.length); // ES6 >> >> There has been some talk about how we can use arrow function >> expressions in PHP. In PHP using the same syntax would have some >> ambiguities: >> >> // Does this mean: >> // 1. Create an array key with the result of ($x) >> and a value >> with $x * 2 >> // 2. Create an array with one value that is an anonymous function >> [($x) => $x * 2] >> >> // Does this mean: >> // 1. Yield a key with the result of ($x) and a >> value with `$x * >> 2` >> // 2. Yield an anonymous function >> yield ($x) => $x * 2; >> >> This is why Bob Weinand [proposed][2] using ~> instead >> of =>. >> However, if we allow type declarations there is another issue. In the >> definition (Type &$x) => expr the >> (Type &$var) part can parse as >> "take constant Type and variable >> $var and do a bitwise and & >> operation." After that the => will be an unexpected >> token. Even >> though the rule would be invalid the parser doesn't know that far >> ahead it will error and it doesn't know which rule to pick. Changing >> the token from => to C >> Y��?�����=Y�~> doesn't affect this issue. >> >> We could solve the first ambiguities with prefering the current >> meaning with key => value and requiring the meaning with >> closures to >> wrap them in (). We could solve the latter ambiguity with a >> backtracking parser since it will eventually error and then know to >> pick the other rule. However, I really think this is a bad idea. >> >> So how can we have shorter closures without this mess? One simple way >> is to require the function prefix: >> >> // clearly an array with an anonymous function >> [function($x) => $x * 2]; >> >> // clearly yields an anonymous function >> yield function($x) => $x * 2; >> >> // clearly an anonymous function >> function(Type &$x) => expr; >> >> Requiring the function prefix mitigates one of the value >> parts of >> arrow functions: they are short. >> >> Another option would be to resolve the ambiguities with keys and >> values but to change the type information in parameters: >> >> (&$input: array) => expr >> >> By putting the type after the variable (similar to how we declare >> return types) we no longer have the issues with mis-parsing. Of >> course, that's not how we declare parameter types currently. I think >> we would need to permit it everywhere and deprecate the current syntax >> with the type being prefixed. (By deprecate I mean in PHP 8 and not >> remove it until PHP 9 or later) >> >> I would prefer that we shorten the function keyword to >> fn: >> >> [fn($x) => $x * 2] >> >> This preserves the shortness of the expression while providing >> unambiguous, simple parsing. Of course, now we have a similar issue: >> we have both fn and >> function. >> >> What concerns do you have about fn($x) => $x * 2 or >> `function($x) => >> $x * 2`? I will be writing a proper RFC later but I wanted to get >> discussion going now. >> >> [1]: >> >> https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions >> [2]: https://wiki.php.net/rfc/short_closures >> > > If my opinion is worth anything, I actually like how fn($x) => $x * 2 looks > the most. It's fairly short like the original proposal, but has the > advantage of *clearly* appearing to be a function. That was a large > complaint on the whole "short closures" idea in the first place, and PHP > usually does a good job at making code very obvious and clear. > > So yeah, an "fn" prefix (and requiring parenthesis always) looks very > consistent, but still is short. > >> I would prefer that we shorten the function keyword to >> fn: > > Do you mean generally, or just in short closures? Turning the keyword > everywhere would be a huge BC break (though pretty easy to fix in code: > "s/function\s/fn /g" :-) ). I'd be OK with allowing both everywhere for > consistency though: > > fn square(int $x) { > return $x * $x; > } > > $squaresPlusOne = array_map(function(int $x) => square($x) + 1, [1, 2, > 3, 4]); > > class Foo { > public fn __construct() {} > } > > You get the idea... > > I actually really like that + your idea. Kudos. I am definitely not proposing to remove function at this time. That would be a huge BC break, indeed! I meant only that fn can be used for brevity if that is preferred and it *could* be used in other places as well. This is my preference, but I know other people don't like that, which is why I would intend to keep it as a separate vote from the arrow part (=> expr).