Re: Arrow function expressions in PHP

On Sat, Sep 26, 2015 at 10:17 AM, Levi Morrison <levim@php.net> 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 > $var and do a bitwise and & > operation." After that the => will be an unexpected > token. Even > though the rule would be invalid the parser doesn't know that far > ahead it will error and it doesn't know which rule to pick. Changing > the token from => to ~> > doesn't affect this issue. > > We could solve the first ambiguities with prefering the current > meaning with key => value and requiring the meaning with > closures to > wrap them in (). We could solve the latter ambiguity with a > backtracking parser since it will eventually error and then know to > pick the other rule. However, I really think this is a bad idea. > > So how can we have shorter closures without this mess? One simple way > is to require the function prefix: > > // clearly an array with an anonymous function > [function($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 Bumping. I feel like perhaps people aren't aware that this is an important conversation to have *right now* before the vote on short_closures end. This is because depending on the interpretation of the rules it may not be possible to have another vote on the short closure idea for at least 6 months.