Arrow functions

[antoineveldhoven]

Aiming to resolve #652

I'm not (yet) quite convinced if this is the way to go; would really like to get some discussion going on how to approach this and eventually get this fixed and merged.

Implementation of arrow functions in filters:

sort
filter
map
reduce

Object manipulation is not really supported; this implementation accepts a arrowFunction (Twig syntax) which is compiled and parsed as a new (runtime) template. The return values (e.g. output) of the arrowFunction are delegated to their respective JS functions.

As previously stated: this is merely a functional (read dirty) workaround; but for most of the use cases it will do the trick.

[lodewijkbovy]

Would be great to see this one merged too!

[willrowe]

I'm in the process of reviewing this, but I'm not sure if this approach is going to work. I think the body of the arrow function will need to be parsed as an expression. I'm trying to come up with a more general way for it to be compiled so a simple function can be called by the filters that support it.

[ericmorand]

I'm not sure this will work.

It is perfectly possible to register filters (and I assume functions and tests) that return promises:

import Twig from "twig";

import pkg from 'twig';
const {twig} = pkg;

Twig.extendFilter('asyncUpper', (value) => Promise.resolve(value.toUpperCase()));

const t = twig({
    data: '{{ "a"|asyncUpper}}'
});

t.renderAsync().then(console.log); // outputs "A"

How would the arrow function behave in this case?

{{ [4, 5]|sort((a, b) => a|asyncUpper < b ? -1 : 1)|join() }}

I think some tests need to be added to ensure that the arrow function support expression and async expressions specifically. The current tests suite provided by this PR is far from covering all the possible expressions that Twig accepts as the body of the arrow function. Moreso because Twig.js supports asynchronous expressions.

Note that this issue applies to every element of the language that accepts an arrow function as parameter: has every, has some, map from the top of my head.

[michaeldietiker]

Any updates on arrow function support?