Support for ECMAScript 6 and 7

[erikd]

This implements features for es 6 & 7.

• [x] const and let
• [x] arrow expressions
• [ ] import
• [ ] export
• [ ] default arguments
• [ ] object matching (destructuring, single name bind)
• [x] spread operator
• [ ] classes + extends
• [ ] template strings

Each of these should be implemented in a separate PR and each item needs tests. There are likely to be three tests needed, one for the appropriate XParser.hs test and one in each if Minify.hs and RoundTrip.hs.

[erikd]

const was already supported and let as added in PR #72.

[erikd]

Support for the spread operator was added in #73.

[erikd]

Arrow expressions were added in #74.

[Cmdv]

@erikd would it be ok to work on import and export individually? I spotted the work was done in the original big PR. Just don't want to do it if you are already working on it 😄

[erikd]

Absolutely fine.

Unfortunately, I have not done much more than merge patches for some time.

[hdgarrood]

I'd like to pick up where @Cmdv left off on import parsing.

[fare]

Also missing, it seems:

• [ ] trailing commas: {x,y,}
• [ ] unparenthesized single-argument arrow functions: x => 42
• [ ] export for function: export function f() {} (looks like export const f = () => undefined works)

[kl0tl]

Support for unparenthesized single-argument arrow functions was added in https://github.com/erikd/language-javascript/pull/90 and export for function declarations in https://github.com/erikd/language-javascript/pull/88.

[jeapostrophe]

const [a, b] = f(); is not supported. It should be easy to just change the VariableDeclaration case to allow ArrayLiteral

edit: I updated to the last checkout and I think it is now supported.

[rhendric]

I knocked off a bunch of the outstanding ES7 items in some work that's now released in 0.6.0.14:

• single name bind (#94)
• template literals (#95, #106)
• variable destructuring: const [a, b] = f() (#96)
• computed property names: {[a + b]: c} (#99)
• default arguments (#100)
• rest parameters (also #100)
• destructuring in parameters (also #100)
• generators and yield (#105)
• method syntax in object literals (#107)
• class, extends, static, and super (#108)

[ronyhe]

Hi all, I'd like to contribute some work here, but I'm unfamiliar with the ! syntax in the AST module: https://github.com/erikd/language-javascript/blob/38e48dd49ccea05675515e8323c0e0035ff97bea/src/Language/JavaScript/Parser/AST.hs#L55 Can someone please refer me to an article/explanation? Thanks in advance!

[erikd]

Its a strictness annotation. See eg: https://wiki.haskell.org/Performance/Strictness

[georgefst]

Any reason to stop at ES7? I'm personally missing ?? and ?.