[longlho]

this is already covered in basically any AST transform systems (babylon, acorn, TS...)

[zeckalpha]

This predates those, and is hygienic.

[andreypopp]

That's not just "AST transform system", Sweet.js provides a way to extend grammar with new productions. That allows to introduce new syntax constructs.

[adgasf]

I believe that Babel lacks custom infix operators whilst Sweet.js provides them.

[btown]

You're absolutely right; Babel's parser is very much hand-coded, and adding syntax requires deep knowledge of the internals. Not impossible, but not for the faint of heart. My dreams of having the safe navigation operator from Coffeescript (a?.b?.c) must yet remain unfulfilled...

https://github.com/babel/babylon/blob/master/src/parser/expr...

[chocolateboy]

> My dreams of having the safe navigation operator from Coffeescript (a?.b?.c) must yet remain unfulfilled...

Not for long: https://github.com/babel/babel/pull/5813

[btown]

You just made my day :)

[scribu]

That raises a question: What's stopping Babel from using Sweet.js instead of a custom parser?

Are there any transpilers that do use Sweet.js?

[zeckalpha]

Contracts.coffee IIRC, uses Sweet for Contracts.

[djfm]

Well "a.b.c" violates the principle that you should never access a property more than one level deep anyway ;)

But in this case it's pretty straightforward to write a function like "get('b', 'c')(a)" or use a lib like lodash [1]

[1]: https://lodash.com/docs/#get

[btown]

The Law of Demeter doesn't always apply neatly, especially when you're introspecting JSON sent over the wire and you don't have a "class" with object oriented behavior for each sub object (at best, they're strongly typed). And the runtime overhead AND verbosity of a function may not be ideal, with the string parsing required in lodash and the awkward syntax of the curried function. Something that compiles natively in a DRY way to a && a.b ... would be used widely imo.