JS syntax future-proofing, Macros, the "Reader" (was: Performance concern with let/const)

Waldemar Horwat waldemar at google.com
Mon Sep 24 17:05:25 PDT 2012

On 09/18/2012 09:47 AM, Brendan Eich wrote:
> 2. Tim Disney with help from Paul Stansifer (Mozilla grad student interns) have figured out how to implement a Reader (Scheme sense) for JS, which does not fully parse JS but nevertheless correctly disambiguates /-as-division-operator from /-as-regexp-delimiter. See
> https://github.com/mozilla/sweet.js
> This Reader handles bracketed forms: () {} [] and /re/. Presumably it could handle quasis too. Since these bracketed forms can nest, the Reader is a PDA and so more powerful than the Lexer (a DFA or equivalent), but it is much simpler than a full JS parser -- and you need a Reader for macros.

That's not possible.  See, for example, the case I showed during the meeting:

boom = yield/area/

Is /area/ a regexp or two divisions and a variable?  You can't tell if you're using a purported universal parser based on ES5.1 and are unaware that yield is a contextual keyword which will be introduced into the language in ES6.  And yes, you can then get arbitrarily out of sync:

boom = yield/a+"3/ string?"  ...


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