Lazy, composable, and modular JavaScript | Hacker News Reader

Lazy, composable, and modular JavaScript

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75 pointschenglou6 comments

A problem I found with generators in Javascript is that you (apparently) can only yield from within the generator function itself, not from any function called by that generator. This affects the composability of functions. For example, say I want to model blocking i/o on top of generators. In that case, there is a low-level asynchronous mechanism (function) that yields whenever there is input. A parser function could call this mechanism. And another function could call the parser. This pattern seems logical, but isn't possible because suddenly the complete calling hierarchy has to be generator-aware.

I don't think this is strictly true: if your parser module also implemented the iterator/iterable protocols[0], then you could compose them that way. That may or may not be good enough for a particular use case.

In order to get my head around all this stuff, I made a trivial little toy library using generators for basic lazy sequences, and providing some basic stuff from the lodash/underscore library for them.

I found it was enough to make a kind of wrapper class that would hold a reference to the generator function, but lazily initialise it only when client code consumed the iterator, or explicitly forced evaluation. At that point, clients are aware only of the iterable interface, and not at all of the implementation details. (Or, they wouldn't be, if the particular implementation details didn't require you to use a polyfill on 90% of browsers and VMs ;-) )

[0] https://developer.mozilla.org/en/docs/Web/JavaScript/Referen...

Nice article but, am I the only one shocked by the lack of semicolons in the given examples?

Try it! If you learn about why semicolons were recommended for js, it's pretty easy to avoid writing code that would land you in trouble in the semicolon-free world. Most likely you wouldn't be getting in trouble in the first place since most of the examples had me thinking "why would I ever write code like this?". Besides, I think that recommendation maybe came from a time when the ASI was unreliable and linters were not common. My eslint is awesome at keeping me out of trouble.

For good reading, have a look at the linked articles at the bottom here http://eslint.org/docs/rules/semi.html

There are a few scenarios where you can get bitten by missing out semicolons.

However, this eslint rule should save you: http://eslint.org/docs/rules/no-unexpected-multiline

I stopped using semicolons a year ago, have not had a single error because of it.

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