Proposal: Make the project generalized

[Naddiseo]

Currently, this project is geared towards ES, but since the interface is so nice for building asts, I'd like to use it in other parsing projects.
Would it be possible to make the project more generalized so that it doesn't load all the types in def/* unless requested?

[benjamn]

@Naddiseo That's a great point. I've been thinking there would be a lot of value in allowing users of the library to instantiate their own copy of the type system, so that when (for example) Recast adds the custom File type (see https://github.com/benjamn/recast/blob/master/lib/types.js), that doesn't interfere with any other users of the ast-types library.

Something like this?

var types = require("ast-types").fork(/* pass true to start fresh? */);
types.registerDefs("ast-types/def/core.js");
types.registerDefs("./custom/defs");
types.registerDefs("./more/defs");

[Naddiseo]

That would work.
I was also thinking something like providing a language or something.

If the files under /def were moved to say /langs/ecma
Maybe something like:

var types = require('ast-types');
/*
languages.ECMA is defined as an array containing what is currently in `def`
*/
var langs = types.languages; 
types.registerDefs(langs.ECMA)

This would allow ast-types to possible build up defs of other language to aid other parser builders (or a separate repository for language defs).

Some other things that need to be thought about

• I think the builder expects there to be at least def("Node") / def("SourceLocation") / def("Position") already defined. So they might need to be predefined and loaded.
• It might be useful to expose /lib/shared

[Naddiseo]

Another thing that I've found using the builders is that I want to be able to pass in the current location. I think one of the arguments in the builder function is a SourceLocation it should be used as the current location.

[Naddiseo]

Actually, that fork() idea is really good. My use case is for transpiling, so I want to use an ast tree/builder for the source language, then a separate for the target language (javascript in my case).

I may have a look tomorrow to see if there's an easy way to accomplish this. Do you have any ideas where to start?

[benjamn]

I still think this is a great idea, by the way. Hope to work on it for the next minor version (0.8.0).

[jamestalmage]

bump!
0.9.0 maybe?

Personally, I think the core components should be split out into another library (ast-types-base for sake of argument).

I like the syntax, but I think you should pass actual modules (or a custom function) instead of strings. The problem with strings would be accurately resolving them (especially if you do split into a separate module):

var types = require('ast-types-base')(
  require('ast-types/def/core'),
  require('./myCustomDefs'),
  function(types) {
   types.def('MyType')
     //...
  }
);

I do like the fork option discussed, and that seems appropriate for ast-types, but not ast-types-base as I'm proposing. Perhaps strings would make sense there for specifying some of the built in stuff.

This would be the equivalent, less verbose version of above:

require('ast-types').fork(
  'core',
  require('./myCustomDefs'),
  function(types) {
   types.def('MyType')
     //...
  }
);

One problem with the fork method as currently being discussed would be with browserify builds. Browserify will pick up and package everything in def/*, even if you aren't using any of it and creating an entirely custom AST. Splitting into a separate module eliminates that problem for people implementing ground up AST definitions, and the top syntax above could still be used by those targeting browserify to ensure only what they want from def/* will be packaged in the bundle.