Re: Typechecker plugins: request for review and another workflow question

On Nov 10, 2014, at 09:46, Adam Gundry wrote:

On 10/11/14 15:58, Austin Seipp wrote:

...

- 2) I am kind of not a fan of having separate 'plugins for core2core' and 'plugins for typechecking' flags, AKA -ftc-plugin and -fplugin. Ideally I would think all plugins could be uniformly specified by simply saying '-fplugin'. This mostly avoids the need for duplication and a naming convention/slew of flags for each case (which we have to catalog and document). There may be an easy way to make this the case; I haven't looked closely yet (it has been some time since I starred at the plugin code, even though Max wrote it and I helped get it merged!)

FWIW, I originally envisaged reusing the existing plugins machinery, and I don't think there are any great problems in doing so (see https://ghc.haskell.org/trac/ghc/wiki/Plugins/TypeChecker). In fact, I had an early implementation of typechecker plugins that did exactly this.

I have been wondering, however, about another possible approach. We could:

1. make the constraint solver changes use a *hook*, instead of the plugins directly, and

2. make it possible for plugins to install/modify hooks at some suitable point in the compilation pipeline.

I don't know the hooks machinery very well, but if this is feasible it seems like it would provide more power (plugins could modify any part of GHC for which a hook is available) and avoid having multiple overlapping ways of extending GHC. In the future, I can imagine wanting plugins to hook into other parts of GHC (e.g. error message postprocessing for domain-specific error reporting), and this seems like a good way to achieve that.

I think we could merge the TcPlugin type into the existing Plugin type without much difficulty, but I also agree that combining both plugin types with Hooks to present a more uniform interface would be nice. Eric