Typechecker plugins: request for review and another workflow question

[Eric Seidel]

> On Nov 10, 2014, at 07:58, Austin Seipp <austin at well-typed.com> wrote:
> 
> Hi Iavor;
> 
> I took a very, very cursory glance. Naturally I am not a typechecker
> guru, but I did look over the mechanical changes/extensions to thread
> things around. Two things popped out to me:
> 
> - 1) Ugh, a new .hs-boot file. I assume this wasn't added without
> good reason, but ideally we would be eliminating them quicker than we
> add them. :) I want to take a closer look at this; perhaps we can
> refactor something for you to remove the need for this.
> 
> - 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!)

I looked into this again a bit today and recall now why we made a separate
Plugin type and flag; adding the TcPlugin to the existing Plugin type defined
in CoreMonad creates a circular dependency between CoreMonad and TcRnTypes.

We could (and perhaps should) move all of the plugin types into a separate
module, but that would require pulling other types (e.g. CoreToDo) out of
CoreMonad to break the circularity.