[ghc-steering-committee] Plugin recompilation avoidance interface (#108)

[Ben Gamari]

Joachim Breitner <mail at joachim-breitner.de> writes:

> Hi,
>
Sorry for the late response; this was buried in my inbox.

> we are all in agreement that we want plugins the ability to avoid
> recompilation. The open question is: Should this be on a per module
> basis? Or a per module-and-pass bases?
>
> The former has a simpler and backward-compatible API, the latter sets
> the stage for some potential future where the compiler might typecheck
> a program, but then abort and not code generate.
>
> … seriously, I still don’t see how that future can be. Can someone
> please give a concrete example of circumstances where that might
> happen?
>
> Ben hints at sub-module granularity, but that slices it orthogonal to
> the current proposal. He also write
>
>> Lumping together all of the recompilation checks into a
>> single hash would expose a relatively ugly facet of GHC's implementation
>> to plugin authors and force authors to tie together logic from
>> conceptually-distinct compilation phases.
>
> but that is also not quite true. Note that the hash is not generated
> _during_ the passes, but needs to be calculated before. The type
> signature of the recalculation check, namely
>
>    pluginRecompile :: [CommandLineOption] -> IfG PluginRecompile
>
> clearly indicates that this is a per-module check (otherwise, you’d see
> an ModGuts input, or such).
>
>
> And even if, in the future, we have ability to provide a finer grained
> mechanism, we can add the necessary functions _then_ when we know how
> it looks.
>
Your argument is pretty compelling. I'll admit that part of my dislike
of the the simple approach that you advocate is purely aesthetic; it
just feels terribly ad-hoc. That being said, you do raise a valid point
that we really don't know enough to be able to say what a more precise
interface would look like.

Moreover, there is something to be said for the nice
backwards-compatibility story that the simple approach allows.

I would be fine accepting the simple approach. Either way, I think we
should move this proposal forward to unblock our contributor.

Cheers,

- Ben
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