Matthew Pickering writes:

4 years ago Edsko proposed a patch to add support for frontend plugins to GHC. You can read his proposal here - https://ghc.haskell.org/trac/ghc/wiki/FrontendPluginsProposal#no1

Something like core to core plugins but for typechecked code would be useful for tool writers. Does anyone know why the patch was never merged?

This was before I started following closely, however, knowing how these things tend to go, I doubt that there was any strong objection to merging. It probably just got dropped. If we were to pick up this proposal again today it would certainly help if it provided some concrete motivation for the new plugin type.

Confusingly Edward Yang implemented another feature called frontend plugins which does something quite different (allowing users to write custom major modes rather than hook into the frontend). These features are quite different to each other.

To be fair, Edsko's proposal refers to the proposed plugin type as a "source plugin" not a "frontend plugin", despite the Wiki page name. I think "source plugin" is a better name anyways. Cheers, - Ben