Derek Lam
Hi ghc-devs,
Hi Derek,
I’m making a first attempt to make a frontend plugin, to resolve cabal packages in the GHC API. However I’m running into troubles with module resolution in the GHC API, because I can’t control where it will search for modules at all. I've attached a minimal example, with a frontend plugin definition that can’t find modules (Plugin.hs), and an equivalent standalone program that does (Main.hs).
Specifically, I'm following a solution Edward Yang published in 2017 (http://blog.ezyang.com/2017/02/how-to-integrate-ghc-api-programs-with-cabal/), where the frontend plugin is called through a helper script that passes flags forwarded from `cabal repl`. To test the plugin directly with GHC, I collected the args through the helper script and filtered them to the minimal set that made the plugin run:
ghc --frontend Plugin -itarget -package-db dist-newstyle/packagedb/ghc-8.6.5 Plugin -plugin-package sandbox -hide-all-packages
This, as well as the full argument set, would complain that it can't find the target module under `./target/A.hs`:
<no location info>: error: module ‘A’ cannot be found locally
It does when the import path arg `-itarget` is absolute.
By "it does" do you mean "it still fails"?
Still, its `importPaths` are what I expect: [".", "target"], and the standalone program finds the target module with the same `importPaths`. I've tested this in GHC 8.6.5, 8.4.2 and 8.2.2, making me sure I'm just missing something, but I haven’t found help in the docs yet. I really appreciate some help to draw my hours over this to a close!
Hmm, very interesting. If I recall correctly, the relevant codepath in GHC is Finder.findImportedModule which should find the module via Finder.findHomeModule. Unfortunately, in my cursory look I didn't see any obvious issues; it looks like this might require a build of GHC and a bit of debugging. If you can produce a minimal, concrete reproducer (e.g. your plugin and a set of specific instructions to reproduce the issue) it's possible I can have a look. Cheers, - Ben