Cannot use modules from same package

[tfausak]

Consider a simple package like this:

-- example.cabal
cabal-version: 3.8
name: example
version: 0
library
  build-depends: base, imp
  ghc-options: -fplugin=Imp
  exposed-modules: A B

-- A.hs
module A where
mainA = System.IO.print 'a'

-- B.hs
module B where
mainB = A.mainA

This should work since B should just import A, but if you actually try to build it you'll get an error:

B.hs:1:1: error: [GHC-58427]
    attempting to use module ‘example-0:A’ (A.hs) which is not loaded
  |
1 | module B where
  | ^
Error: cabal: Failed to build example-0.

I'm guessing that this happens because GHC figures out the topographic sort of modules before actually compiling them. Since the import isn't actually in the file, GHC is free to compile them in either order. Or, more to the point, it doesn't have to make A available to B.

Relevant documentation for getImports: https://hackage.haskell.org/package/ghc-9.8.2/docs/GHC-Parser-Header.html#v:getImports

[tfausak]

I initially thought this could be related to "implicit" imports (#5), but I get the same error either way. This GHC issue is kind of related: https://gitlab.haskell.org/ghc/ghc/-/issues/21730

I could, as that issue suggests, replace the import with an Exact name. However that wouldn't fix this problem.

[tfausak]

I think there's nothing that Imp can do about this. The earliest a plugin can do anything is in driverPlugin, and by the time that's called the modules have already been sorted.

Either this can be considered a GHC bug, or it's just a limitation of what Imp can do. I'll open an upstream ticket to see what GHC devs have to say.

[tfausak]

I created an upstream issue for this: https://gitlab.haskell.org/ghc/ghc/-/issues/24579

In the meantime, it might suffice to say that Imp can't be used to import modules from the same unit (component).

[tfausak]

The title of this issue says "same package" but really it means "same component". You can work around this problem by referring to modules from another component, like a sub-library. For example:

-- example.cabal
cabal-version: 3.8
name: example
version: 0

common library
  build-depends: base, imp
  ghc-options: -fplugin=Imp

library a
  import: library
  hs-source-dirs: a
  exposed-modules: A

library b
  import: library
  hs-source-dirs: b
  build-depends: example:a
  exposed-modules: B

-- a/A.hs
module A where
mainA = System.IO.print 'a'

-- b/B.hs
module B where
mainB = A.mainA

[tfausak]

Upstream has confirmed that this is simply a limitation of GHC plugins. Imp cannot support this, so all I need to do is update the documentation to say that.