Ghcide allows to import private modules without complaining

[fendor]

Example:

cabal-version:       >=1.10
name:                mp-cabal
version:             0.1.0.0
build-type:          Simple

library
  exposed-modules:     MyLib
  other-modules:       OtherLib
  build-depends:       base
  hs-source-dirs:      src
  default-language:    Haskell2010

executable mp-cabal
  main-is:             Main.hs
  build-depends:       base, mp-cabal
  default-language:    Haskell2010

Main.hs

module Main where

import MyLib
import OtherLib

main :: IO ()
main = do
    print foo
    print bar

MyLib.hs

module MyLib (foo) where

foo :: Integer
foo = 3

OtherLib.hs

module OtherLib where

bar :: Integer
bar = 4

---

Opening Main.hs will result in an error. However, opening the library afterwards, will resolve the error.
If you open the library first and then the executable, no error will be shown at all, but if you try to compile it via cabal build, there will be errors such as:

Main.hs:4:1: error:
    Could not load module ‘OtherLib’
    it is a hidden module in the package ‘mp-cabal-0.1.0.0’
    Use -v (or `:set -v` in ghci) to see a list of the files searched for.
  |
4 | import OtherLib
  | ^^^^^^^^^^^^^^^

This makes perfect sense, if you consider how multiple components loading is implemented: Essentially we act like it is one big package. Ghci has no notion of private modules, afaict, and it had not reason to until now.

I expect to run into this problem as well when implementing support for multiple home units in GHC (https://gitlab.haskell.org/ghc/ghc/-/merge_requests/935/diffs#dbce6cb5e8f3d5287103c66d1a56ad63bbbd11a9) where we also don't have a notion of private modules when loading a unit.

What could be a possible solution to this problem? We could teach GHCi about private modules that can not be imported by other units. Would this break any abstractions?

[michaelpj]

Is this a problem with multi-component as it got impemented? @wz1000 ?

[fendor]

That should be fixed by multiple home unit support, as GHC understands now the concept of private modules and cabal gives us the module visibility.

I think it makes sense to close this issue, once we have a release for cabal and HLS.

[michaelpj]

This should be fixed with the new multi-component support.