I’m trying to build an app based on gitlib and want to write a little additional function with this type signature: MonadGit r m => m [RefName]. It’s the same as gitlib‘s own listReferences, so I’ll be using that as an example:

module Foo where

import Conduit
import Git.Types

listReferences :: MonadGit r m => m [RefName]
listReferences = runConduit $ sourceReferences .| sinkList

This compiles as expected using $ ghc Reference.hs within nix-shell -p 'ghc.withPackages (p: with p; [ gitlib-libgit2 ])'

However, in the same file I also need to use Git.Libgit2 which I do as a qualified import like this:

module Foo where

import Conduit
import Git.Types
import qualified Git.Libgit2               as LG

listReferences :: MonadGit r m => m [RefName]
listReferences = runConduit $ sourceReferences .| sinkList

This no longer compiles, throwing the following error:

$ ghc Reference.hs
[1 of 1] Compiling Foo              ( Reference.hs, Reference.o ) [Source file changed]

Reference.hs:7:19: error: [GHC-25897]
    • Couldn't match type ‘r’ with ‘LG.LgRepo’
        arising from a functional dependency between constraints:
          ‘MonadGit LG.LgRepo m’
            arising from a type ambiguity check for
            the type signature for ‘listReferences’ at Reference.hs:7:19-45
          ‘MonadGit r m’
            arising from the type signature for:
                           listReferences :: forall r (m :: * -> *).
                                             MonadGit r m =>
                                             m [RefName] at Reference.hs:7:19-45
      ‘r’ is a rigid type variable bound by
        the type signature for:
          listReferences :: forall r (m :: * -> *).
                            MonadGit r m =>
                            m [RefName]
        at Reference.hs:7:19-45
    • In the ambiguity check for ‘listReferences’
      To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
      In the type signature:
        listReferences :: MonadGit r m => m [RefName]
  |
7 | listReferences :: MonadGit r m => m [RefName]
  |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^

What is even going on here; why is it trying to match the type with some other type that I’m not even using anywhere? (Linter confirms the import is unused.)

How can I solve this issue?