Attempt to build the conda-forge recipe for the latest git-annex version fails like this: https://circleci.com/gh/conda-forge/git-annex-feedstock/124?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link + stack --local-bin-path /home/conda/feedstock_root/build_artifacts/git-annex_1544129313702/h_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_p/bin --extra-include-dirs /home/conda/feedstock_root/build_artifacts/git-annex_1544129313702/h_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_p/include --extra-lib-dirs /home/conda/feedstock_root/build_artifacts/git-annex_1544129313702/h_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_p/lib --stack-root /home/conda/feedstock_root/build_artifacts/git-annex_1544129313702/h_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_p/share/git-annex-7.20181205-0/stackroot setup Downloading lts-12.19 build plan ... Downloaded lts-12.19 build plan. No setup information found for ghc-8.4.4 on your platform. This probably means a GHC bindist has not yet been added for OS key 'linux64-gmp4'. Supported versions: ghc-7.8.4, ghc-7.10.2, ghc-7.10.3, ghc-8.0.1, ghc-8.0.2, ghc-8.2.1, ghc-8.2.2, ghc-8.4.2
Is there a way to specify that ghc-8.4.2 is ok (if it is ok)?
The way that stack works is the "resolver" value in stack.yaml points to a particular version of ghc and libraries that work with it.
I don't think that stack allows overriding the version of ghc. What you can do is go to http://stackage.org/ and find a snapshot that includes the ghc version you have available, and edit stack.yaml's "resolver" to use that. You may also need to pass --system-ghc to stack to make it use your installed ghc rather than trying to download it.
Building with cabal, while more error prone, is another way to use whatever version of ghc you have installed already.