Potentially conflicting ghc in stack.yaml and shell.nix

[alexvorobiev]

stack.yaml requests resolver: lts-6.12 which assumes ghc-7.10.3 but shell.nix has inherit ghc; which can resolve to any version of ghc potentially. Should it be set to haskell.packages.ghc7103.ghc instead?

[mboes]

I believe this is as intended: inherit ghc; means Stack gets to choose which GHC version to use when using the --nix flag (just as it does when not using the --nix flag). By default, it'll select the GHC version for LTS 6.12. But if you override the resolver say by passing --resolver nightly on the command-line, then Stack will tell Nix to provision some other version of GHC.

[alexvorobiev]

The main reason I asked was I was getting an error when I tried to run stack build --nix about missing ghc attribute so I had to edit shell.nix. After I commented the inherit ghc; line and added

ghc = haskell.packages.ghc7103.ghc;

the build worked. So it looks like the default behavior you described (which is what I expected as well) doesn't work. It looks like this is related to a bug (commercialhaskell/stack#2243) which is supposed to be fixed in Stack 1.2.0 so I am waiting for it to become available in Nix (pending this NixOS/nixpkgs#18942).

[mboes]

Ok yes, that's an upstream issue. Stack >= 1.2 highly recommended if you're going to use --nix.