Use Stack's GHC version when using --nix and a custom shell file

[mboes]

Stack's Nix support has two modes:

1. get Stack to auto-generate a shell.nix shell file based on stack.yaml data.
2. write the shell.nix file manually and point Stack to that.

In the former case, Stack generates a shell file that pulls in whatever version of GHC it sees fit (i.e. something matching the current snapshot). But in the latter case reproducibility can be compromised, because the GHC version used is whatever corresponds to the ghc top-level attribute in Nixpkgs. That version changes regularly. The user can always hardcode a specific version, but best to DRY and let Stack decide, as in (1).

The right way to do this is expect the shell.nix file to express a function. Then Stack can use

$ nix-shell --arg ghc <some-ghc-version-that-stack-chooses>

when entering the shell. This is completely backwards compatible with existing shell.nix files that are not function expressions. In that case the --arg ghc just gets ignored.

[YPares]

Given it's backward-compatible, I directly pushed on master a commit implementing this and updating the docs

[YPares]

Ok, I actually didn't take into account the current behaviour of haskell.lib.buildStackProject nix function. See NixOS/nixpkgs#15989 PR that should fix it.
So in the meantime I'm reopening the issue

[YPares]

Ok, this has been statued in NixOS/nixpkgs#15989, so I'm closing the issue.
I updated stack's doc accordingly. For now, until the NixOS PR is merged the ghc field will just be ignored if users make their shells using buildStackProject instead of the raw mkDerivation, so nothing breaks.