Handling of GHCs not present in Nixpkgs

[qrilka]

Trying out stack-to-nix + nix build on https://github.com/qrilka/xlsx/blob/master/stack-lts-12.yaml resulted in

error: attribute 'ghc843' missing, at /nix/store/15f66d3d57hd018sx781bvdm6cxl87q4-source/modules/hackage.nix:69:15

GHC 8.4.3 is no longer present in nixpkgs, I could use a later snapshot with GHC 8.4.4 in this case but probably this situation should be dealt somehow by nix-tools as well. E.g. printing some error about not available GHC could help (and some advice how to make it available)

[puffnfresh]

ghc865 is not in the pinned nixpkgs so I'm getting an error for it.

[angerman]

@puffnfresh ha, that's on my todo list for today, courtesy to @erikd.

[rvl]

Here's a PR to improve the error message at least: #194.

It would be good if we could support more ghc versions in Nixpkgs. I saw that someone made an overlay for this, but it used the binary releases.

[michaelpj]

I vaguely recall @angerman saying he had a plan to pull the GHC expressions into haskell.nix too, so we controlled them and could regularize them/support more versions.

[angerman]

@michaelpj right, it's just not as easy as I had hoped. As it's all quite intertwined (boot tools, compiler), ... maybe the better way is to just regularize the expressions in <nixpkgs> to support a much wider range of versions.

If someone has a better idea I'm all ears. This might all change with hadrian as well. So maybe we need to bite the bullet an pull all that bootstrapping logic int haskell.nix as well. Maybe even some proper ghc.nix is in order. I know @mpickering and @alpmestan (and @bgamari?) have done been working in that direction.

[bgamari]

I would recommend looking at @mpickering's https://github.com/mpickering/ghc-artefact-nix. This is what we use in our head.hackage CI infrastructure.

[mpickering]

Perhaps https://github.com/mpickering/old-ghc-nix is what you want. It packages binaries though, not building from source.

[angerman]

haskell.nix now ships with it's own set of ghc expressions. This makes me quite sad, but will have to do for the time being. I hope that both of @mpickering's projects could be added via an overlay though.

[bgamari]

You might also be interested to in my ghcs-nix repository, which packages GHC built from source all the way back to GHC 8.4.