Stack: Stack on nixos fails when ghc not available in nixpkgs, e.g. ghc802

[duog]

Please follow the steps below for reporting a bug:

Make sure that you are using the latest release (currently stack-1.9.3).
See the upgrade instructions to upgrade.

Please use the following schema for your bug report:

General summary/comments (optional)

LTS-9.21 requires ghc-8.0.2 which is not available in current nixpkgs.

Steps to reproduce

nix-shell -p stack
stack unpack transformers
cd transformers
cat >stack.yaml <<< "resolver: 9.32"
stack build

Expected

It would be nice if Stack would fall back to installing ghc-8.0.2 as it does on other platforms. If that's too hard, an error message saying something like "resolver 9.21 requires ghc-8.0.2. your does not have a derivation for this version, probably because it's too old. Sorry."

Actual

error: attribute 'ghc802' missing, at (string):1:43
(use '--show-trace' to show detailed location information)

Stack version

$ stack --version
1.9.3 x86_64 hpack-0.31.1

Method of installation

nix-shell -p stack

[dbaynard]

@nh2 Can you take a look at this? I'm not sure what ought to happen.

[ketzacoatl]

@dbaynard I think the simple first step would be to catch the error and provide a more useful explanation to the user. As @duog pointed out, a simple message like the following would be really helpful:

resolver 9.21 requires ghc-8.0.2. your does not have a derivation for this version, probably because it's too old. Sorry.

Do we know where the relevant code is?

[nh2]

Perhaps it should be some form of wrapper around getCompilerVersion from

https://github.com/commercialhaskell/stack/blob/217fa42a62f00cb42d4c40995fded5ec3a7cccc7/src/Stack/Nix.hs#L64

?

[mpickering]

IMO if stack insists on using this wrapper then it should pin the version of nixpkgs (for the GHC version) rather than using the system nixpkgs.

[maxkrieger]

Are there any workarounds/tips for this? It's difficult to configure IDE tools etc to use --no-nix, system-ghc, etc. I have the latest stack (1.9.3). I don't think the solutions here helped either: https://github.com/commercialhaskell/stack/issues/4196.

EDIT: a workaround that fixed it for me temporarily is adding the following lines to ~/.stack/config.yaml, but that doesn't work for builds that require nix packages like zlib:

system-ghc: true
nix:
  enable: false

[qrilka]

@maxkrieger I suppose you may install older GHC from @mpickering 's repo - https://discourse.nixos.org/t/new-nur-repo-for-ghc-binaries/2660 and then you should be ablo to use nix mode

[maxkrieger]

@qrilka I don't think it helped, here's my attempt:

~/.stack/config.yaml:

system-ghc: true
nix:
        enable: true
        pure: true
        packages: [llvm, zlib, ncurses, ghc]
        add-gc-roots: true

~$ nix-env -e ghc-8.0.2-binary
~$ nix-env -iA nixpkgs.nur.repos.mpickering.ghc.ghc802
~$ cd myProject/
myProject/$ stack build
error: attribute 'ghc802' missing, at (string):1:65
(use '--show-trace' to show detailed location information)

Alternatively, through nix-shell:

myProject/$ nix-shell -p nur.repos.mpickering.ghc.ghc802 stack zlib
> ghc --version
The Glorious Glasgow Haskell Compilation System, version 8.0.2
> stack build
error: attribute 'ghc802' missing, at (string):1:65
(use '--show-trace' to show detailed location information)

[qrilka]

@maxkrieger sorry for that incorrect assumption, I should have tried it first. The situation is a bit more complicated actually but it looks like there is a workaround: use a custom shell.nix file.
E.g. on my laptop I have NixOS 18.09 and it looks to contain the main latest GHC versions, so I tried not that widely used 8.4.1 and with the following shell.nix:

with (import <nixpkgs> {});
let ghc = nur.repos.mpickering.ghc.ghc841;
in haskell.lib.buildStackProject {
  inherit ghc;
  name = "myEnv";
  buildInputs = [ pcre ];
}

I get:

stack-1.9.3 --nix-shell-file=shell.nix exec -- ghc --version

and building also appears to work.
The main problem here is that you need to map properly GHC version in your stack.yaml and in shell.nix. Without a shell file that's done automatically but that method relies on GHC being available from nixpkgs.
BTW there is an alternative method - you could just add nur.repos.mpickering.ghc.ghc841 as nixpkgs.haskell.compiler.ghc841 in your configuration.nix. With that you won't need shell.nix

[maxkrieger]

@qrilka Thanks so much, your shell solution worked--couldn't quite deduce the syntax for "adding" the repo "as" the compiler package.

Also, what makes GHC unavailable from nixpkgs? I can install haskell.compiler.ghc802 just fine. Is it a naming mismatch?

[qrilka]

@maxkrieger the idea is to add missing attribute from Mathew's repo. Without knowing details of your setup I can't say for sure why it's missing. Example scenario is that you could use unstable channel or the recently released 19.0.3 which don't include this version, e.g. see https://github.com/NixOS/nixpkgs-channels/blob/nixos-unstable/pkgs/top-level/haskell-packages.nix
At the same time I don't quite understand how could you install haskell.compiler.ghc802 (as you're saying it above) if it's not supposed to be available