[nix] "error: undefined variable `xxx' at (string):1:yyy"

[xor-xor]

I'm getting some strange behavior from stack build when trying to install some extra deps via nix section.

Steps to reproduce:

1. cd /tmp && stack new test-project
2. cd test-project && vim stack.yaml - change resolver to let's say lts-3.16 (with 3.19 and 3.20 there seems to be a problem with terminfo not picking up curses headers, which is a whole different story), and add nix section defined as:

nix:
  enable: true
  packages: [brick]

(it doesn't have to be this particular package, though - I've got the same error with different ones)
3. run stack build

Expected:
Minimal, working environment containing brick-0.3.1.

Actual:

error: undefined variable ‘brick’ at (string):1:216
(use ‘--show-trace’ to show detailed location information)

stack build --verbose gives:

Version 0.1.10.0 x86_64
2016-01-01 13:35:58.166018: [debug] Checking for project config at: /tmp/test-project/stack.yaml @(stack_BImf5flmxwxISqhhaJ4o3e:Stack.Config src/Stack/Config.hs:576:9)
2016-01-01 13:35:58.166313: [debug] Loading project config file stack.yaml @(stack_BImf5flmxwxISqhhaJ4o3e:Stack.Config src/Stack/Config.hs:599:13)
2016-01-01 13:35:58.168133: [debug] Using a nix-shell environment with nix packages: brick, haskell.packages.lts-3_16.ghc @(stack_BImf5flmxwxISqhhaJ4o3e:Stack.Nix src/Stack/Nix.hs:89:6)
2016-01-01 13:35:58.168257: [debug] Run process: nix-shell -E "with (import <nixpkgs> {}); runCommand \"myEnv\" { buildInputs=lib.optional stdenv.isLinux glibcLocales ++ [ brick haskell.packages.lts-3_16.ghc ]; STACK_IN_NIXSHELL =1 ; STACK_IN_NIX_EXTRA_ARGS='' --extra-lib-dirs=${brick}/lib --extra-include-dirs=${brick}/include  --extra-lib-dirs=${haskell.packages.lts-3_16.ghc}/lib --extra-include-dirs=${haskell.packages.lts-3_16.ghc}/include  '' ; } \"\"" --command "'/nix/store/ypidmrljwdmdnj0b7xkjm8flycpx5lfh-stack-0.1.10.0/bin/stack' '--internal-re-exec-version=0.1.10.0' 'build' '--verbose' $STACK_IN_NIX_EXTRA_ARGS" @(stack_BImf5flmxwxISqhhaJ4o3e:Stack.Exec src/Stack/Exec.hs:51:5)
error: undefined variable ‘brick’ at (string):1:216
(use ‘--show-trace’ to show detailed location information)

After changing nix section to:

nix:
  enable: true
  packages: []

...running stack build --verbose for the second time gives:

Version 0.1.10.0 x86_64
2016-01-01 13:44:52.981475: [debug] Checking for project config at: /tmp/test-project/stack.yaml @(stack_BImf5flmxwxISqhhaJ4o3e:Stack.Config src/Stack/Config.hs:576:9)
2016-01-01 13:44:52.981762: [debug] Loading project config file stack.yaml @(stack_BImf5flmxwxISqhhaJ4o3e:Stack.Config src/Stack/Config.hs:599:13)
2016-01-01 13:44:52.983593: [debug] Using a nix-shell environment with nix packages: haskell.packages.lts-3_16.ghc @(stack_BImf5flmxwxISqhhaJ4o3e:Stack.Nix src/Stack/Nix.hs:89:6)
2016-01-01 13:44:52.983713: [debug] Run process: nix-shell -E "with (import <nixpkgs> {}); runCommand \"myEnv\" { buildInputs=lib.optional stdenv.isLinux glibcLocales ++ [ haskell.packages.lts-3_16.ghc ]; STACK_IN_NIXSHELL =1 ; STACK_IN_NIX_EXTRA_ARGS='' --extra-lib-dirs=${haskell.packages.lts-3_16.ghc}/lib --extra-include-dirs=${haskell.packages.lts-3_16.ghc}/include  '' ; } \"\"" --command "'/nix/store/ypidmrljwdmdnj0b7xkjm8flycpx5lfh-stack-0.1.10.0/bin/stack' '--internal-re-exec-version=0.1.10.0' 'build' '--verbose' $STACK_IN_NIX_EXTRA_ARGS" @(stack_BImf5flmxwxISqhhaJ4o3e:Stack.Exec src/Stack/Exec.hs:51:5)
[rest of the output truncated for brevity]

...and the whole command finishes successfully (although w/o installing aforementioned dependency).

The difference between these two cases boils down to this fragment, which is not present in the latter:

--extra-lib-dirs=${brick}/lib --extra-include-dirs=${brick}/include

(and of course, [ brick haskell.packages.lts-3_16.ghc ] vs [ haskell.packages.lts-3_16.ghc ], but that's not where the error comes from).

I've tested that on my other machine, which runs OS X + Nix + stack 1.0.0 (instead of NixOS + stack 0.1.10.0, as reported here) and I get exactly the same error.

[mgsloan]

Pinging @YPares

[YPares]

@xor-xor Actually, it seems the package brick doesn't exist in Nix packages. (what you're seeing is a Nix error, not Stack)

Updating the nix channel (nix-channel --update) and trying to launch a shell with it (nix-shell -p brick) juste gives the same error.

Do you mean by any chance brickd?

[YPares]

@xor-xor Ok, I think I get it. You're referring to this package : http://hackage.haskell.org/package/brick right?
The packages: section under nix: is only for non-Haskell packages, the brick package should be handled by Stack as usual. If that's your sole dependency you don't even need nix support.

[xor-xor]

@YPares Yes, that's the one. And thanks for the clarification re: packages: section vs Haskell packages.

On the other hand, I can think of at least one use case where supporting Haskell packages via packages: section would be useful: circumventing some obscure build problems.

For instance, I've been trying to build a project which uses terminfo package, but Stack is unable to build this dependency with lts-3.20 and lts-3.19 resolvers - it cannot find curses headers (configure: error: curses headers could not be found, so this package cannot be built), despite providing paths to them via extra-include-dirs and extra-lib-dirs in stack.yaml (it may be just a bug in terminfo's configure scripts, I don't know). So I thought that instead of being blocked by this, I can just get terminfo's binary from Nix and continue with it.

What do you think..?

[mboes]

The nix support allows you to do just that. If you have a package depending on terminfo, then add

nix:
  enable: true
  packages: [ncurses]

to your project's stack.yaml. I just tested building a fresh project with terminfo-0.4.0.2 as a dependency that way. It works fine.

[xor-xor]

0.4.0.2..? Which resolver did you use to get that..? I had to use lts-3.16 (which gives terminfo-0.4.0.1), b/c anything higher than that produces an error like this one: error: attribute ‘lts-3_20’ missing, at (string):1:291 (I'm on nixos-unstable and I did nix-channel --update).

[mboes]

I added it as an extra-dep. I used that version for my test because terminfo-0.4.0.1 ships by default with GHC so there was no particular issue building that one - it's there out-of-the-box.

[xor-xor]

Allright... Everything is clear to me now. Thanks for the explanations! 😀