Getting envvars into Haskell builds

2019-11-27 21:26:29

When building on TravisCI we pull in the values of a few of Travis envvars (https://docs.travis-ci.com/user/environment-variables/#default-environment-variables). It seems they aren't pushed into the nix-build environment. Any pointers on how to do that would be most appreciated.

Travis CI Documentation

Environment Variables

Vaibhav Sagar

2019-11-27 23:38:50

By default Nix doesn't have dependencies on your existing environment without them being explicitly specified. If you want to do this, I would suggest adding a parameter to the .nix file and then passing them in when you build using --arg

Magnus Therning

2019-11-27 23:46:58

How can I explicitly specify that an envvar should be passed through?

I don't succeed in getting anything I pass in via --arg to actually end up as an envvar in the build environment.

Vaibhav Sagar

2019-11-27 23:53:55

Here's an example:

default.nix

{ environmentVariable ? null }: let
  pkgs = import <nixpkgs> {};
in pkgs.runCommand "example" {
  inherit environmentVariable;
}
''
  echo $environmentVariable
  touch $out
''

and you can use it as follows:

$ nix-build
these derivations will be built:
  /nix/store/mcwn62zwczay4a06mk12zrs77424zgrj-example.drv
building '/nix/store/mcwn62zwczay4a06mk12zrs77424zgrj-example.drv'...

/nix/store/pa9p2zzmdgp8bgxw6qz42kdbrxdm9vr1-example

$ nix-build --argstr environmentVariable "hello!"
these derivations will be built:
  /nix/store/2d5yzznb8wg721lblrcv5pv8wrxy2871-example.drv
building '/nix/store/2d5yzznb8wg721lblrcv5pv8wrxy2871-example.drv'...
hello!
/nix/store/11gn4sppncwzqp9dnrbk9snvisvy3xlf-example

Magnus Therning

2019-11-28 00:05:21

Yes, that's how I can get an envvar into a runCommand, which takes envvars as an argument. I need the envvar to end up in a haskellPackage.developPackage which doesn't take envvars as an argument :thinking:

Vaibhav Sagar

2019-11-28 00:14:42

What is using these environment variables?

Magnus Therning

2019-11-28 00:17:29

They are used during the build itself. Template Haskell picking them up and embedding them in the binary.

Vaibhav Sagar

2019-11-28 00:45:13

You can use preBuild to set those environment variables:

default.nix

{ environmentVariable ? "" }:
let
  pkgs = import <nixpkgs> {};
  modifier = drv: pkgs.haskell.lib.overrideCabal drv (old: {
    preBuild = ''
      export environmentVariable=${environmentVariable}
    '';
  });
in pkgs.haskellPackages.developPackage {
  inherit modifier;
  root = ./.;
}

Main.hs

{-# LANGUAGE TemplateHaskell #-}

module Main where

import Language.Haskell.TH
import System.Environment (lookupEnv)
import Data.Maybe (fromMaybe)

value :: String
value = $(runIO $ do
  var <- lookupEnv "environmentVariable"
  pure $ LitE $ StringL $ fromMaybe "" var)

main :: IO ()
main = putStrLn value

I also needed a .cabal file but I haven't included it here. This is how you would use it:

$ nix-build --argstr environmentVariable "hello!"
<...>
$ result/bin/environment-example
hello!

Magnus Therning

2019-11-28 00:54:14

I can only have one preBuild right?

Vaibhav Sagar

2019-11-28 01:05:00

No, you can have multiple ones as long as you prepend previous overrides by doing something like

preBuild = (old.preBuild or "") ++ ''
  <whatever>
''

Magnus Therning

2019-11-28 01:08:44

Apparently the following works too:

modifier = drv: drv.overrideAttrs (old: old // {
    envvar = 17;
  });

Cool!

Thanks for your help. You put me on track to that overrideAttrs solution, which I rather like (easier to compose).

Getting envvars into Haskell builds - Nix