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If nix-shell <args> fails, Stack just outputs the Nix error #6861
Description
With Nix integration, Stack executes nix-shell with arguments. If the command fails, Stack does not attempt to put that error in any context.
For example, with shell.nix:
{ghc}:
with (import <nixpkgs> {});
haskell.lib.buildStackProject {
inherit ghc;
name = "myEnv";
buildInputs = [ zlib glpk pcre ];
duff;
}The output of stack build --nix is just (coloured by Nix):
error: syntax error, unexpected ';', expecting '.' or '='
at /home/mpilgrem/Code/Haskell/testNic/shell.nix:8:7:
7| buildInputs = [ zlib glpk pcre ];
8| duff;
| ^
9| }
rather than something like:
Error: [S-1264]
While using nix-instantiate, with arguments:
* /home/mpilgrem/Code/Haskell/testNic/shell.nix
* --arg
* ghc
* with (import <nixpkgs> {}); haskell.compiler.ghc9103
* --argstr
* ghcVersion
* ghc9103
* --show-trace
Stack encountered the following error:
error: syntax error, unexpected ';', expecting '.' or '='
at /home/mpilgrem/Code/Haskell/testNic/shell.nix:8:7:
7| buildInputs = [ zlib glpk pcre ];
8| duff;
| ^
9| }
Unfortunately, there is no way to obtain coloured output from Nix commands:
Activity
Re #6861 Prettify existing Nix integration exceptions
Re #6861 Prettify existing Nix integration exceptions
Fix #6861 Add nix-shell command failure exception
Merge pull request #6862 from commercialhaskell/re6861
Fix #6861 Use nix-instantiate command, with failure exception
Fix #6861 Use nix-instantiate command, with failure exception
Fix #6861 Use nix-instantiate command, with failure exception
Fix #6861 Use nix-instantiate command, with failure exception
Fix #6861 Use nix-instantiate command, with failure exception
Fix #6861 Use nix-instantiate command, with failure exception
Fix #6861 Use nix-instantiate command, with failure exception
Fix #6861 Use nix-instantiate command, with failure exception
mpilgrem commentedon Mar 18, 2026
To address this, I separated an initial
nix-instantiatestep from the finalnix-shellstep, and introduced separate--nix-instantiate-options=OPTIONS, just in case.maxigit commentedon Mar 25, 2026
Not related to this issue, but I've seen on discourse that you asked if people where using nix integration : I do and this is one of the (many) reason I am using stack. So please, don't remove it.
Fix #6861 Use nix-instantiate command, with failure exception
Fix #6861 Use nix-instantiate command, with failure exception
Fix #6861 Use nix-instantiate command, with failure exception
mpilgrem commentedon Jul 7, 2026
@maxigit, as a user of Stack's Nix integration, you may be interested in the associated pull request:
EDIT: I wrote too soon. My testing has revealed a problem with locales, which I am looking into.
maxigit commentedon Jul 8, 2026
@mpilgrem Thanks for letting me know. However I moved away from Nix and do my prod build inside a docker container instead ...
Re #6861 Specify LANG=C.UTF-8 rather than en_US.UTF-8
Merge pull request #6947 from commercialhaskell/re6861
Fix #6861 Use nix-instantiate command, with failure exception
Merge pull request #6945 from commercialhaskell/fix6861