How to add environment variables to a haskell flake

[newton-migosi]

I have a haskell project that is failing nix build because the app expects an environment variable to be set before execution. How are environment variables added to haskell flakes? cabal build and cabal test in a nix shell after setting the env vars.

For more context, I first asked this question on the nix os discourse

[srid]

Build-time environment variable? You want to override it in overrides just like any dependency, https://haskell.flake.page/dependency

Apply this on https://github.com/srid/haskell-template and do nix run or nix build:

diff --git a/flake.nix b/flake.nix
index 15f87d2..ea02116 100644
--- a/flake.nix
+++ b/flake.nix
@@ -26,12 +26,17 @@
         # See https://github.com/srid/haskell-flake/blob/master/example/flake.nix
         haskellProjects.default = {
           # packages.haskell-template.root = ./.;  # Auto-discovered by haskell-flake
-          overrides = self: super: { };
+          overrides = self: super: {
+            haskell-template = super.haskell-template.overrideAttrs (_: _: {
+              FOO = "bar";
+            });
+          };
           devShell = {
             tools = hp: {
               treefmt = config.treefmt.build.wrapper;
             } // config.treefmt.build.programs;
             hlsCheck.enable = false;
+            mkShellArgs.shellHook = "export FOO=bar";
           };
           autoWire = [ "packages" "apps" "checks" ]; # Wire all but the devShell
         };
diff --git a/haskell-template.cabal b/haskell-template.cabal
index 770a028..cc3a397 100644
--- a/haskell-template.cabal
+++ b/haskell-template.cabal
@@ -81,6 +81,7 @@ common shared
     , directory
     , filepath
     , mtl
+    , th-env
     , optics-core
     , profunctors
     , relude        >=1.0
diff --git a/src/Main.hs b/src/Main.hs
index fc5ea8d..6586dcb 100644
--- a/src/Main.hs
+++ b/src/Main.hs
@@ -1,7 +1,13 @@
+{-# LANGUAGE TemplateHaskell #-}
+
 module Main where
 
+import Language.Haskell.TH.Env (envQ')
 import Main.Utf8 qualified as Utf8
 
+foo :: String
+foo = $$(envQ' "FOO")
+
 {- |
  Main entry point.
 
@@ -12,3 +18,4 @@ main = do
   -- For withUtf8, see https://serokell.io/blog/haskell-with-utf8
   Utf8.withUtf8 $ do
     putTextLn "Hello 🌎"
+    putStrLn foo

[newton-migosi]

Thanks, this adds the needed env vars for nix build