[Chore] Add weeder check

Problem: We want to be able to check our Haskell applications for dead
code. This can be done using weeder.

Solution: Use weeder check from serokell.nix.

haskell.nix/README.md

@@ -21,7 +21,9 @@ Haskell application and library templates for Buildkite, Gitlab or GitHub CI usi
 
     3. If you're using `hpack` or `stack2cabal` in your project, make sure to uncomment the related lines in both `flake.nix` and pipeline configuration files. To avoid version mismatches, use `nix develop .#ci -c hpack` or `nix develop .#ci -c stack2cabal`.
 
-    4. Make sure to clean up your `flake.nix` and pipeline configuration files by removing any optional code that is left commented out.
+    4. You might want to add a [`weeder`](https://github.com/ocharles/weeder/) check to check for dead code in your application. To do this uncomment the related lines in both `flake.nix` (do not forget to uncomment `writeHieFiles = true;`) and pipeline configuration files, then copy `weeder.toml` (for more advanced setup, please refer to the [corresponding section](https://github.com/ocharles/weeder/?tab=readme-ov-file#calling-weederUsually) of the `weeder` README) to your repository. For a library, code is usually considered dead unless it is exported; such cases are already detected by GHC warnings, so no `weeder` is required.
+
+    5. Make sure to clean up your `flake.nix` and pipeline configuration files by removing any optional code that is left commented out.
 
 - Enjoy working CI!
 

haskell.nix/application/.github/workflows/check.yml

@@ -39,6 +39,10 @@ jobs:
       #   run: nix build -L .#checks.x86_64-linux.hpack
       #   if: success() || failure()
 
+      # - name: check-weeder
+      #   run: nix build -L .#checks.x86_64-linux.weeder
+      #   if: success() || failure()
+
   build:
     runs-on: [self-hosted, nix]
     steps:

haskell.nix/application/.gitlab-ci.yml

@@ -47,6 +47,11 @@ check-trailing-whitespace:
 #   script:
 #     - nix build -L .#checks.x86_64-linux.hpack
 
+# check-weeder:
+#   stage: validate
+#   script:
+#     - nix build -L .#checks.x86_64-linux.weeder
+
 # build haskell components
 build-all:
   stage: build

haskell.nix/application/flake.nix

@@ -18,6 +18,7 @@
     stackage = {
       flake = false;
     };
+    serokell-nix.inputs.haskell-nix.follows = "haskell-nix";
   };
 
   outputs = { self, nixpkgs, haskell-nix, hackage, stackage, serokell-nix, flake-compat, flake-utils, ... }:
@@ -43,6 +44,8 @@
         # haskell.nix configuration
         modules = [
           (serokell-nix.lib.haskell.optionsLocalPackages {
+            # Uncomment if your project uses weeder check
+            # writeHieFiles = true;
             ghcOptions = [
               # fail on warnings
               "-Werror"
@@ -91,6 +94,11 @@
         # cabal-check = pkgs.build.haskell.cabal-check ./.;
         # Uncomment if your project uses hpack to generate cabal files
         # hpack = pkgs.build.haskell.hpack ./.;
+        # Uncomment to check for dead code
+        # weeder = pkgs.build.haskell.weeder {
+        #   weederToml = ./weeder.toml;
+        #   inherit hs-pkgs;
+        # };
       };
     });
 }

haskell.nix/application/weeder.toml

@@ -0,0 +1 @@
+roots = [ "^Main.main$", "^Paths_.*" ]