From f2409bd62f6c3179f344921452189bf8720cdc50 Mon Sep 17 00:00:00 2001
From: jappeace-sloth <sloth@jappie.me>
Date: Sat, 6 Jun 2026 23:27:41 +0200
Subject: [PATCH] Filter source with lib.fileset to stop nix files invalidating
 builds
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Replaces `callCabal2nix "template-project" ../. {}` with an explicit
`lib.fileset.toSource` that unions only app/, src/, test/, the cabal
file, LICENSE, Readme.md and Changelog.md. This addresses the existing
note that nix/tooling files were considered for the build, and so the
cabal2nix-driven derivation no longer rebuilds when shell.nix,
nix/hpkgs.nix, .github/, makefile, .hlint.yaml etc. change.

Verified locally with `nix-build` — template-1.0.0 builds and the
unit test suite (5 tests) passes.

Prompt: go update my haskell-template-project, I want find grained
source filtereing like this: [lib.fileset.toSource snippet]

Tokens: ~25k
---
 nix/hpkgs.nix | 23 ++++++++++++++++++++---
 1 file changed, 20 insertions(+), 3 deletions(-)

diff --git a/nix/hpkgs.nix b/nix/hpkgs.nix
index 2b64da0..001bee7 100644
--- a/nix/hpkgs.nix
+++ b/nix/hpkgs.nix
@@ -1,13 +1,30 @@
 { pkgs ? import ./pkgs.nix { }
 ,
 }:
+let
+  # Decision: use lib.fileset.toSource for fine grained source filtering.
+  # Alternatives considered: passing ../. directly (rebuilds on any nix/tooling
+  # change), cleanSource (still includes too much). fileset.unions makes the
+  # build inputs explicit so editing nix/, .github/, makefile, .hlint.yaml,
+  # etc. does not invalidate the haskell build cache.
+  src = pkgs.lib.fileset.toSource {
+    root = ../.;
+    fileset = pkgs.lib.fileset.unions [
+      ../app
+      ../src
+      ../test
+      ../template.cabal
+      ../LICENSE
+      ../Readme.md
+      ../Changelog.md
+    ];
+  };
+in
 # you can pin a specific ghc version with
 # pkgs.haskell.packages.ghc984 for example.
 # this allows you to create multiple compiler targets via nix.
 pkgs.haskellPackages.override {
   overrides = hnew: hold: {
-    # NB this is a bit silly because nix files are now considered for the build
-    # bigger projects should consider putting haskell stuff in a subfolder
-    template-project = hnew.callCabal2nix "template-project" ../. { };
+    template-project = hnew.callCabal2nix "template-project" src { };
   };
 }