refactor({modules,pkgs}/by-name-overlay): De-duplicate in a `nixLib`

1 files changed, 37 insertions, 0 deletions

diff --git a/lib/default.nix b/lib/default.nix
new file mode 100644
index 00000000..7c86aba1
--- /dev/null
+++ b/lib/default.nix
@@ -0,0 +1,37 @@
+{}: let
+  # Taken from `nixpkgs/lib`.
+  # Is here to avoid a dependency on `lib` (making this file easy to test with `nix eval`)
+  warn =
+    # Since Nix 2.23, https://github.com/NixOS/nix/pull/10592
+    builtins.warn
+    or (
+      # Do not eta reduce v, so that we have the same strictness as `builtins.warn`.
+      msg: v:
+      # `builtins.warn` requires a string message, so we enforce that in our implementation, so that callers aren't accidentally incompatible with newer Nix versions.
+        assert builtins.isString msg;
+          builtins.trace "evaluation warning: ${msg}" v
+    );
+in {
+  mkByName = import ./by-name-overlay.nix {inherit warn;};
+
+  # Only merge two attrsets if the RHS does not share names with the LHS.
+  # Example:
+  # ```nix
+  # let
+  # attr1 = {atuin = "first"; default = "second";};
+  # attr2 = {atuin = "hi"; other = "hi2";};
+  # in
+  # maybeMerge attr1 attr2 "Failed to merge"
+  # ```
+  maybeMerge = attr1: attr2: message: let
+    list1 = builtins.attrNames attr1;
+
+    check = name2: value:
+      if builtins.elem name2 list1
+      then warn "Overriding ${name2} while merging two attrs sets: ${message}" value
+      else value;
+
+    checkedAttr2 = builtins.mapAttrs check attr2;
+  in
+    attr1 // checkedAttr2;
+}