feat: allow optional arbitrary filtering of generated gitignore

lib/default.nix

@@ -2,6 +2,8 @@
   downloadGitignore = arguments @ {
     languages ? [],
     hash ? lib.fakeSha256,
+    # Allow variadic arguments so we have one API
+    ...
   }:
     builtins.fetchurl {
       url = "https://www.toptal.com/developers/gitignore/api/${lib.concatStringsSep "," arguments.languages}";
@@ -10,8 +12,21 @@
       sha256 = hash;
     };
 in {
-  # Pass-through the function
+  # Pass-through the function in case people want plain gitignores
   inherit downloadGitignore;
+  sourceGitignoreList = arguments @ {
+    # Default this to a no-op processing where every list item is retained.
+    # TODO: Allow this to take a list of functions and recurse down to progressively apply them.
+    filterFunction ? (_: true),
+    ...
+  }: let
+    gitignoreFile = downloadGitignore arguments;
+    rawText = builtins.readFile gitignoreFile;
+    splitList = builtins.split "\n" rawText;
+    pureList = builtins.filter (x: x != []) splitList;
+    filteredList = builtins.filter filterFunction pureList;
+  in
+    filteredList;
   allAttrsSet = x: (builtins.all (v: lib.stringLength v > 0) (lib.attrValues x));
   getPublicKeys = forge: username: fileHash:
   # For some reason we get not one but two trailing empty lines
@@ -21,11 +36,4 @@ in {
       url = "https://${forge}.com/${username}.keys";
       sha256 = fileHash;
     }));
-  sourceGitignoreList = arguments @ {...}: let
-    gitignoreFile = downloadGitignore arguments;
-    rawText = builtins.readFile gitignoreFile;
-    splitList = builtins.split "\n" rawText;
-    pureList = builtins.filter (x: x != []) splitList;
-  in
-    pureList;
 }