add lib.lists.groupByMult

nix/lib/default.nix

@@ -11,6 +11,7 @@ nixpkgs.lib
 # groups
 // mapAttrs (autoExtend nixpkgs.lib) {
   attrsets = ./attrsets.nix;
+  lists = ./lists.nix;
   math = ./math.nix;
   modules = ./modules.nix;
   network = ./network.nix;

nix/lib/lists.nix

@@ -0,0 +1,42 @@
+{ lib, ... }@flakeArg:
+let
+  inherit (builtins)
+    deepSeq
+    foldl'
+    groupBy
+    mapAttrs
+    ;
+  inherit (lib.lists) singleton;
+  inherit (lib.trivial) flip pipe;
+in
+{
+
+  groupByMult =
+    groupers:
+    # TODO (maybe) make more efficient by building up grouping function from right
+    # from left
+    # 0= (x: x) vals
+    # 1= groupBy values[0] <0>
+    # 2= mapAttrs (_: groupBy values[1]) <1>
+    # 3= mapAttrs (_: mapAttrs (_: groupBy values[2])) <2>
+    # from right
+    # 1= (groupBy values[0])
+    # 2= ...
+    let
+      nul = {
+        mapper = x: x;
+        result = [ ];
+      };
+      op =
+        { mapper, result }:
+        grouper: {
+          mapper = fun: mapAttrs (_: mapper fun);
+          result = result ++ singleton (mapper grouper);
+        };
+      list = map groupBy groupers;
+      pipeList = (foldl' op nul list).result;
+    in
+    # catch errors while building groupers before values are passed through
+    deepSeq pipeList (flip pipe pipeList);
+
+}