lib: lists: add countValues

lib/lists.nix

@@ -1,8 +1,22 @@
 { lib, ... }:
 let
-  inherit (lib) filter;
+  inherit (lib) filter foldl';
 in
 {
+  # Count the number of appararitions of each value in a list.
+  #
+  # countValues ::
+  #   [ any ] -> ({ any = int; })
+  countValues =
+    let
+      addToCount = acc: x:
+        let
+          v = toString x;
+        in
+        acc // { ${v} = (acc.${v} or 0) + 1; };
+    in
+    foldl' addToCount { };
+
   # Filter a list using a predicate function after applying a map.
   #
   # mapFilter ::