lib: add groupBy (#38612)

lib/lists.nix

@@ -250,6 +250,42 @@ rec {
       else { right = t.right; wrong = [h] ++ t.wrong; }
     ) { right = []; wrong = []; });
 
+  /* Splits the elements of a list into many lists, using the return value of a predicate.
+     Predicate should return a string which becomes keys of attrset `groupBy' returns.
+
+     `groupBy'' allows to customise the combining function and initial value
+
+     Example:
+       groupBy (x: boolToString (x > 2)) [ 5 1 2 3 4 ]
+       => { true = [ 5 3 4 ]; false = [ 1 2 ]; }
+       groupBy (x: x.name) [ {name = "icewm"; script = "icewm &";}
+                             {name = "xfce";  script = "xfce4-session &";}
+                             {name = "icewm"; script = "icewmbg &";}
+                             {name = "mate";  script = "gnome-session &";}
+                           ]
+       => { icewm = [ { name = "icewm"; script = "icewm &"; }
+                      { name = "icewm"; script = "icewmbg &"; } ];
+            mate  = [ { name = "mate";  script = "gnome-session &"; } ];
+            xfce  = [ { name = "xfce";  script = "xfce4-session &"; } ];
+          }
+
+
+     groupBy' allows to customise the combining function and initial value
+
+     Example:
+       groupBy' builtins.add 0 (x: boolToString (x > 2)) [ 5 1 2 3 4 ]
+       => { true = 12; false = 3; }
+  */
+  groupBy' = op: nul: pred: lst:
+    foldl' (r: e:
+              let
+                key = pred e;
+              in
+                r // { ${key} = op (r.${key} or nul) e; }
+           ) {} lst;
+
+  groupBy = groupBy' (sum: e: sum ++ [e]) [];
+
   /* Merges two lists of the same size together. If the sizes aren't the same
      the merging stops at the shortest. How both lists are merged is defined
      by the first argument.