1 files changed, 36 insertions, 0 deletions
diff --git a/lib/lists.nix b/lib/lists.nix
index 5ec97f5a07f3..194e1c200ec9 100644
--- a/lib/lists.nix
+++ b/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.