Add Enum.tally/2

lib/elixir/lib/enum.ex

@@ -1147,6 +1147,27 @@ defmodule Enum do
     end)
   end
 
+  @doc """
+  creates a new map with keys as unique elements from enumerable,
+  and set values as the count of every element
+
+  The order of elements within each list is preserved from the `enumerable`.
+
+  ## Examples
+
+      iex> Enum.tally(~w{ant buffalo ant ant buffalo dingo})
+      %{"ant" => 3, "buffalo" => 2, "dingo" => 1}
+
+      iex> Enum.tally(~w{aa aA bb cc}, fn x -> String.downcase(x) end)
+      %{"aa" => 2, "bb" => 1, "cc" => 1}
+  """
+  @spec tally(t, (element -> any)) :: map
+  def tally(enumerable, func \\ fn x -> x end) do
+    group_by(enumerable, func)
+    |> map(fn {key, val} -> {key, count(val)} end)
+    |> Map.new()
+  end
+
   @doc """
   Intersperses `element` between each element of the enumeration.
 

lib/elixir/test/elixir/enum_test.exs

@@ -279,6 +279,16 @@ defmodule EnumTest do
     assert Enum.group_by([1, 2, 3], &rem(&1, 2)) == %{0 => [2], 1 => [1, 3]}
   end
 
+  test "tally/2" do
+    assert Enum.tally(~w{a a a b c c}) == %{"a" => 3, "b" => 1, "c" => 2}
+
+    assert Enum.tally(~w{aa aA bb cc}, fn x -> String.downcase(x) end) == %{
+             "aa" => 2,
+             "bb" => 1,
+             "cc" => 1
+           }
+  end
+
   test "intersperse/2" do
     assert Enum.intersperse([], true) == []
     assert Enum.intersperse([1], true) == [1]