Add Enum.sum_by/2 (#12885)

Author: sabiwara

lib/elixir/lib/enum.ex

@@ -3447,6 +3447,8 @@ defmodule Enum do
 
   Raises `ArithmeticError` if `enumerable` contains a non-numeric value.
 
+  If you need to apply a transformation first, consider using `Enum.sum_by/2` instead.
+
   ## Examples
 
       iex> Enum.sum([1, 2, 3])
@@ -3473,6 +3475,40 @@ defmodule Enum do
     reduce(enumerable, 0, &+/2)
   end
 
+  @doc """
+  Maps and sums the given enumerable in one pass.
+
+  Raises `ArithmeticError` if `fun` returns a non-numeric value.
+
+  ## Examples
+
+      iex> Enum.sum_by([%{count: 1}, %{count: 2}, %{count: 3}], fn x -> x.count end)
+      6
+
+      iex> Enum.sum_by(1..3, fn x -> x ** 2 end)
+      14
+
+      iex> Enum.sum_by([], fn x -> x.count end)
+      0
+
+  Filtering can be achieved by returning `0` to ignore elements:
+
+      iex> Enum.sum_by([1, -2, 3], fn x -> if x > 0, do: x, else: 0 end)
+      4
+
+  """
+  @doc since: "1.18.0"
+  @spec sum_by(t, (element -> number)) :: number
+  def sum_by(enumerable, mapper)
+
+  def sum_by(list, mapper) when is_list(list) and is_function(mapper, 1) do
+    sum_by_list(list, mapper, 0)
+  end
+
+  def sum_by(enumerable, mapper) when is_function(mapper, 1) do
+    reduce(enumerable, 0, fn x, acc -> acc + mapper.(x) end)
+  end
+
   @doc """
   Returns the product of all elements.
 
@@ -4770,6 +4806,11 @@ defmodule Enum do
     {:lists.reverse(acc), []}
   end
 
+  ## sum_by
+
+  defp sum_by_list([], _, acc), do: acc
+  defp sum_by_list([h | t], mapper, acc), do: sum_by_list(t, mapper, acc + mapper.(h))
+
   ## take
 
   defp take_list(_list, 0), do: []

lib/elixir/pages/cheatsheets/enum-cheat.cheatmd

@@ -297,6 +297,15 @@ iex> cart |> Enum.map(& &1.count) |> Enum.sum()
 10
 ```
 
+Note: this should typically be done in one pass using `Enum.sum_by/2`.
+
+### [`sum_by(enum, mapper)`](`Enum.sum_by/2`)
+
+```elixir
+iex> Enum.sum_by(cart, & &1.count)
+10
+```
+
 ### [`product(enum)`](`Enum.product/1`)
 
 ```elixir

lib/elixir/test/elixir/enum_test.exs

@@ -1321,6 +1321,24 @@ defmodule EnumTest do
     end
   end
 
+  test "sum_by/2" do
+    assert Enum.sum_by([], &hd/1) == 0
+    assert Enum.sum_by([[1]], &hd/1) == 1
+    assert Enum.sum_by([[1], [2], [3]], &hd/1) == 6
+    assert Enum.sum_by([[1.1], [2.2], [3.3]], &hd/1) == 6.6
+    assert Enum.sum_by([[-3], [-2], [-1], [0], [1], [2], [3]], &hd/1) == 0
+
+    assert Enum.sum_by(1..3, &(&1 ** 2)) == 14
+
+    assert_raise ArithmeticError, fn ->
+      Enum.sum_by([[{}]], &hd/1)
+    end
+
+    assert_raise ArithmeticError, fn ->
+      Enum.sum_by([[1], [{}]], &hd/1)
+    end
+  end
+
   test "product/1" do
     assert Enum.product([]) == 1
     assert Enum.product([1]) == 1