Add Enum.product_by/2 (elixir-lang#13683)

lib/elixir/lib/enum.ex

@@ -3476,9 +3476,9 @@ defmodule Enum do
   end
 
   @doc """
-  Maps and sums the given enumerable in one pass.
+  Maps and sums the given `enumerable` in one pass.
 
-  Raises `ArithmeticError` if `fun` returns a non-numeric value.
+  Raises `ArithmeticError` if `mapper` returns a non-numeric value.
 
   ## Examples
 
@@ -3514,6 +3514,8 @@ defmodule Enum do
 
   Raises `ArithmeticError` if `enumerable` contains a non-numeric value.
 
+  If you need to apply a transformation first, consider using `Enum.product_by/2` instead.
+
   ## Examples
 
       iex> Enum.product([])
@@ -3530,6 +3532,40 @@ defmodule Enum do
     reduce(enumerable, 1, &*/2)
   end
 
+  @doc """
+  Maps and computes the product of the given `enumerable` in one pass.
+
+  Raises `ArithmeticError` if `mapper` returns a non-numeric value.
+
+  ## Examples
+
+      iex> Enum.product_by([%{count: 2}, %{count: 4}, %{count: 3}], fn x -> x.count end)
+      24
+
+      iex> Enum.product_by(1..3, fn x -> x ** 2 end)
+      36
+
+      iex> Enum.product_by([], fn x -> x.count end)
+      1
+
+  Filtering can be achieved by returning `1` to ignore elements:
+
+      iex> Enum.product_by([2, -1, 3], fn x -> if x > 0, do: x, else: 1 end)
+      6
+
+  """
+  @doc since: "1.18.0"
+  @spec product_by(t, (element -> number)) :: number
+  def product_by(enumerable, mapper)
+
+  def product_by(list, mapper) when is_list(list) and is_function(mapper, 1) do
+    product_by_list(list, mapper, 1)
+  end
+
+  def product_by(enumerable, mapper) when is_function(mapper, 1) do
+    reduce(enumerable, 1, fn x, acc -> acc * mapper.(x) end)
+  end
+
   @doc """
   Takes an `amount` of elements from the beginning or the end of the `enumerable`.
 
@@ -4811,6 +4847,11 @@ defmodule Enum do
   defp sum_by_list([], _, acc), do: acc
   defp sum_by_list([h | t], mapper, acc), do: sum_by_list(t, mapper, acc + mapper.(h))
 
+  ## product_by
+
+  defp product_by_list([], _, acc), do: acc
+  defp product_by_list([h | t], mapper, acc), do: product_by_list(t, mapper, acc * mapper.(h))
+
   ## take
 
   defp take_list(_list, 0), do: []

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

@@ -313,6 +313,15 @@ iex> cart |> Enum.map(& &1.count) |> Enum.product()
 18
 ```
 
+Note: this should typically be done in one pass using `Enum.product_by/2`.
+
+### [`product_by(enum, mapper)`](`Enum.product_by/2`)
+
+```elixir
+iex> Enum.product_by(cart, & &1.count)
+18
+```
+
 ## Sorting
 {: .col-2}
 

lib/elixir/test/elixir/enum_test.exs

@@ -1360,6 +1360,27 @@ defmodule EnumTest do
     end
   end
 
+  test "product_by/2" do
+    assert Enum.product_by([], &hd/1) == 1
+    assert Enum.product_by([[1]], &hd/1) == 1
+    assert Enum.product_by([[1], [2], [3], [4], [5]], &hd/1) == 120
+    assert Enum.product_by([[1], [-2], [3], [4], [5]], &hd/1) == -120
+    assert Enum.product_by(1..5, & &1) == 120
+    assert Enum.product_by(11..-17//-1, & &1) == 0
+
+    assert_raise ArithmeticError, fn ->
+      Enum.product_by([[{}]], &hd/1)
+    end
+
+    assert_raise ArithmeticError, fn ->
+      Enum.product_by([[1], [{}]], &hd/1)
+    end
+
+    assert_raise ArithmeticError, fn ->
+      Enum.product_by(%{a: 1, b: 2}, & &1)
+    end
+  end
+
   test "take/2" do
     assert Enum.take([1, 2, 3], 0) == []
     assert Enum.take([1, 2, 3], 1) == [1]