Use case for Enum.reduce_while without accumulator

Hi Everybody,

Recently I discovered interesting use case for reduce_while that does not use acc as accumulator -> it doesn’t carry / use the variable in subsequent loops.

Could you please take a look and tell me if it’s clever or stupid?

Enum.reduce_while(some_enumerable, {:error, :show_this_when_empty_input}, fn some_item, _ ->
  with {:ok, something} <- something(some_item),
       {:ok, something} <- something_else(something) do
    {:halt, {:ok, something}}
  else
    # known error - continue OR finish with this error
    {:error, :expected_message} = error -> {:cont, error}

    # unknown error - halt immediately
    {:error, _} = error -> {:halt, error}
  end
end)

What I can get from that is:

  1. {:error, :show_this_when_empty_input} when enumerable is an empty list
  2. {:error, :expected_message} when no element in enumerable passes the test (the function) in 3rd argument
  3. {:ok, result} first positive result from the function -> first element that passes and then stop processing
  4. {:error, anything} for results / errors that are not expected

I’ve written code similar to this, so yeah, I think that’s a perfectly reasonable usage of Enum.reduce_while. :slight_smile:

1 Like

Your code looks good except one point. Calling Enum.reduce* functions does not makes sense if you do not need an accumulator. I would rather write something like:

func = fn
  # return error if needed - equivalent of: {:halt, {:error, error}}
  x when rem(x, 2) != 0 -> {:error, 2}
  # skip item if needed - equivalent of: {:cont, acc}
  x when rem(x, 3) != 0 -> nil
  # return ok tuple for desired item - equivalent of: {:halt, {:ok, item}}
  x -> {:ok, x}
end

iex> Enum.find_value(1..3, func)
{:error, 2} # error returned
iex> Enum.find_value([2, 4], func)
nil # all skipped
iex> Enum.find_value([2, 4, 6], func)
{:ok, 6} # ok returned

For more information please take a look at Enum.find_value/2 documentation.

1 Like

I would also try to move the code to Enum.find, or Enum.find_value, but the condition is more complex, and not really a truthy kind of condition.

It’s hard to capture the with, with Enum.find :slight_smile:

1 Like

Sorry, I did not get it. Since we accept any function and expect not nil all mentioned clauses could be changed to return “something” like {:error, error} or {:ok, result} and optionally for {:error, :expected_message} we may want to return nil in order to skip specific item.

If you want you can also write a really simple few-lines function like:


defmodule Example do
  def sample(list, default \\ nil, func)

  def sample([], default, _func), do: func

  def sample([head | tail], default \\ nil, func) do
    case func.(head) do
      {:ok, something} -> something
      {:error, :expected_message} = error -> sample(tail, error, func)
      {:error, _} = error -> error
    end
  end
end

and use it like:

Example.sample(some_enumerable, {:error, :show_this_when_empty_input}, fn some_item, _ ->
  with {:ok, something} <- something(some_item),
       {:ok, something} <- something_else(something) do
    {:ok, something}
  else
    {:error, _} = error -> error
  end
end)

Look that in case clauses you may simply add support for {:halt, acc} and {:cont, acc} if you do not want to change anonymous function from an original post. You may even combine Example.sample/3 with such anonymous function.

1 Like

I know recursion could solve this problem quite elegantly.

What I meant is Enum.find_value is not working well in that case.

Because the condition should exit only if ok, or unexpected error… and continue if the error is known. But You need to return nil/or false instead, to continue iteration :slight_smile:

1 Like

Thank you very much @Eiji, I took you example sample and I created a functional example / comparison of my old cold and your recommended new code with recursion.

Just one small correction - second sample has do: default and I didn’t use default value for default argument.

defmodule DoSomething do
  def function_that_shoud_not_fail(value) do
    # dummy code

    # {:error, :this_is_not_expected_terminate_me_now}
    {:ok, value <> "-abc"}
  end

  def another_function(value) do
    # dummy code

    # {:error, :this_is_normal}
    {:ok, value <> "-xyz"}
  end

  #####################################################
  # old way

  def test_this_using_reduce_while(enumerable) do
    Enum.reduce_while(enumerable, {:error, :enumerable_is_empty}, fn single_element, _ ->
      with {:ok, value} <- function_that_shoud_not_fail(single_element),
           {:ok, _} = result <- another_function(value) do
        # sucess -> finish (halt)
        {:halt, result}
      else
        # continue processing other elements / finish with this error
        {:error, :this_is_normal} = result -> {:cont, result}
        # unknown exception -> halt immediately
        {:error, _} = result -> {:halt, result}
      end
    end)
  end

  #####################################################
  # new way

  defp sample([], default, _func),
    do: default

  defp sample([head | tail], _default, func) do
    case func.(head) do
      # sucess -> finish (halt)
      {:ok, _} = result -> result
      # continue processing other elements / finish with this error
      {:error, :this_is_normal} = error -> sample(tail, error, func)
      # unknown error -> finish (halt)
      {:error, _} = error -> error
    end
  end

  def test_this_using_recursion(enumerable) do
    sample(enumerable, {:error, :enumerable_is_empty}, fn single_element ->
      with {:ok, value} <- function_that_shoud_not_fail(single_element),
           {:ok, _} = result <- another_function(value) do
        result
      else
        {:error, _} = error -> error
      end
    end)
  end

  #####################################################
end

result = DoSomething.test_this_using_reduce_while(["aaa", "bbb", "ccc", "ddd", "eee", "fff"])
IO.puts("result: #{inspect(result)}")

result = DoSomething.test_this_using_recursion(["aaa", "bbb", "ccc", "ddd", "eee", "fff"])
IO.puts("result: #{inspect(result)}")

That would basically create Enum.reduce_while that doesn’t have acc. Very cool :wink: .