Passing an updated struct to each iteration of Enum

Dusty

I am trying to iterate over a map, and repeatedly update a struct using each {k, v} pair in the map. But I am having difficulty figuring out how to pass the new/updated struct to the next iteration of the function. So I have something like:

struct = %MyStruct{...}
map = %{key1: value1, key2: value2,...}

def my_func(struct, key, value) do
  struct = # some code that updates the struct using the key and value
  struct # return the new struct
end

and I am trying to do something like:


struct = Enum.each(map, fn {key, value} -> my_func(struct, key, value) end)
{:ok, struct}

Obviously that code does not work, because Enum.each/2 returns :ok, but doesn’t carry the updated struct into the next iteration of the function, and doesn’t return the struct itself. It seems like I need a form of Enum.reduce/3 where the struct acts as the accumulator. How would you approach this?

5 comments

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2020-07-22 13:29:00 UTC

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henrik

Hi Dusty!

You could indeed use Enum.reduce/3:

iex> fake_struct = %{x: 0}
%{x: 0}
iex> map = %{key1: 1, key2: 2}
%{key1: 1, key2: 2}
iex> Enum.reduce(map, fake_struct, fn ({_k, v}, s) -> %{s | x: s.x + v} end)
%{x: 3}

Post #1

jmurphyweb

Usually you would reduce over the map, and use a struct as the accumulator.

Enum.reduce(map, struct, fn {key, val}, new_struct -> 
  # ... perform some logic
  %{new_struct | key: value}
end)

Post #2

jmurphyweb

@Dusty as a general rule, if you are trying to transform one enumerable into another and there is not a direct mapping (where Enum.map would be useful), then it can be done with Enum.reduce.

Often the shape/type of what you want to return will be passed in as the accumulator.

There’s a nice practice exercise that you can try, which is to ‘recreate’ Enum.map, Enum.filter etc using Enum.reduce (which I think is more or less how most Enum functions are built if you check the Elixir source code on Github).

Post #3

Dusty

@henrik @jmurphyweb

Thank you both for your time. It appears I just need a broader mental definition of an accumulator. Examples of reduce that I’ve seen always seem to use simple structures (like integer values) as an accumulator, but if the struct itself can be the accumulator then I think I’m in good shape.

Post #4

Oliver

Hi, Dusty.

Maps make useful accumulators for Enum.reduce/3, especially if you are interested in doing something for some of the items, or if the previously worked on items are needed in some form.

Another application is passing the whole state of my OTP process into the reduce so it can be used when modifying the enum or reading from it and even be updated while doing so, allowing state-sensitive functions to work on an enumerable. It depends what information you need to use and retain.

def handle_info({:message_with_list, list}, state) do
  state = Enum.reduce(list, state, &function_that_uses_state_and_list/2)
  {:noreply, state}
end

def handle_info({:more_complex, list}, state) do
  {state, modifed_list} = Enum.reduce(list, state, &function_that_uses_state_and_list/2)
  [...]
  {:noreply, state}
end

Etc.

Post #5

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