How would you accumulate errors?

In “Elixir in Action”, there is an example about using pattern matching with the “with” special form:

       {:ok, login}  <- extract_login(user),
       {:ok, email} <- extract_mail(user),
       {:ok, pw}     <- extract_pw(user)
  {:ok,   %{login: login, email: email, pw: pw}}

If each validation of user is successful, the “ok” tuple is returned. Otherwise, the first failing validation’s “error” tuple is returned.

But suppose I wanted to run all the validations, returning the same “ok” if they all pass, but returning a list of all the “error” tuples for all failing validations.

Is there an idiom for this in Elixir? afaik this is like an applicative functor.

One way how to do it: 7) ElixirConf US 2018 – Architecting Flow in Elixir - From Leveraging Pipes to Designing Token APIs – René Föhring

You could reduce each function call… then check if Enum.all?, Enum.any?, whatever tests You may want to run on the list.


result = ~w(extract_login extract_mail extract_pw)a
|> Enum.reduce(&apply(__MODULE__, &1, [user]))

But your code looks like a user validation, and usually this can be done with a changeset.


Something like this, (I haven’t run the code though)

validations = [
  login: &extract_login/1,
  mail: &extract_mail/1,
  pw: &extract_pw/1

Enum.reduce(validatations, [], fn {key, validate_fun}, acc ->
  case validate_fun.(user) do
    {:ok, value} ->
      [{:ok, key, value} | acc]

    :error ->
      [{:error, key} | acc]
|> Enum.reverse()
1 Like

This looks like what I was doing in Java, where functional programming idioms are at last available. I was hoping there were a macro or special form like with that might accomplish what that reduce is doing there. I’m still learning the bare basics. Thank you!

1 Like

Whoa your sigil just blew my mind. I didn’t know that was a thing yet. Thank you. I’ll need to read about changesets.

you can use

for {key, validate_fun} <- validation, reduce: [] do
  acc ->

I just started with Elixir - this is way over my head. I’ll revisit in a few months. Thanks