How would you accumulate errors?

Questions / Help
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1

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

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

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.

2

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

3

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

# NOT TESTED!

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.

2 Likes
4

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]
  end
end)
|> Enum.reverse()
1 Like
5

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
6

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.

7

you can use

for {key, validate_fun} <- validation, reduce: [] do
  acc ->
   ....
end
8

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