What is the idiomatic way for input validations?

Hello @nmcalabroso,

in Elixir and other functional languages, you cannot »return early« as you would do in Python or Ruby.
In your particular case, you could use the with keyword in order to avoid nesting and chain pattern-matched conditions together:

with {:ok, raw_password} <- PasswordManager.valid?(user_attrs["password"]),
     {:ok, changeset} <- create_changeset(user_attrs, raw_password),
     {:ok, user} <- Repo.insert(changeset) do
     #whatever you want to do if everything went okay
else
  {:error, reason} -> #whatever you want to do if there was an {:error, reason} tuple
end

You can read more about the with keyword in the Elixir Getting Started Guide: http://elixir-lang.org/getting-started/mix-otp/docs-tests-and-with.html#with

And here is a nice blog article about the with keyword and what’s called »Railway-oriented programming« in the F# world: http://www.scottmessinger.com/2016/03/25/railway-development-in-elixir-using-with/

Maybe extract the call to PasswordManager.valid? to your schema?
This post explains how to implement a custom validation.

Edit: I think this stackoverflow answer demonstrates better how to use add_error.

Agreed. Maybe I just chose a poor example. But what I’m trying to get at is how do we validate and “return early” in elixir? Of course, we can’t. But what’s the idiomatic way for this scenario?

I would say the most natural way of emulating “early return” would be to split the logic into multiple functions each one returning or continuing to the next one. The with construct is kind of a “get out of jail free card” - it allows you to encode those patterns less verbosely in a single expression. I’d say that’s the most “common” way to do this.

In case of ecto, the most idiomatic way of doing validations is to use, as mentioned, changesets.

Hi @michalmuskala,

Can you give an example for this? Apologies if this is considered spoon-feeding but I genuinely interested to know.

How would you emulate this the elixir-way?

validate_one
if true, continue else return error
validate_two
if true, continue else return error

Definitely reading this. Thanks a lot.

Nope, I think its meant like this:

with {:ok, data} <- validate_one(data),
     {:ok, data} <- validate_two(data) do
  {:ok, data}
else
  {:error, reason} -> {:error, reason}
end