What is the idiomatic way for input validations?

Questions / Help
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1

I’m new to elixir and functional programming, in general. I want to ask what’s the idiomatic way to validate our input parameters? Say I want to do this in my UserController.create.

Check if password is valid by PasswordManager.valid?/1
If not, render error
else, continue

This is what I’ve come up with:

defmodule MyApp.UserController do
  def create(conn, %{"data" => data}) do
    case PasswordManager.valid? user_attrs["password"] do
      {:ok, raw_password} ->
        # make changeset with hashed password
        case Repo.insert(changeset) do
          {:ok, user} ->
            # success
          {:error, invalid_changeset} ->
            # render failure  
      {:error, reason} ->
        render(conn, :errors, data: reason)
    end
  end
end

The issue here is the “ugly” nesting seems unnecessary. I’m looking for a “return early” statements but I guess elixir wants us to write pure functions as much as possible and utilize pattern matching accordingly. How do I refactor this?

Coming from python, I would do this:

def create(user_attrs):
  if not PasswordManager.is_valid(user_attrs['password']):
    # display error
    return
  
  # continue

But what’s the best practice in elixir?

Feel free to do some nitpicks to my code so I can improve writing in this language.

Also, please do note that I’m asking this as a generic question. I hope you don’t dwell too much with the password validation as an example. Thank you.

1 Like
2

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/

4 Likes
3

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.

4

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?

5

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.

1 Like
6

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

7

Definitely reading this. Thanks a lot.

1 Like
8

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
2 Likes