Para is an Elixir library that provides structured and declarative way to parse and validate parameters.

Para uses Ecto under the hood and therefore inherits most of its utilities such as changeset and built-in validators.

Usage

Let's imagine that you have a controller named Web.UserController and wanted to validate the parameters for its :create and :update actions.

First, let's define your parameters schema.

defmodule Web.UserPara do
  use Para

  validator :create do
    required :name, :string
    required :age, :integer
    required :email, :string
    optional :phone, :string
  end

  validator :update do
    required :name, :string
    required :age, :integer
    required :email, :string
    optional :phone, :string
  end
end

This will generate two validate/2 functions for your module with action name and params as arguments.

defmodule Web.UserController do
  use Web, :controller
  alias Web.UserPara

  def create(conn, params) do
    with {:ok, data} <- UserPara.validate(:create, params) do
      # ...
    end
  end

  def update(conn, params) do
    with {:ok, data} <- UserPara.validate(:update, params) do
      # ...
    end
  end
end

The validate/2 function will return either an {:ok, map} or {:error, changeset} tuple.

Inline validators

Inline validator is a convenient way to validate your fields. This is especially useful when you need to perform some basic validation using Ecto.Changeset's built-in validators.

defmodule UserPara do
  use Para

  validator :update do
    required :name, :string, validator: {:validate_length, [min: 3, max: 100]}
  end
end

You can also use custom inline validators by supplying the function name as an atom. Similar to most Ecto's built-in validators, the function will receive changeset, key, and opts as the arguments.

defmodule UserPara do
  use Para

  validator :update do
    required :age, :string, validator: :validate_age
    required :gender, :string, validator: {:validate_gender, [allow: :non_binary]}
  end

  def validate_age(changeset, key, opts) do
    # ...
  end
end

Callback validator

Sometimes, you might want to use custom validators or need to perform additional data manipulations. For this, you can use the callback/1 macro.

The callback/1 macro will always be the last function to be called after the validator has parsed and validated the parameters.

defmodule Web.UserPara do
  use Para

  validator :create do
    required :name, :string
    required :age, :integer
    required :email, :string
    optional :phone, :string
    callback :create_validators
  end

  def create_validators(changeset, params) do
    changeset
    |> format_email(params)
    |> format_phone(params)
    |> validate_age()
  end

  def format_email(changeset, params) do
    # ...
  end

  def format_phone(changeset, params) do
    # ...
  end

  def validate_age(changeset) do
    # ...
  end
end