# Para
[](https://hex.pm/packages/para)
[](https://github.com/syamilmj/para/actions)
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.
## Why use Para?
When building API endpoints that deal with a lot of parameters, it is sometimes not enough to just rely on database schema for parsing and validation. A lot of times the HTTP parameters do not always represent the final form of the data that gets sent to the database.
Para is meant to be implemented as part of your data processing pipeline, typically between *external source to internal services*. Using Para in this way avoids having to contaminate your controllers and internal services with repetitive inputs parsing and validations. It should also make testing a lot easier.
Para also allows you to define *multiple schemas* inside the same module to promote consistency between your schema and controller files, resulting in better code organizations.
## Usage
Add Para as a dependency in your `mix.exs` file.
```
def deps do
[{:para, "~> 0.4"}]
end
```
### Examples
First, let's define your parameters schema
```elixir
defmodule Web.UserParams 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, droppable: true
end
end
```
You can now use this module as a validator in your controller
```elixir
defmodule Web.UserController do
use Web, :controller
alias Web.UserParams, as: Params
def create(conn, params) do
with {:ok, data} <- Params.validate(:create, params) do
# ...
end
end
def update(conn, params) do
with {:ok, data} <- Params.validate(:update, params) do
# ...
end
end
end
```
## Advanced Usage
Para supports all of Ecto's built-in validators, while also allowing custom validator functions and callbacks.
### Using Ecto's built-in validator as inline validator
To use an inline validator, use the `:validator` option, as such:
```elixir
validator :create do
required :name, :string, [validator: {validate_length: [max: 30]}]
end
```
In the example above we're using Ecto's [`validate_length/3`](https://hexdocs.pm/ecto/Ecto.Changeset.html#validate_length/3) functions to validate the length of of the `:name` parameter.
### Custom inline validator
A custom validator function accepts 3 arguments
```elixir
def validate_age(change, field, opts) do
...
end
```
You can then use it as a custom inline validator:
```elixir
validator :update do
required :age, :integer, validator: :validate_age
end
```
or supply it with options:
```elixir
validator :update do
required :age, :integer, validator: {:validate_age, [less_than: 60]}
end
```
For more advanced usage, please refer to the [docs](https://hexdocs.pm/para/)
