Allow param transformation with transproc

[waiting-for-dev]

A new :params extension has been added (which in turns removes #params implementation from :url extension).

This new extension adds a Conn#params method which can perform any number of transformations to the request parameters.

When no transformations are given, #params just returns request parameters (both GET and POST) as a hash:

 # http://www.example.com?foo=bar
conn.params #=> { 'foo' => 'bar' }

Further processing can be specified thanks to transproc gem (you need to add it yourself to the Gemfile). All hash transformations in transproc are available:

  # http://www.example.com?foo=bar
conn.params([:deep_symbolize_keys]) #=> { foo: 'bar' }

Extra needed arguments can be provided as an array:

 # http://www.example.com?foo=bar&zoo=zoo
conn.params([:deep_symbolize_keys, [:reject_keys, [:zoo]]) #=> { foo: 'bar' }

Instead of injecting transformations at the moment #params is called, you can configure them to be automatically used.

 # http://www.example.com?foo=bar
conn.
  add_config(:param_transformation, [:deep_symbolize_keys]).
  params #=> { foo: 'bar' }

You can register your own transformation functions:

 # http://www.example.com?foo=bar
fake = ->(_params) { { fake: :params } }
WebPipe::Params::Transf.register(:fake, fake)
conn.params([:fake]) #=> { fake: :params }

Your own transformation functions can depend on the Conn instance at the moment of execution. For that, just place it as the last argument of the function and it will be curried automatically:

 # http://www.example.com?foo=bar
add_name = ->(params, conn) { params.merge(name: conn.fetch(:name)) }
WebPipe::Params::Transf.register(:add_name, add_name)
conn.
  add(:name, 'Joe').
  params([:deep_symbolize_keys, :add_name]) #=> { foo: 'bar', name: 'Joe' }

Inline transformations can also be provided:

 # http://www.example.com?foo=bar
fake = ->(_params) { { fake: :params } }
conn.
  params(fake) #=> { fake: :params }

Besides, router params has also been removed from :url extension and it is now a standalone extension called :router_params.

This extension adds a :router_params transformation to merge router params into Conn#params.

require 'web_pipe'

WebPipe.load_extensions(:router_params)

class MyApp
  include WebPipe

  plug :config
  plug :get_params

  private

  def config(conn)
    conn.add_config(:param_transformation, [:router_params])
  end

  def get_params(conn)
    # http://example.com/users/1/edit
    conn.params #=> { id: 1 }
    conn
  end
end