Default param values from endpoint variables

You can access things like cookies and request from inside of an endpoint definition, but for my particular application, I need to give the requester the ability to override some params that should otherwise default to Rack environment values.

Example:

# config.ru
require 'grape'

class API < Grape::API
  format :json
  rescue_from :all

  params do
    optional :id, type: Integer, default: -> { cookies[:id] }
    optional :ip, default: -> { request.ip }
  end

  get do
    {
      params_id: params[:id],
      endpoint_id: cookies[:id],
      params_ip: params[:ip],
      endpoint_ip: request.ip,
    }
  end
end

run API

$ rackup
$ curl 'localhost:9292' && echo
{"error":"undefined local variable or method `cookies' for #<Grape::Validations::ParamsScope:0x007faec2846fe8>"}
$ curl 'localhost:9292?id=1' && echo
{"error":"undefined local variable or method `request' for #<Grape::Validations::ParamsScope:0x007faec2846fe8>"}
$ curl 'localhost:9292?id=1&ip=whatever' && echo
{"params_id":1,"endpoint_id":null,"params_ip":"whatever","endpoint_ip":"127.0.0.1"}

I could accomplish this with one-off lines like params[:id] ||= cookies[:id] in the endpoint, but I have to do this for a bunch of different endpoints, so I'd like to just be able to reuse the params (via use :such_and_such).

How hard would it be to allow defaults access to these Grape::Endpoint instance variables? It looks like you'd have access to cookies, headers, env, request, etc. at the point where you run the validators, and default values are populated via validation. But I don't want to just kludge a patch in there myself if there's a more elegant way of doing it than passing every-variable-under-the-sun to the validate! method.

Let me know what you think!

Start in lib/grape/endpoint.rb around the validator call linked in the issue, then read lib/grape/validations/validators/default.rb where default values are populated. Trace how validation context is passed and compare the documented cookies, request, headers, and env examples. Done means reusable parameter defaults can access endpoint request values while explicitly supplied parameters still take precedence.