Params with default values [feature request?]

Hi folks, consider the following example:

resource :companies do
  params do
    requires :name, allow_blank: false
  end
  post do
    # where `current_user` is the signed in user
    Company.create(declared(params).merge!(seller_id: current_user.id))
  end
end

Wouldn't be nice if we could write something like:

resource :companies do
  params do
    requires :name, allow_blank: false
    # `default` is an example, maybe another name makes more sense
    default :seller_id, default_value: ->() { current_user.id }
  end
  post do
    Company.create(declared(params))
  end
end

It would help keeping our code DRY.

What do you think? Is there some way to do something similar it today?

Note after opening this issue:

I just discover it works:

resource :companies do
  params do
    requires :name, allow_blank: false
    optional :seller_id, default: 1
  end
  post do
    Company.create(declared(params))
  end
end

declared(params) contains seller_id with the proper value 1.

but all these dont work:

optional :seller_id, default: @current_user.id

optional :seller_id, default: ->() { @current_user.id }

module ApiHelpers
  extend Grape::API::Helpers

  def current_user
    @current_user
  end
end

helpers ApiHelpers
...
optional :seller_id, default: current_user.id
# neither
optional :seller_id, default: ->() { current_user.id }
...

Start by reproducing the examples in the params block, comparing static defaults with defaults that reference current_user or a lambda. Trace how optional parameters are processed before declared(params) is built. Done would require an agreed approach for context-dependent defaults, documented behavior, and tests covering the supported form.