How to set type and desc params with a function with Grape/Rails API

Question

I am writing an API with Grape for may Rails 6 application.

I have a model with custom columns (depending on specification). I generate a patch request parameters list, and I would like to generate desc and type with a function. But I do not know if it is possible, and if yes, where to put the function (all I tried do not work).

This is a part of my Api class.

class MyObject < Grape::API
  # ...

  desc 'Update a MyObject.'
  params do
    requires :id, type: Integer, desc: 'MyObject ID.', documentation: {param_type: 'body'}
    optional :title, type: String, desc: 'MyObject Title.', documentation: {param_type: 'body'}

    optional :custom_field_attributes, type: Hash, documentation: {param_type: 'body'} do
      CustomField.column_names.each do |cf|
        next unless cf.include? 'custom_'

        optional cf.intern,
                 type: custom_field_type(cf), # Here I would like to call a function
                 desc: custom_field_desc(cf) # Here too
      end
    end
  end
  patch do
    # ...
  end
end

Is it possible to do this ?