example1.rb

class MailboxInputValidator < Dry::Validation::Schema
  key(:per_page) { |pp| pp.int? & pp.gt?(0) & pp.lteq?(100) }
  key(:page) { |p| p.int? & p.gt?(0) }
end

class MailboxInput < Dry::Data::Struct
  attribute :page,     'coercible.int'
  attribute :per_page, 'coercible.int'

  def offset
    (page - 1) * per_page
  end
end

# Functions is a transproc registry
MailboxInputMapper = Functions[:accept_keys, ['page', 'per_page']] >> Functions[:symbolize_keys!]

# in your controller / endpoint

input = {page: 1, per_page: 10}.merge!(MailboxInputMapper[params])
errors = validator.messages(input)

if errors.empty?
  # everything is fine
  input = MailboxInput.new(input)
  
  mails = mailbox_store.load(limit: input.per_page, offset: input.offset)
else
  # render bad request
end