I dare you to change { } to do ... end. I double dare you, even.

gistfile1.rb

private def sanitise_recursively(params)
  params.map { |key, value|
    safe_value = case value
      when Hash;   Hash[sanitise_recursively(value)]
      when Array;  sanitise_recursively(value)
      when String; @sanitiser.call(value)
      else         value
    end
  
    [ key, safe_value ]
  }
end