Dear all, <br /><br />Supposing that I have a FlowObject datatype such as:<br /><br />data FlowObject = Activity { ...  }  | Start | End | Proceed | ...<br /><br />I could make "open" this datatype by refactoring this definition through a type class + one datatype for each kind of FlowObject. Such as:<br /><br />data Activity = Activity { ... }<br />data Start = Start <br />... <br /><br />class FlowObject a <br /><br />instance FlowObject Activity<br />instance FlowObject Start <br />.... <br /><br /><br />Although the second approach opens the FlowObject datatype, it does not resemble a grammar anymore. So: <br /><br />- Is there any other idiom that opens a datatype keeping its similarities with grammars? <br />- Does exist any idiom that allows a developer to introduce new fields into an existing datatype?<br /><br />Thanks in advance,<br /><br />Rodrigo.<br /><br />