I'm getting an unexpected "Program error:" in a situation where I don't think "error" is being called. Moreover, the error message is bizarre, though I can see in the gobbledy-gook the structure of the piece of my program where things are going awry. The exact message (except for addition of some line breaks) is:
Program error: {thenP_v1700 parser_v1798 ((initialSymbols [] (Num_fromInt instNum_v33 0),Num_fromInt instNum_v33 0),ParseResult_Failed (Posn_Pn_v1551 (Num_fromInt instNum_v33 0) (Num_fromInt instNum_v33 0) (Num_fromInt instNum_v33 0)) "Parse error")}
IIRC, "Program error:" means that a branch was missing from a case expression. It looks like it's in your parsing library, perhaps in its definition of thenP. If you posted the whole program (if it's not too big) we might be able to help further. --KW 8-)