I errorneously specified categories as class (Eq object, Eq morphism) => Category id object morphism | id ->, id -> morphism where o :: id -> morphism -> morphism -> Maybe morphism dom, cod :: id -> morphism -> object it should have been class (Eq object, Eq morphism) => Category id object morphism | id -> object, id -> morphism ... ^^^^^^ - but ghci 5.02.2 does not complain. Why? Till Mossakowski -- Till Mossakowski Phone +49-421-218-4683 Dept. of Computer Science Fax +49-421-218-3054 University of Bremen till@tzi.de P.O.Box 330440, D-28334 Bremen http://www.tzi.de/~till