| class (Monad m) => MRef m r | r->m, m->r | class (Monad m) => Stream m h | h->m | newtype StringReader r = StringReader r | instance (Monad m, MRef m r) => Stream m (StringReader r) | | coverage condition requires that monad type 'm' should occur in stream | handle type 'StringReader r'. With MRef defined as bijective type | function between m and r that is, in fact, true. may be compiler is just not smart | enough to figure this? The coverage condition is certainly conservative -- there are cases where nothing goes wrong if you drop it, and that's what -fallow-undecidable-instances does. The worst that can happen with -fallow-undecidable-instances is that the type checker loops (and even then it'll usually emit an error message); if the program typechecks it'll run ok. | is using -fallow-undecidable-instances safe in this situation? | what of MRef fundeps, r->m or m->r, is really necessary in this case - | i think it is r->m ? I think you are right. You could also try adding 'm' as a phantom type parameter to StringrReader. Simon