Hello Simon, Wednesday, September 20, 2006, 7:35:39 PM, you wrote:
Quite right; that's just as specified. The coverage condition is documented in the manual. (GHC 6.4 and Hugs are too liberal, and thereby risk divergence during type checking.) If you want this program to work, use -fallow-undecidable-instances
let's look at the slightly modified program: 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? 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 ? -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com