to be fair, it also don't work with Hugs 03 and Hugs 05 data UnboxedMutableArray i e = UMA !i !i type IOUArray i e = UnboxedMutableArray i e data Dynamic a i e = Dynamic (a i e) type DynamicIOUArray s = Dynamic IOUArray if second line substituted with the following type IOUArray = UnboxedMutableArray then all compilers are happy. i don't know, may be i should appeal to Haskell' committee instead of compiler developers (but you are the same persons, anyway :) btw, the kinds are trickiest language part, especially in Hugs what don't supports kind annotations. in one case compiler don't accept partial type (as here), in other case compiler don't accept "extraneous" parameters. as net result, making type definition that will work everywhere is a kind of shamanism -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com