| The error above is telling you that there's no instance for 'MArray b | FlatVector IO' - this is because you haven't specified the array type | 'b' into which thaw should convert the array. The error message is a | bit misleading; it's really an ambiguity rather than a missing instance | (I don't know whether it could be reasonably improved, though). That is indeed horrible. Duly improved. Foo.hs:16:7: Ambiguous type variable `b' in the constraint: `MArray b FlatVector IO' arising from use of `thaw' at Foo.hs:16:7-10 Probable fix: add a type signature that fixes these type variable(s) Simon