| instance MArray IOUArray a IO => MArray IOUArray (S a) IO where | However, GHC complains: | | /nfs/isd/hdaume/projects/MESumm/Linear2/Linear2.hs:76: | Non-type variables in constraint: MArray IOUArray a IO | (Use -fallow-undecidable-instances to permit this) | But I don't see why this is undecidable. As I understand it, undecidable | instances basically arise when context reduction might not | terminate. Here, it always will (AFAICS). In this case it's not undecideable, but GHC isn't clever enough to spot that. The point is that that each of the types on the left of the => are smaller than the types on the right MArray IOUArray a IO Is smaller than MArray IOUArray (S a) IO But GHC isn't clever enough to spot that. In general it's the halting problem. Doubtless there are useful special cases. Simon