In the case of overlapping instance declarations, GHC currently requires the less specific instance to be compiled with OverlappingInstances for the more specific instance to be usable. This means that, for example, if you write type ChessBoard = Array (Int,Int) Piece there is no way to instance Show ChessBoard where show board = ... because Array is already an instance of Show, and was not compiled with OverlappingInstances. http://hackage.haskell.org/trac/ghc/ticket/3877 requests that OverlappingInstances be required for only the most specific instance declarations, as suggested in the documentation. "Perhaps the rule should instead say that the overlapping instance declaration should be compiled in this way, rather than the overlapped one ... We are interested to receive feedback on these points." SPJ offered to make this change, if the list users agree. (See comment 6.) Proposed deadline for discussion is 16th November.