On Tue, Oct 26, 2010 at 09:41:58PM +0200, John Smith wrote:

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.

I always expected that such programs as above must be valid. On the other hand, I do not understand these possible ways of compilation, these words about "compiled in this way, rather than the overlapped one". Now, as I already have overlapping instances work in my programs, for a long time, it is difficult for me to predict the consequences of the change. Can the team issue the corresponding GHC pre-release for testing, without making a decision for future before the users report their impression? With kind regards, ----------------- Serge Mechveliani mechvel@botik.ru