The ticket has been updated with the new deadline. (My message to glasgow-haskell-users@haskell.org should have gone to libraries as well, perhaps gmane's NNTP interface doen't like cross-posting?) On 27/10/2010 09:46, Simon Peyton-Jones wrote:
Libraries folk: see John's proposal below, which would affect libraries.
John: I suggest a longer deadline, say end Nov. Unlike libraries, this one has global implications.
Simon
-----Original Message----- From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell-users-bounces@haskell.org] On Behalf Of John Smith Sent: 26 October 2010 20:42 To: glasgow-haskell-users@haskell.org Subject: Proposal: Require OverlappingInstances for the most specific instance only
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.