Proposal: Require OverlappingInstances for the most specific
Christian Höner zu Siederdissen
choener at tbi.univie.ac.at
Tue Nov 16 11:02:09 EST 2010
The change should not affect working programs, as it just allows you to
define further places where you say that you want an overlapping
* Serge D. Mechveliani <mechvel at botik.ru> [16.11.2010 16:47]:
> 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
> Can the team issue the corresponding GHC pre-release for testing,
> without making a decision for future before the users report their
> With kind regards,
> Serge Mechveliani
> mechvel at botik.ru
> Glasgow-haskell-users mailing list
> Glasgow-haskell-users at haskell.org
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