Proposal: Require OverlappingInstances for the most specific instance only

[Christian Höner zu Siederdissen]

The change should not affect working programs, as it just allows you to
define further places where you say that you want an overlapping
instance.

Gruss,
Christian

* 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 
> 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 at botik.ru
>  
> 
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