FW: Proposal: Require OverlappingInstances for the most specific
simonpj at microsoft.com
Wed Oct 27 03:46:16 EDT 2010
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.
From: glasgow-haskell-users-bounces at haskell.org [mailto:glasgow-haskell-users-bounces at haskell.org] On Behalf Of John Smith
Sent: 26 October 2010 20:42
To: glasgow-haskell-users at 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
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