#1241: Functional dependency Coverage Condition is lifted, and should not be
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Reporter: guest | Owner:
Type: bug | Status: new
Priority: normal | Milestone: _|_
Component: Compiler (Type checker) | Version: 6.6
Keywords: | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Difficulty: Unknown
Testcase: |
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Comment(by iavor.diatchki@…):
commit fe61599ffebb27924c4beef47b6237542644f3f4
{{{
Author: Iavor S. Diatchki
Date: Sun Jan 13 16:29:10 2013 -0800
Use a version of the coverage condition even with
UndecidableInstances.
This fixes bug #1241 and #2247. When UndecidableInstances are on,
we use the "Liberal Coverage Condition", which is what GHC used to do
in
the past. This is the gist of the check:
class C a b | a -> b
instance theta => C t1 t2
we check that `fvs t2` is a subset of `fd-closure(theta,fvs t1)`.
This is strictly more general than the coverage condition, while
it still guarantees consistency with the FDs of the class. This
check is completely orthogonal to termination (it by no means
guarantees
it).
I am not sure of the role of the "coverage condition" in
termination---
the comments suggest that it is important. This is why, for the
moment,
we only use this check when UndecidableInstances are on.
compiler/typecheck/TcValidity.lhs | 8 ++++-
compiler/types/FunDeps.lhs | 65
++++++++++++++++++++++++++++++++++++-
2 files changed, 71 insertions(+), 2 deletions(-)
}}}
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Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/1241#comment:20
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler
