RE: ghc-CVS 001024: minor hslibs dependency bug
The dependency "concurrent" should be added to line 11 of /hslibs/net/Makefile so that it becomes: HSLIB_DEPS = lang text concurrent After that fix, 'make boot' goes through. Thanks, I've applied your fix. Cheers, Simon ___ Glasgow-haskell-bugs mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs
False duplicate or overlapping instances message
When I compile the attached file on Linux with 4.08.1: /home/ger/ghc-4.08.1-binary/bin/ghc -c Instances.hs -fglasgow-exts -fallow-overlapping-instances -fallow-undecidable-instances I get the message: Instances.hs:1: Duplicate or overlapping instance declarations for `HasConfig (arcTypeConfig value) (arcTypeParms value)' defined at Instances.hs:19 and defined at Instances.hs:11 This is not fair, because while the instances for HasConfig have the potential to conflict in the future, they will only do so should the classes NodeTypeConfigParms and ArcTypeConfigParms overlap. Instances.hs
Re: False duplicate or overlapping instances message
Instances.hs:1:
Duplicate or overlapping instance declarations
for `HasConfig (arcTypeConfig value) (arcTypeParms value)'
defined at Instances.hs:19 and defined at Instances.hs:11
This is not fair, because while the instances for HasConfig have the
potential to conflict in the future, they will only do so should the classes
NodeTypeConfigParms and ArcTypeConfigParms overlap.
It is fair. Let's rename your type variables to make what's going on a little clearer:
main = return ()
class HasConfig a b where
($$) :: a - b - b
class NodeTypeConfigParms a b where
nodeTypeConfig :: a c - b c - b c
instance (NodeTypeConfigParms a b) = HasConfig (a c) (b c) where
($$) = nodeTypeConfig
class ArcTypeConfigParms a b where
arcTypeConfig :: a c - b c - b c
instance (ArcTypeConfigParms a b) = HasConfig (a c) (b c) where
($$) = arcTypeConfig
You can now see that the two instance declarations overlap: their
right hand sides are in fact *identical*. Remember that the
typechecker simply matches on the right-hand sides ("heads") of the
instance declarations.
If you do -fallow-undecidable-instances, I think your program will
work (in the Prolog-ish backtracking way), but note that if
NodeTypeConfigParms and ArcTypeConfigParms are ever given instances at
the same pair of types, the value of ($$) will be undefined.
--KW 8-)
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Re: False duplicate or overlapping instances message
Keith Wansbrough wrote:
Instances.hs:1:
Duplicate or overlapping instance declarations
for `HasConfig (arcTypeConfig value) (arcTypeParms value)'
defined at Instances.hs:19 and defined at Instances.hs:11
This is not fair, because while the instances for HasConfig have the
potential to conflict in the future, they will only do so should the classes
NodeTypeConfigParms and ArcTypeConfigParms overlap.
It is fair. Let's rename your type variables to make what's going on a little
clearer:
main = return ()
class HasConfig a b where
($$) :: a - b - b
class NodeTypeConfigParms a b where
nodeTypeConfig :: a c - b c - b c
instance (NodeTypeConfigParms a b) = HasConfig (a c) (b c) where
($$) = nodeTypeConfig
class ArcTypeConfigParms a b where
arcTypeConfig :: a c - b c - b c
instance (ArcTypeConfigParms a b) = HasConfig (a c) (b c) where
($$) = arcTypeConfig
You can now see that the two instance declarations overlap: their
right hand sides are in fact *identical*. Remember that the
typechecker simply matches on the right-hand sides ("heads") of the
instance declarations.
No they do not overlap, unless there is an a b satisfying NodeTypeConfigParms a b
and ArcTypeConfigParms a b. Which there aint.
If you do -fallow-undecidable-instances, I think your program will
work (in the Prolog-ish backtracking way), but note that if
NodeTypeConfigParms and ArcTypeConfigParms are ever given instances at
the same pair of types, the value of ($$) will be undefined.
As a matter of fact I DID use -fallow-undecidable-instances, as you can see
from my original contribution . . .
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Re: False duplicate or overlapping instances message
Keith Wansbrough wrote:
You can now see that the two instance declarations overlap: their
right hand sides are in fact *identical*. Remember that the
typechecker simply matches on the right-hand sides ("heads") of the
instance declarations.
No they do not overlap, unless there is an a b satisfying NodeTypeConfigParms a b
and ArcTypeConfigParms a b. Which there aint.
Haskell makes the "open world" assumption... there may be new
instances added later, and the behaviour of a well-typed program
should not change if this occurs.
I'll defer further discussion of this to the experts (Mark Jones?).
I think I've worked out what's going on now. But I don't like it.
When I use -fallow-undecidable-instances and -fallow-overlapping-instances
(as I did) I was assuming (like Keith Wansbrough did) that GHC would do a
Prolog-style backtracking search when it was time to resolve an overloading,
and would only complain if there were more or fewer than one chain of inferences.
Instead Haskell eagerly tries to anticipate possible conflicts, which is a nuisance
when it is obvious (as it is to me in this case) that such conflicts are unlikely
to arise. For a simpler example, imagine that we have two classes
Integral a (things corresponding to integers) and String a (things corresponding to
strings). It is a pity that we cannot write
instance Integral a = Show a
and
instance String a = Show a
just because someone may come along later on and try to show something which is an
instance of both Integral and String. (Though obviously if they do, we DO need an
error message.)
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