ghc is confused about instances of Ord
When I compile the attached file _without_ -fallow-incoherent-instances
on ghc 6.4 on Fedora Core Linux, I receive the following errors:
PartialOrder.hs:19:0:
Overlapping instances for PartialOrder a
arising from the superclasses of an instance declaration at
PartialOrder.hs:19:0
Matching instances:
PartialOrder.hs:11:0: instance (Ord a) => PartialOrder a
PartialOrder.hs:19:0: instance PartialOrder (LineList a)
(The choice depends on the instantiation of `a'
Use -fallow-incoherent-instances to use the first choice above)
In the instance declaration for `PartialOrder (LineList a)'
PartialOrder.hs:20:53:
Overlapping instances for PartialOrder a
arising from use of `lte' at PartialOrder.hs:20:53-55
Matching instances:
PartialOrder.hs:11:0: instance (Ord a) => PartialOrder a
PartialOrder.hs:19:0: instance PartialOrder (LineList a)
(The choice depends on the instantiation of `a'
Use -fallow-incoherent-instances to use the first choice above)
In the first argument of `zipWith', namely `lte'
In the first argument of `and', namely `(zipWith lte c d)'
In the definition of `lte':
lte (LineList c) (LineList d) = and (zipWith lte c d)
However, when I compile it _with_ -fallow-incoherent-instances, I
receive _these_ errors:
PartialOrder.hs:19:0:
No instance for (Ord a)
arising from the superclasses of an instance declaration at
PartialOrder.hs:19:0
Probable fix: add (Ord a) to the instance declaration superclass
context
In the instance declaration for `PartialOrder (LineList a)'
PartialOrder.hs:20:53:
Could not deduce (Ord a)
from the context (PartialOrder (LineList a), Eq (LineList a))
arising from use of `lte' at PartialOrder.hs:20:53-55
Probable fix: add (Ord a) to the class or instance method `lte'
In the first argument of `zipWith', namely `lte'
In the first argument of `and', namely `(zipWith lte c d)'
In the definition of `lte':
lte (LineList c) (LineList d) = and (zipWith lte c d)
So in the _first_ compilation run, ghc is suggesting that an instance of
Ord a (which I believe is a figment of its imagination) collides; but in the
_second_ compilation run, it complains that it can't _find_ the instance for
Ord a. Surely an inconsistency.
Trac metadata
Trac field |
Value |
Version |
6.4 |
Type |
Bug |
TypeOfFailure |
OtherFailure |
Priority |
normal |
Resolution |
ResolvedInvalid |
Component |
Compiler (Type checker) |
Test case |
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Differential revisions |
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BlockedBy |
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Related |
|
Blocking |
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CC |
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Operating system |
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Architecture |
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