[ ghc-Bugs-1277810 ] ghc is confused about instances of Ord

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Bugs item #1277810, was opened at 2005-08-31 23:39
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Category: Compiler (Type checker)
Group: 6.4
Status: Open
Resolution: None
Priority: 5
Submitted By: Robin Green (greenrd)
Assigned to: Nobody/Anonymous (nobody)
Summary: ghc is confused about instances of Ord

Initial Comment:
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. 

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