[Haskell-cafe] Problem with overlapping class instances
gk at ninebynine.org
Tue Nov 23 09:01:05 EST 2004
>At 22:05 22/11/04 +0000, Keean Schupke wrote:
>The trick here is to use a type to represent the constraint rather
>than a class, if possible.
Hmmm, I'm not sure that I understand what you mean.
Considering my example (repeated below), is it that 'AtomicConcept' should
be an algebraic datatype rather than just a type synonym? Or is there more?
Or... I just found John Hughes 1999 paper on Restricted Data Types in
Haskell , which talks about representing class constraints by the type
of its associated dictionary. Is this is what you mean?
> type AtomicConcepts a = [(AtomicConcept,[a] )]
> type AtomicRoles a = [(AtomicRole ,[(a,a)])]
> type TInterpretation a = ([a],AtomicConcepts a,AtomicRoles a)
> class (Eq c, Show c) => ConceptExpr c where
> iConcept :: Ord a => TInterpretation a -> c -> [a]
> type AtomicConcept = String -- named atomic concept
Declare AtomicConcept and AtomicRole as instances of ConceptExpr and RoleExpr
(AtomicRole is used by AL, and including AtomicConcept here for completeness).
> instance ConceptExpr AtomicConcept where
> iConcept = undefined
To allow a common expression to support multiple description logics,
we first define a wrapper class for DLConcept and DLRole:
> class ConceptExpr c => ConceptWrapper cw c | cw -> c where
> wrapConcept :: c -> cw c -> cw c
> getConcept :: cw c -> c
Using this, a ConceptWrapper can be defined to be an instance of
This is line 30:
> instance (ConceptWrapper cw c, ConceptExpr c) => ConceptExpr (cw c) where
> iConcept = iConcept . getConcept
Reading file "D:\Cvs\DEV\HaskellDL\spike-overlap-conceptexpr.lhs":
ERROR "D:\Cvs\DEV\HaskellDL\spike-overlap-conceptexpr.lhs":30 - Overlapping
ances for class "ConceptExpr"
*** This instance : ConceptExpr (a b)
*** Overlaps with : ConceptExpr AtomicConcept
*** Common instance : ConceptExpr [Char]
>Ralf Laemmel wrote:
>>Instance selection and thereby overlapping resolution
>>is *independent* of constraints. It is defined to be purely
>>syntactical in terms of instance heads. See the HList paper
>>for some weird examples.
>>Graham Klyne wrote:
>>>The reported overlapping instance is [Char], which I take to be derived
>>>from the type constructor  applied to type Char, this yielding a form
>>>that matches (cw c). But the instance ConceptExpr (cw c) is declared to
>>>be dependent on the context ConceptWrapper cw c, which has *not* been
>>>declared for the type constructor .
>>>GHCi with -fglasgow-exts is no more informative.
>>>What am I missing here?
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