Re: [Haskell-cafe] Problem with overlapping class instances

[Keean Schupke]

The problem is that (cw c) overlaps with String. It will still ovarlap 
if you use data decl.
it is the CW that needs to be a datatype. See Below:

   Keean.
Graham Klyne wrote:
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 [1], which talks about representing class constraints by the 
type of its associated dictionary.  Is this is what you mean?

#g
--
[1] http://www.cs.chalmers.se/~rjmh/Papers/restricted-datatypes.ps
spike-overlap-ConceptExpr.lhs
-----------------------------
> 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
Do this:
data CW cw = CW cw
class ConceptWrapper cw c | cw -> c
   wrapConcept :: c -> (CW cw) c -> (CW cw) c
   getConcept :: (CW cw) c -> c
Using this, a ConceptWrapper can be defined to be an instance of
ConceptExpr:
This is line 30:
> instance (ConceptWrapper cw c, ConceptExpr c) => ConceptExpr (cw c) 
where
>     iConcept             = iConcept . getConcept

instance ConceptWrapper (CW cw) c,ConceptExpr c) => ConceptExpr ((CW cw) 
c) where

Error message:
Reading file "D:\Cvs\DEV\HaskellDL\spike-overlap-conceptexpr.lhs":
ERROR "D:\Cvs\DEV\HaskellDL\spike-overlap-conceptexpr.lhs":30 - 
Overlapping inst
ances for class "ConceptExpr"
*** This instance   : ConceptExpr (a b)
*** Overlaps with   : ConceptExpr AtomicConcept
*** Common instance : ConceptExpr [Char]

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