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.
Keean
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
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
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]
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.
Ralf
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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