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{-# OPTIONS -XMultiParamTypeClasses -XFunctionalDependencies -XFlexibleInstances #-}
module HaskellBug where
data Relation c -- The basic Relation
= Rel { relnm :: String -- The name of the relation
, relsrc :: c -- Source concept
, reltrg :: c -- ^Target concept
}
deriving Eq
-- This declaration is ok; should not get an error here
class (Eq concept)=> Association rel concept | rel -> concept where
source, target :: rel -> concept
-- e.g. Declaration Concept -> Concept
sign :: rel -> (concept,concept)
sign x = (source x,target x)
homogeneous :: rel -> Bool
homogeneous s = source s == target s
instance (Eq c)=>Association (Relation c) c where
source = relsrc
target = reltrg
-- This declaration has a kind error
-- The error should be reported here
class (Eq c, Association r c) => Morphic r c where
multiplicities :: r c -> [c]
multiplicities _ = []
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