[Haskell-cafe] Problem with overlapping class instances
Ralf.Laemmel at cwi.nl
Mon Nov 22 15:40:56 EST 2004
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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