[Haskell-cafe] Problem with overlapping class instances

Ralf Laemmel 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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