[Haskell-cafe] Problem with overlapping class instances
k.schupke at imperial.ac.uk
Mon Nov 22 17:05:02 EST 2004
The trick here is to use a type to represent the constraint rather
than a class, if possible.
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.
> 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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