On Sat, 20 Jun 1998, Lennart Augustsson wrote:
> > > class Aggregate a where
> > > toList::(Num b)=>a->[b]
> > > fromList::(Num b)=>[b]->a
> >
> > > data MyAgg =Agg Int Int
> > > instance Aggregate MyAgg where
> > > toList (Agg x y) = [x,y]
> > > fromList [x,y] = (Agg x y)
> The type of toList in the class declaration claims that it can
> produce a list of any type in Num, but with MyAgg you can only get
> a list of Int. So the type of toList in the class is more general
> than the one you have given.
> fromList has a similar problem.
I understand what it is saying. I don't understand why it is a problem.
MyAgg abides by the rules of Aggregate and will never cause a type error
when passed to a function that requires an Aggregate. Why should MyAgg be
illegal?
-Alex-
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S. Alexander Jacobson i2x Media
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