On Saturday 27 March 2004 16:33, S. Alexander Jacobson wrote:
> Alternatively, is there another way to
> figure out how to create an instance of
> a class like this?
>
> class MyClass a b where emptyVal::a b
>
> Here are one of the thing that don't work:
>
> type MyType a = [a]
> instance MyClass (MyType a) a where emptyVal=[]
>
> And I have no idea why...
Well, you've been told why.
Anyway, why do you insist on using a two parameter type class? The more
general
class MyClass a where emptyVal::a
would do exactly the same. To make type constructor B applied to type C an
instance you just write
instance MyClass (B C) where ...
for instance
instance MyClass [a] where emptyVal=[]
If you enable overlapping instances you could even add
instance MyClass [Int] where emptyVal=[0]
The only difference to your 2-parameter class is that now you can declare
instances for simple types like Integer too. Is that a problem?
Ben
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