> I have a base class,Organization, with name and address functions.
> I want classes Buyer and Seller from Organization.
>
> Now if I want to create an 2 instances of Seller
> > data Yahoo = Yahoo
> > instance Organization Yahoo where
> > name _= "Yahoo"
> > addreess = ...
>
> > data DoubleClick= DoubleClick
> > instance Organization DoubleClick where
> > name _ = "DoubleClick"
> ...
>
> Why is [Yahoo,DoubleClick] illegal?
It's illegal because we can't give it a type. It hasn't got
type [Yahoo], nor [DoubleClick]. What you want is to say
"it's a list of things in class Organization, and that's all I will
ever ask of them. In hbc (and soon in GHC) you can say:
data OrganizationThing = forall a. Organisation a => MkOrg a
(Some people would write "exists" instead of "forall", and that's
reasonable. But the type of MkOrg really is
MkOrg :: forall a. Organization a => a -> OrganizationThing
)
Notice that OrganizationThing isn't parameterised.
Now you can say
foo :: [OrganizationThing]
foo = [MkOrg Yahoo, MkOrg DoubleClick]
Further, you can say, for example
toName :: OrganizationThing -> String
toName (MkOrg o) = name o
In effect, an OrganizationThing is anything that in in class
Organization. The MkOrg constructor is a bit annoying (like newtype)
but no more.
Does that help? I'm about to put existentials into GHC. It's
been delayed while I ripped its entrails out; adding existentials
and then keeping types throughout the compiler caused a non-trivial
change that I took advantage of to perform a heart-liver-and-lung
transplant.
Simon
