Tim Docker wrote:
> Steve Downey wrote:
> > So, I've been working on a Composite example. I've used
> > existential types to have a generic proxy to the base
> > type, rather than a simple algebraic type, since adding
> > new leaves to the algebraic type means modifying the whole
> > type, a violation of the Open-Closed principle (open for
> > extension, closed for modification)
>
> Rather than using existential types, a simple record of
> functions can be often be useful. ie:
>
> data Component = Component {
> draw :: String
> add :: Component -> Component
> }
>
> It might be worth comparing this approach with the (more
> complex) one you have described.
The point about existential types is that every class like IComponent
that allow as useful existential like
data Component =
forall e.(IComponent e) => Component e
can be put into the record form Tim mentions. See the old wiki pages at
http://haskell.org/hawiki/ExistentialTypes
This is because every such IComponent has to look like
class IComponent e where
foo1 :: e -> ... -> e
...
bar1 :: e -> ...
...
where the dots in "-> ..." must not contain the type variable e.
Regards,
apfelmus
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