Existential types, object-orientation.

[rinus plasmeijer]

Hi Nick,

At 16:48 -0800 05-11-1997, Nick Kallen wrote:
>Existential types are a spiffy feature for psuedo-object orientation as is
>clearly seen in the Clean book in the OO Drawing program chapter. I have a
>few questions related to object-orientation as seen in some of the more
>modern mixed languages like Java and Oberon:
>    Can one can use inheritence or class polymorphism with Existential type=
s
>in Clean? If not, are these concepts even compatible?

I don't understand this question. Can you be more precise?

>    Could dynamic types be implemented in a functional language like clean?
>Exactly how would it work? Could one simulate prototypes in a functional
>language?

Dynamic types are currently being implemented in the new Clean compiler.
Any static type T can be converted to a dynamic type. Via pattern matching=
 one can inspect a dynamic type and unify with other dynamic types.

>My only real use of existential types is seeing the Clean book (I'm pretty
>new to functional programming). It is probable that their potential uses fa=
r
>exceed the limited scope of usage in the book. If anyone can provide any
>interesting problems or data structures implemented with existential types
>(in a manner distinct from that of the Clean book), I'd be appreciative.

You can have a look in the Object I/O library which makes use of existential=
 types.

>Furthermore, if anyone can provide any critical information, relating
>existential types to object-orientation, their limitations, etc.--I'd be
>appreciative.

One of the important limitations of existential typed structures is that the=
 concrete type cannot be inspected from "outside". Dynamic don't have this=
 restriction. Their type can always be inspected but the costs are that the=
 types have to be maintained at run-time, while existential types are static=
 types.

Greetings,

Rinus