subtyping in gephex-0.5

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The simplifications of the unit and link type system make
it possible to get a very cool thing working without
much work: subtyping of link types.

We can simply use the subtyping of c++.

Consider the following relation.
In the C++ implementation the respective classes
use inheritance to model it.
(LinkType and PackedImage are abstract base classes)

~   LinkType -> PackedImage ->  PackedBGRAImage
~                           |
~                            -> PackedYUVImage

The only extra thing we would have to implement is to
tell the core about this subtype relation.
This could be done in the link type spec:

~   link PackedBGRAImage
~   {
~    basetype    = "PackedImage"
~    author      = "..."
~    version     = "1.0"
~    [...]
~    }

While this is somewhat redundant to the inheritance
in C++ I do not see an easy way to avoid it.

I think that the extra complexity in the core would be
very small (maintain subtype relation, type check of
connection uses transitive closure).

Georg
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