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Abstract: The main difference between
LISP and ML comes from their types system: dynamic or static typechecking.
Each community can promote either the freedom from the dynamic typing for
LISP
or the safety given from the static typing for ML. There are some cases where
a full static typing must be opened to integrate new features as
object-oriented features and persistence of values.
For example, the {\OCAML} object extension has some high level features
as the
multiple inheritance and parametric classes. This extension distinguishes
inheritance and subtyping. By using the late binding for method call in
conjunction with the subtyping relation it gives a kind of inclusion polymorphism
from the objects languages.
Nevertheless we can not build a classical classes hierarchy mainly because it
is impossible to ``downcast'' an object to a subclass.
We propose here to present a light extension, realized in {\tt{camlp4}}, to
allow ``cast'' this transtyping between objects belonging to a
same classes hierarchy. With this we can build some ``design patterns'',
which are not allowed in pure {\OCAML}, and we can implement a safe mechanism
for objects persistence..
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