We have shown that several practical examples presented as a justification for
the special treatment of virtual types can in fact be directly programmed in
Ocaml. We achieved this result by decoupling related classes that had to be
recursively defined, previously. Decoupling is made possible in Ocaml by
the use of row variables. A few other languages that have both binary
methods and structural types can also solve virtual types with parametric
polymorphism, as easily.
Grouping can still be allowed, but as a notational convenience rather than
as a fundamental and new construction. Given the simplicity and the
additional flexibility that we have obtained by decoupling classes, we claim
that parametric polymorphism and structural types are preferable to virtual
Indeed, parametric polymorphism is well-established,
easy-to-manipulate and expressive. Its strength is hereby reaffirmed.
Structural types are also commonly used in type theory and almost exclusively
used in higher-order type systems. However, declared types are still common
among programming languages with weaker, usually non parametric, type
Our experience with virtual types certainly reinforces the importance of
structural types. This may be an issue to consider seriously in the design
of type systems for new programming languages.