The example of the FOC system illustrates a common situation that calls for hybrid mechanisms for code structuring that would more elegantly combine the features of modules and classes. This is an active research area, where several solutions are currently explored. Let us mention in particular ``mixin'' modules and objects with ``views''. The former enrich the ML modules with inheritance and a late binding mechanism [18, 4, 5]. The latter provide a better object-encapsulation mechanism, in particular in the presence of binary operations and ``friend'' functions; views also allow to forget or rename methods more freely [68, 71].

Other object-oriented languages, such as CLOS, detach methods from objects, transforming them into overloaded functions. This approach is becoming closer to traditional functional programming. Moreover, it extends rather naturally to multi-methods [13, 22, 8] that allow to recover the symmetry between the arguments of a same algebraic type. This approach is also more expressive, since method dispatch may depend on several arguments simultaneously rather than on a single one in a privileged position. However, this complicates abstraction of object representation. Indeed, overloading makes abstraction more difficult, since the precise knowledge of the type of arguments is required to decide what version of the method should be used.