0.0.0.3 1.4.3 Generics.

A generic module is a template in which some of the imported interfaces are regarded as formal parameters, to be bound to actual interfaces when the generic is instantiated. For example, a generic hash table module could be instantiated to produce tables of integers, tables of text strings, or tables of any desired type. The different generic instances are compiled independently: the source program is reused, but the compiled code will generally be different for different instances.

To keep Modula-3 generics simple, they are confined to the module level: generic procedures and types do not exist in isolation, and generic parameters must be entire interfaces.

In the same spirit of simplicity, there is no separate typechecking associated with generics. Implementations are expected to expand the generic and typecheck the result. The alternative would be to invent a polymorphic type system flexible enough to express the constraints on the parameter interfaces that are necessary in order for the generic body to compile. This has been achieved for ML and CLU, but it has not yet been achieved satisfactorily in the Algol family of languages, where the type systems are less uniform. (The rules associated with Ada generics are too complicated for our taste.)