We identify three programming language abstractions for the construction of reusable components: abstract type members, explicit selftypes, and modular mixin composition. Together, these abstractions enable us to transform an arbitrary assembly of static program parts with hard references between them into a system of reusable components. The transformation maintains the structure of the original system. We demonstrate this approach in two case studies, a subject/observer framework and a compiler front-end.
We present a type system for a language based on F ≤ , which allows certain type annotations to be elided in actual programs. Local type inference determines types by a combination of type propagation and local constraint solving, rather than by global constraint solving. We refine the previously existing local type inference system of Pierce and Turner[PT98] by allowing partial type information to be propagated. This is expressed by coloring types to indicate propagation directions. Propagating partial type information allows us to omit type annotations for the visitor pattern, the analogue of pattern matching in languages without sum types.
A major problem for writing extensible software arises when recursively defined datatypes and operations on these types have to be extended simultaneously without modifying existing code. This paper introduces Extensible Algebraic Datatypes with Defaults which promote a simple programming pattern to solve this well known problem. We show that it is possible to encode extensible algebraic datatypes in an object-oriented language, using a new design pattern for extensible visitors. Extensible algebraic datatypes have been successfully applied in the implementation of an extensible Java compiler. Our technique allows for the reuse of existing components in compiler extensions without the need for any adaptations.
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