This dissertation provides the motivation for and evidence in favour of an approach to language extension via dynamic loading of plug-ins. There is a growing realisation that language features are often a superior choice to software libraries for implementing applications. Among the benefits are increased usability, safety and efficiency. Unfortunately, designing and implementing new languages is difficult and time consuming. Thus, reuse of language infrastructure is an attractive implementation avenue. The central question then becomes, what is the best method to extend languages? Much research has focussed on methods of extension based on using features of the language itself such as macros or reflection. This dissertation focuses on a complementary solution: plug-in compilers. In this approach languages are extended at run-time via dynamic extensions to compilers, called plug-ins. Plug-ins can be used to extend the expressiveness, safety and efficiency of languages. However, a plug-in compiler provides other benefits. Plug-in compilers encourage modularity, lower the barrier of entry to development, and facilitate the distribution and use of experimental language extensions. This dissertation describes how plug-in support is added, to both the front and back-end of a compiler, and demonstrates their application through a pair of case studies.

AspectFun是一個語法近似Haskell語言的函數式剖面導向語言。本論文中探討AspectFun在模組化型態擴充以及泛型程式設計上可扮演的角色。研究首先比較剖面與Haskell語言的type class在處理型態擴充需求議題的可行性，型態擴充議題我們以著名的Expression Problem作為代表(程式語言機制如何確保程式在擴充資料與運算函數的過程中，不需要修改舊有的程式碼，並能確保程式的型態安全)。 我們接著會探討剖面如何以模組化方式實現泛型程式設計。泛型程式設計是指函數接收一額外的型態引數，且函數所執行的運算是依據此型態引數結構來進行的。型態引數是用來表示函數所處理的引數或其回傳值型態為何，但型態引數在函數定義中可能是明確定義的或者隱含的。在此研究中會展示以剖面實現的泛型程式設計更優於使用type class。此外，本研究為使AspectFun可以實現泛型程式設計方法，亦在AspectFun擴充了Existential types與多型互遞迴函數。 / AspectFun is an aspect-oriented functional language with a Haskell-like syntax. This thesis present an study on modular type extension and generic programming using AspectFun. First, we compare the feasibility of using aspects and Haskell's type classes to address the type extension requirements as stated in the famous expression problem (which calls for language mechanisms that can support type-safe program extension in both the dimensions of data types and associated operations, yet neither code duplication nor code rewriting is required.) Second, we investigate how to use aspects to support generic programming in amodular manner. Generic programming means a form of programming in which a function takes a type as argument, and its behavior depends upon the structure of this type. The type argument, which may be explicit or implicit, represents the type of values to which the function is applied, or which the function returns. We show that aspects can do better than type classes in supporting generic programming. In particular, we extend AspectFun with existential types and polymorphic mutual recursion to achieve such a result.