This chapter expands on the implementation of type classes in Haskell using dictionary values as proposed by Wadler and Blott (1989) and sketched in Section 4.5. For brevity, we refer to this approach to the use of type classes as HTC. The main emphasis in this chapter is on concrete implementation and we adopt a less rigourous approach to formal properties of HTC than in previous chapters. In particular, we describe a number of optimisations that are necessary to obtain an efficient implementation of HTC - i.e. to minimise the cost of overloading. We do not consider the more general problems associated with the efficient implementation of non-strict functional languages like Haskell which are beyond the scope of this thesis.

Section 7.1 describes an important aspect of the system of type classes in Haskell which means that only a particularly simple form of predicate expression can be used in the type signature of an overloaded function. The set of predicates in a Haskell type signature is usually referred to as the context and hence we will use the term context reduction to describe the process of reducing the context to an acceptable form. Context reduction usually results in a small context, acts as a partial check of satisfiability and helps to guarantee decidability of predicate entailment. Unfortunately, it can also interfere with the use of data abstraction and limits the possibilities for extending the Haskell system of type classes.

The main ideas used in the implementation of HTC are described in Section 7.2 including the treatment of default definitions which were omitted from our previous descriptions.