Abstract Building a cost calculus for a parallel program development environment is difficult because of the many degrees of freedom available in parallel implementations, and because of difficulties with compositionality. We present a strategy for building cost calculi for skeleton-based programming languages which can be used for derivational software development and which deals in a pragmatic way with the difficulties of composition. The approach is illustrated for the Bird-Meertens theory of lists, a parallel functional language with an associated equational transformation system. Keywords: functional programming, parallel programming, program transformation, cost calculus, equational theories, architecture independence, Bird-Meertens formalism.

A requirement is a desired relationship among phenomena of the environment of a system, to be brought about by the hardware/software machine that will be constructed and installed in the environment. A specification describes machine behaviour sufficient to achieve the requirement. A specification is a restricted kind of requirement: all the environment phenomena mentioned in a specification are shared with the machine; the phenomena constrained by the specification are controlled by the machine; and the specified constraints can be determined without reference to the future. Specifications are derived from requirements by reasoning about the environment, using properties that hold independently of the behaviour of

This paper concerns the introduction of an iterator into the refinement calculus. The construct is based on concepts from functional programming, and the work gives an interesting example of cross-fertilisation between the functional and imperative programming worlds. Specifically, the iterator construct it..ti uses the idea of a catamorphism — the unique homomorphism from an initial algebra. The datatype for which the iterator is to be defined is considered as an initial algebra of an appropriate functor. The it..ti construct is formally defined as a recursive procedure, and it is shown that, if the value to be obtained by an iteration can be expressed as a catamorphism, then the it..ti construct provides a very natural implementation. Examples are given to show typical uses of the new construct. 1