Interactive Functional Programs: a Method and a Formal Semantics

In this paper we present a model of interactive programs in a purely functional style. We exploit lazy evaluation in the modelling of streams as lazy lists. We show how programs may be constructed in an ad hoc way, and then present a small set of interactions and combinators which form the basis for a disciplined approach to writing such programs. One of the difficulties of the ad hoc approach is that the way in which input and output are interleaved by the functions can be unpredictable. In the second half of the paper we use traces, i.e. partial histories of behaviour, to explain the interleaving of input and output, and give a formal explanation of our combinators. We argue that this justifies our claim that the combinators have the intuitively expected behaviour, and finally contrast our approach with another