Certain classes of applications are naturally described as a network of
cooperating components where each component reacts to input as and
when it becomes available This paper builds on previous work on ex
pressing IO and statebased algorithms safely in a functional language
and presents a new way of expressing reactive systems in a functional
language that does not violate vital semantic properties The approach
taken is a pragmatic one in that it integrates constructs for expressing
reactive systems within existing framework for writing IO bound state
based programs functionally The original contributions of this paper are twofold rst we show
how the existing monadic IO model can be extended to cope with non
determinism and secondly we introduce primitives for evaluating IO
actions concurrently