Non-strict evaluation improves the expressive power of functional languages at the expense of an apparent loss of efficiency. In this paper we give examples of this expressive power, taking as an example an interactive functional program and describing the programming techniques depending on non-strict evaluation which improved its design. Implementation methods for non-strict languages have delivered poor performance precisely when such programming techniques have been used. This need not be the case, however, and in the second part of the paper we describe Tim, a method of implementing non-strict languages for which the penalty for using lazy evaluation is very small. 1 Introduction Effort in the functional programming community is today divided into two main activities: making efficient implementations of functional languages and exploiting the expressive power of these languages by writing elegant programs. To a large extent these activities are carried out by separate groups of p...