We claim that both audiences can be addressed through stepwise
enhancement.
The method can be explained directly in term of programming techniques
applied to simple programs, and also given a more theoretical basis in
terms of higher order functions. We review stepwise enhancement, sketch how
to capture an enhancement as a higher order function adapted from
foldr,
and then sketch how individual enhancements are specialisations of the
particular foldr predicate. We show how this is closely related to the new
ideas on shape and polytypism being discussed in the functional programming
community (Jay and Cockett, 1994), (Jay, 1995), (Belle et al., 1996)
(Jeuring and Jansson, 1996), (Jansson and Jeuring, 1997).
We go on to generalise this work in several ways, utilising key
features of logic programming such as nondeterminism and flexible modes,
and show how foldl can also be adapted.