University
of Washington • CSE583 • D. Notkin © 2000
94
What do to?
lThe basic
approach to doing better lies in representing reduction as a
graph reduction process, not a string reduction process; this allows sharing of
computations not allowed in string reductions (Wadsworth)
lA graph-based approach to normal-order evaluation in which
recomputation is avoided (by sharing) is called lazy
evaluation, or call-by-need
–One can prove
it has all the desirable properties of normal-order
reduction and it more efficient than applicative order evaluation.
–Still,
performance of the underlying mechanisms isn’t that great,
although it’s improved a ton