Chapter 12
Lazy evaluation & Lists revisited

Aims

• To describe the principles behind Miranda expression evaluation.
• To describe in a more formal way the rules for writing down calculations by hand.
• To develop further the list comprehension notation.
• To examine the consequences for programming style of lazy evaluation. This gives a `data directed' style, as well as the `list of successes' method.

Issues

How important is lazy evaluation?

Lazy evaluation has been postponed until the final part of the text: the principles in Chapters 1-11 would apply equally well in a strict language such as Standard ML, except that calculation in the form we have given fits lazy evaluation more accurately. It would be possible to bring in lazy evaluation at an earlier stage, but I felt that it was only necessary to talk about it in detail at this stage, where it enables

• data-directed techniques which would be grossly inefficient in a strict language;
• infinite lists, which are not available in a strict language.

List comprehensions

We can re-examine earlier examples in the light of the extended list comprehension notation; the delay in introducing the full notation is due to the difficulty I have found in explaining the complete behaviour of list comprehensions with multiple generators and guards.

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