Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!uwm.edu!bionet!ig!arizona!gudeman
From: gudeman@cs.arizona.edu (David Gudeman)
Newsgroups: comp.lang.misc
Subject: Re: What is Lazy Evaluation?
Message-ID: <17909@megaron.cs.arizona.edu>
Date: 14 Feb 90 09:31:31 GMT
Organization: U of Arizona CS Dept, Tucson
Lines: 51

In article  <3109@caesar.cs.montana.edu> icsu8209@caesar.cs.montana.edu (Lou Glassy) writes:
>What does 'lazy evaluation' mean?  Is it the same thing as 'short-circuit evaluation?

No, but they are related.  Short-circuit evaluation, as you said,
involves avoiding some computations.  The most common example is
boolean operations such as you showed, however there are other
examples such as

	x := y * f(x);

where f(x) does not have to be called if y = 0 (ignoring the issue of
side-effects).

Lazy evaluation is usually associated with parameter passing.  For
example, in the function

	function f(x,y) = if x > 1 then x else y + y

given the call

	f(h(z),g(z))

it is not necessary to ever call g(z) if h(z) > 1.  This is like
call-by-name in some respects, in that call-by-name also does not
evaluate arguments that are never needed, but with lazy evaluation
(call-by-need) the expression g(z) is evaluated at most once.  The
evaluation sequence goes something like this:

1	call f(h(z),g(z)) where the arguments are passed unevaluated
2	try to evaluate (x > 1)
3	the value of x is needed, so call h(z) and assign the result to x
4	evaluate (x > 1) to get the result
5	(assume the result is TRUE) return the value stored at x

In call-by-name, the steps are

1	call f(h(z),g(z)) where the arguments are passed unevaluated
2	evaluate (x > 1) as (h(x) > 1)
3	(assume the result is TRUE) return the value of x as h(x)

In the call-by-name version, h(x) is called twice.

There are other forms of lazy evaluation as well, the term generally
means, "when you encounter an expression, don't evaluate the
expression until you actually need for it the first time, then store
the result in case you need it again."
-- 
					David Gudeman
Department of Computer Science
The University of Arizona        gudeman@cs.arizona.edu
Tucson, AZ 85721                 noao!arizona!gudeman