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From: schaut@cat19.cs.wisc.edu (Rick Schaut)
Newsgroups: comp.lang.misc
Subject: Re: What is Lazy Evaluation?
Keywords: expression evaluation, lazy evaluation
Message-ID: <4317@daffy.cs.wisc.edu>
Date: 18 Feb 90 23:34:18 GMT
References: <3109@caesar.cs.montana.edu>
Sender: news@daffy.cs.wisc.edu
Distribution: usa
Organization: U of Wisconsin CS Dept
Lines: 59

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

Lazy evaluation is not the same as short-circuit evaluation.  Lazy evaluation
is where expressions are not evaluated until they are needed.  For example,
if you had the following function definition:

	Func Foo(x,y: integer): integer;
		var z integer;
	begin
		z := y + 5;
		return (z + x / 5);
	end;

and call:

	a = Foo((3+4),7);

the expression, (3 + 4) doesn't get evaluated until the formal parameter, x,
is referenced in the return expression.

This may seem inocuous, but consider the following:

	program

	var a: integer;

	Func Foo(x,y: integer): integer;
	begin
		a = 2;
		return( x * y + a);
	end;

	begin
		a = 3;
		Write(Foo((a + 4),7));
	end.

With normal evaluation, the output is 7 * 7 + 2, or 51.  With lazy evaluation,
however, the expression (a + 4) isn't evaluated until the return is executed
in Foo.  The output is 6 * 7 + 2, or 44.

If you're interested in why someone might want lazy evaluation, see:

Vuillemin, J. [1974].  Correct and optimal implimentation of recursion in
	a simple programming language. _J. Computer and System Sciences
	9:3, 332-354
and

Wadsworth, C [1971].  Semantics and Pragmatics of the Lambda Calculus.
	Ph. D. Thesis, Oxford Univ.

[References from Sethi, R., _Programming Languages: Concepts and Constructs_,
Addison-Wesley, 1989]

--
Rick (schaut@garfield.cs.wisc.edu)

Peace and Prejudice Don't Mix! (unknown add copy)