Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!uunet!mcsun!ukc!edcastle!lfcs!db
From: db@lfcs.ed.ac.uk (Dave Berry)
Newsgroups: comp.lang.misc
Subject: Re: Lazy Evaluation... a simple example
Message-ID: <2434@castle.ed.ac.uk>
Date: 22 Feb 90 14:20:43 GMT
References: <7476@mentor.cc.purdue.edu> <12037@goofy.megatest.UUCP>
Reply-To: db@lfcs.ed.ac.uk (Dave Berry)
Organization: Laboratory for the Foundations of Computer Science, Edinburgh U
Lines: 27

The best explanation I've read of evaluation orders is in Field and
Harrison's book "Functional Programming", pp. 118-130.  Their summary
defines lazy evaluation as:

	Normal-Order Reduction to Weak-Head Normal Form
	+ Sharing
	+ Lazy Constructors

	or, equivalently

	Call By Need + Lazy Constructors

They point out that other combinations are possible, such as Hope+, which
uses Applicative-Order Reduction and Lazy Constructors,


Othis book is a very good introduction to the different kinds of functional
languages around and the different methods of implementing them.  It
also discusses Type Inference and Program Transformation.  I'd recommend
it (even though it resolutely ignores ML throughout).

It's published by Addison-Wesley, ISBN 0-201-19249-7

 Dave Berry, LFCS, Edinburgh Uni.      db%lfcs.ed.ac.uk@nsfnet-relay.ac.uk

"The thought of someone sharing one's own preferences without also sharing
 one's aversions strikes most people as utterly inconceivable." - C. A. Tripp.