Path: utzoo!attcan!uunet!dino!ux1.cso.uiuc.edu!iuvax!purdue!mentor.cc.purdue.edu!ksb
From: ksb@mentor.cc.purdue.edu (Kevin Braunsdorf)
Newsgroups: comp.lang.misc
Subject: Lazy Evaluation... a simple example
Summary: an example
Keywords: lcons, cons, LISP
Message-ID: <7476@mentor.cc.purdue.edu>
Date: 14 Feb 90 17:28:34 GMT
References: <3109@caesar.cs.montana.edu> <12017@goofy.megatest.UUCP>
Reply-To: ksb@mentor.cc.purdue.edu (Kevin Braunsdorf)
Organization: Purdue UNIX Group
Lines: 32


When I was a student here at Purdue I read a few papers about lazy
evaluation and became interested in it.  Since I was writing a LISP
interp.  for a class project I set about to implement a lazy eval feature.


What I did was much simpler than most of the systems I saw described at
the time.  I simply made `cons' defer evaluation of its arguments until
either `car' or `cdr' was called on the resulting CONSBOX (which was
tagged as being LAZY, and had a saved environment hung on it).

Later I made the deferred one be `lcons' and put back the original `cons'
because `lcons' is slow if you do not need the power.


An example:
	(defun ilister (n) (lcons n (ilister (+ '1 n))))

This function will produce an infinite list of integers, starting at
n.  If we had a function `head' (like the UNIX command)

	(head '10 (ilister '0))

would only compute (about) the first 11 elements in the sequence, and
output:
	(0 1 2 3 4 5 6 7 8 9)


If anyone is interested in a silly Pure LISP interp that does this...
--
Lightning strikes!  Maybe once, maybe twice. Oh, and it lights up the night!
Kevin Braunsdorf, ksb@cc.purdue.edu, pur-ee!ksb, purdue!ksb