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From: raja@copper.ucs.indiana.edu (Raja Sooriamurthi)
Newsgroups: comp.lang.scheme
Subject: Jensen's device [Was: How to pass parameter by reference in scheme?]
Message-ID: <raja.668088763@copper>
Date: 4 Mar 91 12:12:43 GMT
References: <JINX.91Mar2101750@chamarti.ai.mit.edu> <9103040333.AA11169@cymbal.reasoning.com.>
Sender: news@bronze.ucs.indiana.edu (USENET News System)
Organization: Indiana University
Lines: 44

gyro@cymbal.reasoning.COM (Scott Layson Burson) writes:

>   This is not quite passing a variable by reference.  It is closer to
>   Algol 60's call-by-name, and mimics some of its implementations.  If
>   you use this pervasively, you will notice subtle differences with
>   passing parameters by reference (e.g. Jensen's device).

>What is Jensen's device?  This sounds interesting.

Jensen's device is a technique to parameterize procedures using 
call-by-name parameter passing. The below Algol 60 code [from
Horowitz's book] is an example

real procedure sigma (x, j, n);
   value n; real x, integer j, n;
   begin real s;
    s := 0;
    for j := 1 step 1 until n
      do s := s + x;
    sigma := s
  end

By default everything is passed by name in Algol 60 and an explicit
declaration is needed for call-by-value (as in value n)

sigma(A(i), i, 10) returns the sum of the 1st 10 elements in A
A(1)+A(2) ... A(10)

The sum of squares of elements as well as the dot product of two
vectors could be obtained by:

sigma (A(i)*A(i), i, 10) and
sigma (A(i)*B(i), i, 10)


While Jensen's device was an illustration of how procedures could be
parameterized, I feel Scheme's 1st class procedures offer a better
mechanism for parameterization.

- Raja

---
Raja Sooriamurthi                          |   Computer Science Department
raja@cs.indiana.edu	                   |       Indiana University