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From: bartlett@WRL.DEC.COM
Newsgroups: comp.lang.scheme
Subject: Re: Internal definitions, DEC Scheme->C
Message-ID: <9006291615.AA28227@jove.pa.dec.com>
Date: 29 Jun 90 16:15:43 GMT
References: <DIGEST.184.900629.021542.6@MC.LCS.MIT.EDU>
Sender: daemon@athena.mit.edu (Mr Background)
Organization: The Internet
Lines: 43

In a previous message, Wayne Schou (math1205@waikato.ac.nz) suggested that
the expression:

	(let ((a 1))
  	     (define (f x)
    	     	     (define b (+ a x))
    		     (define a 5)
    		     (+ a b))
  	     (f 10))

should evaluate to either 20 or 16.

>> As I see the Scheme description given in R^nRS, expressions in the body of f
>> should be evaluated in sequence and therefore the above should return 16.

>> Who agrees/disagrees?

Disagree, according to R3RS,the effect of the program is undefined.
Section 5.2.2 of R3RS defines internal defines in terms of letrec, so
the above expression is really:

	(let ((a 1))
	     (letrec ((f (lambda (x)
	     			 (letrec ((b (+ a x))
	     			 	  (a 5))
	     			 	 (+ a b)))))
		     (f 10)))

The last paragraph of section 4.2.2 of R3RS explains that a letrec of
this form violates an important restriction.  Quoting R3RS "it must be
possible to evaluate each <init> without referring to the value of any
<variable>.  If this restriction is violated, then the effect is
undefined, and an error may be signalled during the evaluation of the <init>s."

>> Also in the DEC Scheme->C implementation the above expression returns the
>> rather confusing value of 15. I assume this is caused by a having the value
>> of 0 when the definition of b is evaluated. I think this is a bug!

>> Who agrees/disagrees?

Disagree as the value of the program is undefined.  I will agree with
you though that error signals on undefined operations are often more
desirable than implementation specific values.