On Wed, 4 May 2011, Andrzej wrote:

> On Tue, May 3, 2011 at 11:35 PM, Andre van Tonder <andre@het.brown.edu> wrote:
>>
>> (define-syntax convert-to-boolean
>>  (syntax-rules ()
>>    ((_ exp)
>>     (cond (exp #t)
>>           (else #f)))))
>>
>>  (let ((else #f))
>>    (convert-to-boolean else))
>>
>> What answer should he get?
>> What answer would he get in your implementation?
>
> My implementation will return #f here (that is once I have macro
> expansion in place) because it expands to:
>
>> (let ((else #f))
>>   (cond
>>     (else #t)
>>     (else #f))) => #f

I am not sure, but I think this might be a syntax error, given that the spec
says:

   The LAST <clause> may be an ``else clause,''

Be that as it may, let me give another example.  I am pretty sure that the
the spec requires the following to evaluate to 1.

(define-syntax nonfalse-identity
   (syntax-rules ()
     ((_ x)
      (cond (x => (lambda (x) x))))))

(let ((else 1))
   (nonfalse-identity else))  ====> 1

but I think your implementation will give the wrong result here (or an error).

Here is anotehr example, which evaluates a given expression (in case it has
side effects) and returns 1.  It is a silly way of doing this, but there is no
doubt that the spec requires the answer to be 1.

(define-syntax map-to-identity
   (syntax-rules ()
     ((_ exp)
      (cond (#t exp 1)))))

(let ((=> 0))
   (map-to-identity =>))   ======> 1

Again, I believe your implementation will return the wrong answer (or an error).

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