On Wed, 4 May 2011, Andrzej wrote:

> On Wed, May 4, 2011 at 12:07 PM, Andre van Tonder <andre@het.brown.edu> wrote:
>>
>> 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).
>
> You're right. An implementation conforming to R5RS should fail here.
> '=>' is not allowed in the 'else' clause.

No, I claimed that a correct implementation must sucxceed and give 1.  Macro
hygiene reequires it to.  See the explanation below.

>> 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
>
> Ditto. A clause with '=>' must contain a single operand procedure (not '1').

Again, I calim that teh spec requires the answer to be 1, again because of
macro hygiene.  I would suggest that you reread the section on macro expansion:

    If a macro transformer inserts a binding for an identifier (variable or
    keyword), the identifier will in effect be renamed throughout its scope to
    avoid conflicts with other identifiers.

In other words, the => in

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

is renamed during expansion to become (a lambda expression) equivalent to

    (let ((g0 0))
      (map-to-identity g0)

where g0 is a generated identifier, all before MAP-TO-IDENTITY is even
encountered.  This then expands to the equivalent of

    (let ((g0 0))
      (cond (#t g0 1)))

which evaluates to 1.

So the relevant part of the spec that you were missinbg had to do with the
expansion algorithm.

_______________________________________________
Scheme-reports mailing list
Scheme-reports@scheme-reports.org
http://lists.scheme-reports.org/cgi-bin/mailman/listinfo/scheme-reports