On Wed, 4 May 2011, Andrzej wrote:
On Tue, May 3, 2011 at 11:35 PM, Andre van Tonder <[email protected]> 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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