Re: [Chicken-users] Redefining macros and special forms

[Peter Bex]

On Tue, Oct 28, 2014 at 09:31:44PM +0100, Michele La Monaca wrote:
> Hi,
> 
> shadowing a macro doesn't seem to work properly in all the cases:
> 
> (define-syntax my-begin (syntax-rules () ((_ x ...) (begin x ...))))
> (let ((my-begin -)) (my-begin 0 1)) ; => -1 (ok)
> (define my-begin -)
> (apply my-begin '(0 1))             ; => -1 (ok)
> (my-begin 0 1)                      ; =>  1 (oops)
> 
> Thus `my-begin' acts as either a procedure or a macro depending on the 
> context.
> 
> Redefining `begin' (or even `##core#begin') has the same
> unsatisfactory behavior:
> 
> (let ((begin -)) (begin 0 1)) ; => -1
> (define begin -)
> (apply begin '(0 1))          ; => -1
> (begin 0 1)                   ; =>  1
> 
> Is this the expected behavior?

Yes, this is according to spec.  Macros aren't first-class, so whenever
you use the same identifier in a non-application context it will look up
the identifier in the runtime environment.  In application context it
will check the syntactic (compile-time) environment first.

I agree this is surprising as Scheme is touted to be a Lisp-1, but this
is just one of those nasty dark corners of the spec.

Cheers,
Peter
-- 
http://www.more-magic.net