I've entered the following example from the Racket Guide into DrRacket
#lang racket
(define-syntax-rule (swap X Y)
(let ([tmp X])
(set! X Y)
(set! Y tmp)))
(let ([tmp 5] [other 6])
(swap tmp other)
(list tmp other))
It gives me the correct result:
> '(6 5)
But when I stepped through it with the macro expander I got:
[Macro transformation]
(module anonymous-module racket
(#%module-begin
(define-syntax-rule
(swap X Y)
(let ([tmp X]) (set! X Y) (set! Y tmp)))
(let ([tmp 5] [other 6])
(let ([tmp tmp]) (set! tmp other) (set! other tmp))
(list tmp other))))
I was expecting to see something like the Racket Guide description in
section 16.1 I.e:
"Racket doesn’t produce the naive expansion for the above use of swap.
Instead, it produces
(let ([tmp 5]
[other 6])
(let ([tmp_1 tmp])
(set! tmp other)
(set! other tmp_1))
(list tmp other)) "
While I do get the correct result, why does the macro expander show me
(let ([tmp tmp] ..... instead of (let ([tmp_1 tmp] .... as the Racket
Guide says I should.
Thanks,
Harry
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