On 05/21/2012 08:13 PM, Harry Spier wrote:
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)) "
The Guide should really say that it produces something *equivalent* to the code above. It doesn't actually change the names; the information is stored "out of band" as Mark and Rename wraps attached to the syntax object. The macro stepper renders Marks using colors: some of the 'tmp's should be red (or some other color besides black).
Ryan ____________________ Racket Users list: http://lists.racket-lang.org/users
