为什么Racket博士不展开嵌套宏？

Question

I am using Dr. Racket, version 5.3.1. I am trying to use the Macro Stepper feature, and am having problems with "nested" macros. (By "nested" macros, I mean macros that expand to a form which contains more (used-defined) macros. I don't know if this is the correct terminology). The macro stepper only expands these macros once, and then doesn't bother to continue expanding.

For example, I type the following into the Dr. Racket definitions area:

#lang racket

(define-syntax foo
  (syntax-rules ()
    ((foo a) 1)
    ((foo a stuff ...) (+ 1 (foo stuff ...)))))

(foo a b c d e)

Running this returns 5, as expected. Furthermore, typing (expand '(foo abcde)) in the Interactions window yields the syntax '(#%app + '1 (#%app + '1 (#%app + '1 (#%app + '1 '1)))) , also as expected. However, going into the Macro Stepper (with standard macro hiding) and pressing the End button, I only see (+ 1 (foo bcde)) . If I disable macro hiding, I get the expected result, but also a whole lot of line noise that I'd rather not see.

Is this a bug, or expected behaviour? I swear that Dr. Racket didn't used to behave like this...

I actual submitted a bug report about this a month ago ( http://bugs.racket-lang.org/query/?cmd=view&pr=13203 ), but then I started having second thoughts about whether it was a bug or not, so I decided to ask here.

PS - other random notes about this:

It seems to depend on whether or not the nested macro is the outer-most expression in the expanded form. For example, if I define (in addition to foo):

(define-syntax bar
  (syntax-rules ()
    ((bar xs ...) (foo xs ...))))

(define-syntax baz
  (syntax-rules ()
    ((baz xs ...) (bar xs ...))))

(baz a b c d e)

Then the macro stepper shows me that (baz abcde) expands to (bar abcde) to (foo abcde) to (+ 1 (foo bcde)) , but then it stops.

The previous example might make you think it has something to do with macros that expand to themselves, but this doesn't appear to be the case. For example, if I redefine foo as follows:

(define-syntax foo
  (syntax-rules ()
    ((foo a) 1)
    ((foo a stuff ...) (+ 1 (blah stuff ...)))))

With

(define-syntax blah
  (syntax-rules ()
    ((blah xs ...) 10)))

Then (foo abcde) expands to (+ 1 (blah bcde)) and then stops.

Answer 1

I'm not sure if this is a bug or not, although the foo in the expanded code is a different color then the original foo, so I'm inclined to think that is what is confusing it.

By default, the macro stepper tries to simplify the expanded macro to show only macros defined in the module, and files required with strings (ie files from your 'project'). This is the feature that seems to be throwing it off.

One way you can work around this (at least in smallish files) is to disable this feature. To do this, go to the bottom left of the window where it says: Macro hiding: , click the drop down menu and select Disable .

Macro hiding: -> Disable

From there, you should see the entire expansion of your module, including the expansion of foo .