Hello,
I recently posted this as a Stackoverflow question, but upon the advice of some
of the commenters, I've decided to ask here instead. The SO question, for
reference, is this one:
http://stackoverflow.com/questions/13449842/why-doesnt-dr-racket-expand-nested-macros
What follows is the text of the
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 a b c d
e)) 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 b c d e)). 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 a b c d e) expands to (bar a b c d e)
to (foo a b c d e) to (+ 1 (foo b c d e)), 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 a b c d e) expands to (+ 1 (blah b c d e)) and then
stops.__________________________________________________________________________________________
Thank you for your help,
Sandro
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