On 08/12/2011 12:08 PM, Sam Tobin-Hochstadt wrote:
On Fri, Aug 12, 2011 at 1:58 PM, Ryan Culpepper<[email protected]> wrote:
On 08/12/2011 11:37 AM, Sam Tobin-Hochstadt wrote:
On Fri, Aug 12, 2011 at 1:25 PM, Danny Yoo<[email protected]> wrote:
In which the first three lines are coming from compile-time, and I see
that my lift-to-toplevel macro is firing off, even though I placed it
in the stop-list of local-expand.
What's happening here is that the `#%module-begin' binding from
`racket/base' calls `local-expand' on each of its forms, to determine
whether its an expression or not. This implements the printing of
top-level expressions. If you replace `#%module-begin' with
`#%plain-module-begin', you should see the desired behavior.
'#%plain-module-begin' also calls 'local-expand' to expose definitions,
requires, etc. When I change Danny's sample code to use
'#%plain-module-begin' instead of '#%module-begin', I get the same output.
`#%plain-module-begin' doesn't seem to explicitly call `local-expand',
in the sense that the Macro Stepper shows. For example, this program:
(module m racket
(#%plain-module-begin (#%expression 3) 4))
when macro-stepped, shows no local-expand steps, but this program:
(module m racket
(#%module-begin (#%expression 3) 4))
does show local-expansion steps.
When a primitive form like '#%plain-module-begin' performs partial local
expansion, the macro stepper shows it as expansion in a context, because
it knows that primitive forms only expand sub-forms, only expand them
once, etc.
In general, when a 'local-expand' happens inside of another 'local-expand',
the outer stop list is discarded. Since '#%plain-module-begin' effectively
calls 'local-expand', I would expect the #'lift-to-toplevel stop list to
never have any effect.
That's not quite true. This program shows the difference between
having it in the stop-list and not:
(module e 'small-lang
(printf "hello world\n")
(#%expression (lift-to-toplevel (printf "ok!"))))
In general, if `lift-to-toplevel' is an expression form, then I'd do
(define-syntax-rule (lift-to-toplevel-aux . rest)
(#%expression (lift-to-toplevel . rest)))
Which I think would fix the problem (even for `racket/base's `#%module-begin').
If it needs to be a top-level form, more tricks are needed.
Ah, that make sense. In pass 1 (uncover definitions), there is a stop
list that includes all primitive forms. In pass 2 (expand expressions),
there is no stop list, so the outer stop list should be in effect again.
So a use of 'lift-to-toplevel' only has to survive pass 1.
Of course, if 'lift-to-toplevel' may contain definitions or require
forms, hiding it from pass 1 by pretending it's an expression may cause
the rest of the module's expansion to go wrong. I assume that's what you
meant by "if it needs to be a top-level form".
Ryan
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