2014-08-08 7:07 GMT+02:00 Matthew Flatt <mfl...@cs.utah.edu>:
> At Wed, 6 Aug 2014 22:03:17 +0200, Jens Axel Søgaard wrote:
>> The program below produce bytecodes for the program that returns a
>> syntax-object representing 42.
>>
>> The syntax-object looks like this:
>>
>> (#s((stx zo 0)
>> #s((wrapped zo 0)
>> 42
>> (#s((top-level-rename wrap 0 zo 0) #f)
>> #s((top-level-rename wrap 0 zo 0) #f)
>> #s((top-level-rename wrap 0 zo 0) #t)
>> #s((phase-shift wrap 0 zo 0) 0 #f #f #f))
>> clean))))
>>
>> Two questions:
>> 1) Why three top-level-renames and not just one?
>>
>> 2) What is the meaning of the boolean flag?
>
> The boolean apparently indicates whether the rename is for phase 0. It
> should instead be the phase of the rename, where using a boolean for
> the phase is probably a leftover from an original implementation where
> imports were either phase 0 or phase 1.
>
> The three renames are from phases 0, 1, and #f.
Got it.
The next part of the syntax object is:
#s((phase-shift wrap 0 zo 0) 0 #f #f #f))
The form of a phase-shift is
(phase-shift amt src dest cancel-id)
where
amt is #f or an integer,
src and dest are module-path-index?
cancel-id is #f or an integer
The documentation on phase-shift is rather brief.
1) Is it correct that a shift of #f means that only bindings at phase level 0
are shifted to the label phase level?
2) Even though src and dest are documented to be module-path-indices
the example show that #f is a valid value too.
Is it safe to assume that the combination #f #f means toplevel and
"self-module" ?
Or do #f mean "use the same value as before" ?
3) The example shows a phase-shift of 0. At first I didn't get what
the purpose of this.
Then I found a comment in add_renames_unless_module :
/* this "phase shift" just attaches the namespace's module registry: */
form = scheme_stx_phase_shift(form, NULL, NULL, NULL,
genv->module_registry->exports, NULL, NULL);
and this comment in scheme_stx_phase_shift :
/* Shifts the phase on a syntax object in a module. A 0 shift might be
used just to re-direct relative module paths. new_midx might be
NULL to shift without redirection. And so on. */
Is this is the explanation behind the zero phase shift?
4) What is the cancel-id? used for ?
/Jens Axel
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