At Sun, 10 Aug 2014 17:01:01 +0200, Jens Axel Søgaard wrote: > 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:
Answers below, but first a big caution: While these details have been the same for a while, I hope and expect the representation of syntax objects to change in the coming months. I have been threatening to rewrite the macro expander (yes, in Racket) for a while, but things are lining up so that I can actually start trying soon. > #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? I'm afraid I could only look for the answer in the same place in the implementation where you must have found the "phase shift to #f shifts only phase-0 bindings" comment. So, yes. > 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" ? I believe it means "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? Yes, that sounds right. > 4) What is the cancel-id? used for ? It's for yet another job of "phase shift" entries: to indicate the boundaries between information for a submodule context and information for the enclosing module's context. Roughly, a cancel ID causes a later module context to be ignored for the purposes of determining an identifier's context. The value of the cancel-ID field is matched against an identity field of each module-rename record. You could see get_old_module_env() for more, but I have trouble understanding and trusting this piece myself. I think it's a hack that has worked just well enough... except for Typed Racket, where submodule expansion doesn't work right, probably due to this part. ____________________ Racket Users list: http://lists.racket-lang.org/users
