2014-08-11 7:44 GMT+02:00 Matthew Flatt <mfl...@cs.utah.edu>: > At Sun, 10 Aug 2014 17:01:01 +0200, Jens Axel Søgaard wrote: ... >> 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.
That is great news. That will make it easier make a version of Whalesong that can run in the browser. >> #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. I better begin reading "phase shift" as "context shift" ;-) > 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. Thanks for the explanation. It makes more sense now. /Jens Axel ____________________ Racket Users list: http://lists.racket-lang.org/users
