Hi Mark, > > > So what we actually need is a procedure of > > two arguments: `(ellipsis? e ctx)' returns `#t' if the identifier `e' > > is the current ellipsis in the lexical environment of the identifier > > `ctx'. > > Hmm. I don't actually see a need for the second argument, do you? I > can't think of a case where I'd want to 'e' to be different from 'ctx'. >
Let's assume we are writing a macro that reimplements syntax (or some variation thereof) and which has to check whether identifiers are ellipses. For example, the following could be given: (with-ellipsis e (my-syntax a e) Now, this could be a result of a macro expansion and e could carry different marks than with-syntax or my-syntax. This is why I have been thinking that one also needs the lexical context of my-syntax and not only the context of e. The issue I raised has to do with the fact that syntax-objects do not > contain their lexical environments. The 'wrap' of a syntax-object > essentially only contains a set of deferred substitutions to be applied > to the identifiers within the syntax object, if they end up outside of a > quoted datum in the expanded code. The wrap is primarily an efficiency > hack, but also enables the implementation of 'datum->syntax'. If we eliminated the efficiency hack, and also 'datum->syntax', we could > implement identifiers more simply as a record containing two symbols: > the original symbol, and the symbol after all substitutions have been > applied. Identifiers found within quoted datums would be interpreted as > their original symbols, and identifiers found anywhere else would be > interpreted as the symbols with the substitutions applied. > Thanks for the explanation. I have been toying with my own implementation of the syntax-case system. In my implementation the (shared) lexical environments are part of the wraps (so the identifiers are in some way self-contained). Anyway, as I wrote above, the good news is that we can get 'r' from the > dynamic environment. > Will ellipsis? also work outside of macros? Say, what would be the result of the following (run-time) code? (with-syntax e (ellipsis? #'e) > I have a draft patch to add a unary predicate to (system syntax), but > I'd like to work on it a bit more before posting it. > That's great news! Thanks! Best, Marc P.S.: By the way, the module (system syntax) and in particular the procedure syntax-local-binding has already helped me a lot because I needed to attach extra information to symbols and Guile doesn't (yet) support Chez's define-property (well, this would be another feature request).
