On Tue, Apr 21, 2009 at 3:04 AM, Derick Eddington <derick.edding...@gmail.com> wrote: > On Mon, 2009-04-20 at 22:04 -0400, Lynn Winebarger wrote: > [...] > But for explicit phasing systems, such as Larceny and PLT, I don't think > a result of #T makes sense because `cons' and `kons' are unbound at > phase -1 and unbound identifiers are compared according to their > spelling. If they are supposed to be bound, that's either implicit > phasing or confusing syntax template semantics.
Right, PLT's explicit phasing is exactly why this is interesting. I should have been clearer that the three points I offered are hypothetical reasons for the inconsistency rather than expressions of my preferences. > I think Larceny's phase mismatch detection is very preferable to > allowing separate namespaces (that would be very unSchemely) for > different phases such that `cons' would be allowed unbound at phase -1 > but bound at the phases it's imported for. And this detection is > consistent with the fact that R6RS requires preventing importing the > same name but different bindings at different phases. Ok, I hadn't noticed the latter restriction in R6RS. >> There are multiple issues: >> 1) The syntax violation is getting triggered too early. > > How can you tell? > >> There's an >> echo of Will's complaint that R6RS requires compiling programs with >> obvious errors to give error messages at run-time > > That temp2.sps is being compiled and run in the same process, so how can > you tell when the error is happening? I think it probably actually is > happening at expand-time. Exactly - evidently the error should wait at least until it appears on the right hand side of a macro definition. R6RS has a lot of requirements like that - the compiler has to generate code that will definitely signal an exception at run-time, even if it's provable at compile time that the exception will be signaled. >> 2) The binding is getting resolved too early. It shouldn't be >> resolved until it appears on the right hand side of a syntax binding >> form. Consider whether the following program should produce 'a or 7: > > Because Scheme is lexically scoped and maintaining that is a primary > purpose of R6RS syntax templates, it's obvious to me that `foo' should > be bound to a syntax object representing the identifier `x' from the > lexical scope where that identifier occurs, and that identifier refers > to the `x' imported from `(defines x)'. > The point of this hypothetical was whether it's the macro that's hygeinic or the syntactic template. You're arguing for the template, but I think it's arguable that the template does not become actual source code until it's on the right hand side of a macro definition, and so referential transparency should be "through" that macro defintion, not the template. > [...] > IIUC, you're saying the syntax object returned from `helper' should > refer to the `do-it' in the program. That would defeat the whole point > of hygienic macros. > Again, you're referring to hygienic templates, not macros as such. >> 3) It doesn't make a lot of sense to have free-identifier=? if you >> can't create a free identifier without triggering an expand time >> error. > > You can do that. Larceny's error is happening because the free > identifiers have imported bindings in scope but not at the right phase > level and so it's preventing separate namespaces from existing. Then this goes back to point 1, the error should be signaled either when the template is actually appears in a macro defintion or when it appears as part of source code as a result of the macro invocation. Before that, maybe it should just be a warning. Why cause an error if the offending identifier never even appears in the output of the macro? Lynn _______________________________________________ Larceny-users mailing list Larceny-users@lists.ccs.neu.edu https://lists.ccs.neu.edu/bin/listinfo/larceny-users
