On Fri, Apr 1, 2011 at 7:45 AM, Matthew Flatt <[email protected]> wrote: > At Fri, 1 Apr 2011 04:10:19 -0500, Casey Klein wrote: >> What exactly is in the `id' field of a `exn:fail:contract:variable' >> structure? It doesn't seem to be the symbolic name of the >> not-yet-defined variable. > > It's the real symbolic name --- the one that would work with > `namespace-variable-value' and similar reflective operations. > > In this case, you get `x.6', because the original symbolic name `x' was > introduced by a macro; the variable name can't be just 'x, since it > must be renamed for hygiene. Also, the symbol that prints as 'x.6' in > this case is a kind of gensym --- not the result of `(string->symbol > "x.6")'. > > The mismtach between `x' and `x.6' reflects is a mismatch between > identifiers (the modern Scheme way of binding) and symbols (the > traditional Lisp way of binding, on which the identifier layer is > currently built and that sometimes shows through in the reflection > API). >
Thanks, that makes sense. I asked this question because I'm having trouble getting Redex to produce a good error message when you use a metafunction before its definition. For example, the program #lang racket (require redex) (define-language L) (apply-reduction-relation (reduction-relation L (--> (any ...) ((f any) ...))) '(a b c)) (define-metafunction L [(f any) (any)]) raises the error "reference to an identifier before its definition: f3.18". I know how to get the error to say "f.18", but I don't know how to get it to say just "f". The problem is the expansion of metafunction definitions. The definition of f expands into something roughly like (begin (define f.18 ---) (define-syntax f (make-term-fn #'f.18))) and the reduction-relation macro walks over the rules' right-hand sides, looking for identifiers bound to term-fns and constructing appropriate calls to the underlying functions. With this approach, the apply-reduction-relation call ends up referencing f.18 (in this case, before it's defined). Maybe mine is a weird case, but it has me wondering why it's better for the exn:fail:variable:contract message to use the real symbolic name. These names are never known to programmers, since they don't appear in source programs, right? _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
