They're different `#%top`s. In the the macro stepper, click on the first `#%top`, and the panel on the right tells you that it comes from "bound.rkt". When you click on the second `#%top`, though, the panel shows that it comes from '#%kernel via `racket`.
At Fri, 7 Feb 2014 08:24:36 -0800, Matthew Butterick wrote: > Last thing. This solution seems to work, but I can't figure out WHY it works. > > If I look at these two lines in the macro expander, where bar is unbound: > > (bar "hello") > ((bound/c bar) "hello"))) > > First step makes sense: bar is replaced with (#%top . bar). > > ((#%top . bar) "hello") > ((bound/c bar) "hello"))) > > Second step makes sense: (#%top . bar) is replaced with my syntax-rule for > #%top to make an x-expression: > > ((λ x `(bar ,@x)) "hello") > ((bound/c bar) "hello"))) > > Third step makes sense: (bound/c bar) is replaced with (#%top . bar). > > ((λ x `(bar ,@x)) "hello") > ((#%top . bar) "hello"))) > > But in the fourth step, I get the "bar: unbound identifier in module" error. > This is what I want. But I don't understand is why this is so. Why wouldn't > the second (#%top . bar) get transformed by the syntax-rule attached to #%top? > > > On Feb 6, 2014, at 9:37 PM, Matthew Butterick <m...@mbtype.com> wrote: > > > Reflecting on it as an issue of detecting bound identifiers, I've come up > with a possible approach — could it be this simple, or am I overlooking some > complication? > > > > bound.rkt: > > > > #lang racket > > > > (provide bound/c (rename-out (top~ #%top))) > > > > (define-syntax-rule (top~ . id) > > (λ x `(id ,@x))) > > > > (define-syntax (bound/c stx) > > (syntax-case stx () > > [(_ x) > > (if (identifier-binding #'x ) > > #'x > > #'(#%top . x))])) > > > > > > test.rkt: > > > > #lang racket > > (require "bound.rkt") > > (define foo displayln) ; foo is now bound > > (foo "hello") > > ((bound/c foo) "hello") > > (bar "hello") ; bar is unbound > > ;((bound/c bar) "hello") > > > > This does the right thing: > > > > hello > > hello > > '(bar "hello") > > > > And then when the last line is uncommented > > > > ((bound/c bar) "hello") > > > > It triggers the usual error on compile: > > > > bar: unbound identifier in module in: bar > > > > > > > > On Feb 6, 2014, at 8:38 PM, Matthew Butterick <m...@mbtype.com> wrote: > > > >> I'm trying to figure out how to make #%top change behavior based on > different kinds of function names, though my experiments keep leading to > infinite loops that blow up DrRacket. Oops. > >> > >> In my #lang project based on Scribble, I've been using this simple > redefinition of #%top for convenience: > >> > >> (define-syntax-rule (#%top . id) > >> (λ x `(id ,@x))) > >> > >> IOW, if the thing in the function position is undefined, it's treated as > the opening tag of an x-expression. This makes it easy to mix undefined and > defined names. > >> > >> But it can make debugging difficult. Because if you expect a name to be > defined as a function and it isn't, then you don't get the syntax error you > ordinarily would. The function call silently gets converted an x-expression. > Spooky side effects follow. > >> > >> I'd like to improve this by making a syntactic prefix that suppresses this > behavior and that can be attached to any function name. For instance, def-*. > So if I write (def-foobar ..) in the code, this will mean: > >> 1) try to invoke a function called (foobar ...); > >> 2) if the function (foobar ...) isn't defined, raise the usual exception + > syntax error. > >> In other words, the standard #%top behavior. > >> > >> Whereas if I just write (foobar ...) without the def-* prefix, this will > mean: > >> 1) try to invoke the function (foobar ...); > >> 2) if the function (foobar ...) isn't defined, turn it into the > x-expression `(foobar ,@ ...). > >> In other words, my special #%top behavior shown above. > >> > >> Having explained the logic, I think the flaw in my experiments so far is > that this actually requires manipulation of #%app too, because once you hit > #%top, you've missed your chance to apply functions. (Right?) But even > supposing one creates two evaluation paths under #%app, it's not clear to me > how to preserve those paths on the way up to #%top. > >> > >> > > > > > ____________________ > Racket Users list: > http://lists.racket-lang.org/users ____________________ Racket Users list: http://lists.racket-lang.org/users
