At Mon, 21 Dec 2015 21:30:45 -0700, Leif Andersen wrote: > Ah, that's a good question. One that I don't really know the answer > too, because when I do: > > > (compile #'(begin > (define-syntax (foo stx) > (displayln "hello") > #'5) > foo)) > > I get back the compiled object.
The compiled object ends with a reference to a top-level `foo` variable, not an expansion of the `foo` macro. > Also foo is not displayed. That's because the `foo` macro was only compiled, not bound, and so the ending `foo` didn't reference it. > And when I do: > > > (eval (compile #'(begin > (define-syntax (foo stx) > (displayln "hello") > #'5) > foo))) > > I get the error: > > foo: undefined; > cannot reference an identifier before its definition That demonstrates the reference to `foo`, which is never defined as a variable. Confusingly, however, `foo` at this point is bound to a macro. To make sense of that, you have to remember that there are two levels to identifier resolution. The first is a binding layer that points `foo` to either a module import, a top-level macro, or a top-level variable. In the case of a variable, a second layer maps the variable name to its value. Compilation commits to a choice in the first layer; it commits to the the `foo` as a variable reference before that first layer is changed to make `foo` a macro. > But, when I do: > > > (expand #'(begin > (define-syntax (foo stx) > (displayln "hello") > #'5) > foo)) > > It displays 'hello' to the console. And expands to what I would expect it too. That example doesn't display anything if you try it in a fresh REPL. But if you continue in the same REPL as the previous `eval`, then it's the other `foo` that gets used as a macro here, not this one. > On Mon, Dec 21, 2015 at 9:15 PM, Alex Knauth <alexan...@knauth.org> wrote: > > > >> On Dec 21, 2015, at 10:53 PM, Leif Andersen <l...@leifandersen.net> wrote: > > > >> But `compile` is not supposed to evaluate any code, it just compiles > >> it. Which is why it fails to compile that code. But if you interleave > >> it with evals, it will run the require code, which gives the phase > >> level 2 code some meaning. > > > > I understand that `compile` isn't supposed to evaluate any _runtime_ code, > but I thought it had to evaluate the compile-time code for it to compile the > program? > > > > Am I misunderstanding what `compile` is supposed to do? Or what > > compile-time > code is, or? > > > >> Does that make any sense, or was it too rambly. > >> > >> ~Leif Andersen > >> > >> > >> On Mon, Dec 21, 2015 at 8:24 PM, Alex Knauth <alexan...@knauth.org> wrote: > >>> I get that `compile` doesn't evaluate the require form, but why doesn't > >>> it > evaluate what's needed for compile time? I thought that was the reason for > needing require as a macro instead of a function. Or am I getting the purpose > of compile wrong? > >>> > >>> Alex Knauth > >>> > >>>> On Dec 21, 2015, at 7:47 PM, Matthew Flatt <mfl...@cs.utah.edu> wrote: > >>>> > >>>> In the REPL, `begin` splices at the granularity of expansion and > >>>> evaluation. That is, the first of the two forms grouped by `begin` is > >>>> compiled and evaluated, and only afterward the second one is compiled > >>>> and evaluated. > >>>> > >>>> The `compile` function can't do that, because it's only supposed to > >>>> compile -- not evaluate. > >>>> > >>>> The `syntax/toplevel` library provides > >>>> > >>>> expand-syntax-top-level-with-compile-time-evals > >>>> > >>>> as an intermediate point between those: it pulls apart `begin` and > >>>> `expands` (which is the interesting part of `compile`) the forms in > >>>> sequence, but it evaluates only compile-time parts of the given forms. > >>>> That works for many cases, including your example. > >>>> > >>>> At Mon, 21 Dec 2015 17:35:22 -0700, Leif Andersen wrote: > >>>>> So, I was under the impression that the REPL was the toplevel (unless > >>>>> you entered a module context or something like that). But I just had > >>>>> something challenge that assumption and so now I'm a bit confused. > >>>>> > >>>>> I have the following top level program: > >>>>> > >>>>> (begin > >>>>> (require (for-meta 1 racket) > >>>>> (for-meta 2 racket)) > >>>>> (begin-for-syntax > >>>>> (begin-for-syntax > >>>>> (define x 5)))) > >>>>> > >>>>> When I run this top level program in the REPL, it works just fine. And > >>>>> I can even reference x in later evaluations. (Provided I'm at the > >>>>> meta-level of 2). > >>>>> > >>>>> However, when I pass this top level program into compile like so: > >>>>> http://pasterack.org/pastes/38556 > >>>>> > >>>>> (compile #'(begin > >>>>> (require (for-meta 1 racket) > >>>>> (for-meta 2 racket)) > >>>>> (begin-for-syntax > >>>>> (begin-for-syntax > >>>>> (define x 5))))) > >>>>> > >>>>> > >>>>> I get the following error: > >>>>> > >>>>> define: unbound identifier at phase 2; > >>>>> also, no #%app syntax transformer is bound in: define > >>>>> > >>>>> Can someone give me the reason (or at least some intuition) as to why > >>>>> this works in the repl, but not at the top level? > >>>>> > >>>>> Thank you. > >>>>> > >>>>> ~Leif Andersen > >>>>> > >>>>> -- > >>>>> You received this message because you are subscribed to the Google > >>>>> Groups > >>>>> "Racket Users" group. > >>>>> To unsubscribe from this group and stop receiving emails from it, send > >>>>> an > >>>>> email to racket-users+unsubscr...@googlegroups.com. > >>>>> For more options, visit https://groups.google.com/d/optout. > >>>> > >>>> -- > >>>> You received this message because you are subscribed to the Google > >>>> Groups > "Racket Users" group. > >>>> To unsubscribe from this group and stop receiving emails from it, send > >>>> an > email to racket-users+unsubscr...@googlegroups.com. > >>>> For more options, visit https://groups.google.com/d/optout. > >>> > > > > -- > You received this message because you are subscribed to the Google Groups > "Racket Users" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to racket-users+unsubscr...@googlegroups.com. > For more options, visit https://groups.google.com/d/optout. -- You received this message because you are subscribed to the Google Groups "Racket Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to racket-users+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
