So, the way I understand it (and please correct me if I'm wrong), is that require is a macro for #%require, which is converted by the compiler to either a top level require form, or part of the module's require field. (Depending on where it goes).
When you put a `begin-for-syntax` or a `define-syntax` in your macro, the bindings of each of the bodies need to become those of one phase level up, which enables the macro expander to further expand the macros in the expanded code. (This is better explained in the "You Want it When" paper. https://www.cs.utah.edu/plt/publications/macromod.pdf ) The problem here is that by default, Racket only loads racket/base at phase level 1, and no language at phase level 2, so racket/base must be explicitly required to give anything two `begin-for-syntax` levels in any meaning. 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 meening. 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.
