Probably more products should have an FHQ (Frequently Hopeless Questions) in addition to an FAQ.
I'm making a web-page generator that uses include-template from web-server/templates. I need to set the template inputs & the template itself at runtime, and collect the output. I tried doing it with module and module* but I didn't see how to make the data go round-trip. A submodule with include-template could require the needed data from the enclosing module, but then the enclosing module can't require the submodule to get the result, so ... ? I could well be overlooking an obvious technique. My original solution was to write a new file and send it through the Racket executable as a system command. (This worked, slowly.) So that's how I ended up using eval. Which does work, now that I've retooled how I set up the namespace. On Tue, May 14, 2013 at 5:13 PM, Robby Findler <ro...@eecs.northwestern.edu>wrote: > The short answer is that the top-level is hopeless, as Matthew has > discussed at some length here and elsewhere. > > If you really need to eval code, can you first put it into a module? > > Robby > > > On Tue, May 14, 2013 at 11:09 AM, Matthew Butterick < > mb.list.a...@gmail.com> wrote: > >> Suppose x-is-foo.rkt is this: >> >> (module x-is-foo racket >> (define x 'foo) >> (provide x)) >> >> If you open another file and try this: >> >> (require "x-is-foo.rkt") >> (define x 'bar) >> >> You'lll get a "identifier already imported" error. OK, that much I >> understand. >> >> Here's the question. When you do this: >> >> (parameterize ([current-namespace (make-base-empty-namespace)]) >> (namespace-require 'racket) >> (namespace-require "x-is-foo.rkt") >> (namespace-set-variable-value! 'x 'bar) >> (eval '(print x) (current-namespace))) >> >> This time, you get 'foo. Why 'foo? Why not another "identifier already >> imported" error? >> >> I assume I'm missing a subtlety of how the namespace environment is >> different. But according to the docs, both namespace-require and >> namespace-set-variable-value! affect the top-level environment of the >> namespace. So I don't see why the require is silently overriding the >> set-variable-value, rather than causing a conflict. >> >> It's not a sequencing issue, because if you swap the two lines: >> >> (parameterize ([current-namespace (make-base-empty-namespace)]) >> (namespace-require 'racket) >> (namespace-set-variable-value! 'x 'bar) >> (namespace-require "x-is-foo.rkt") >> (eval '(print x) (current-namespace))) >> >> You still get 'foo. >> >> Only if you remove the require line: >> >> (parameterize ([current-namespace (make-base-empty-namespace)]) >> (namespace-require 'racket) >> (namespace-set-variable-value! 'x 'bar) >> (eval '(print x) (current-namespace))) >> >> Do you get 'bar. >> >> >> >> ____________________ >> Racket Users list: >> http://lists.racket-lang.org/users >> >> >
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