+1 totally agree. Eg. clojure.core/with-open Sent from phone. Please excuse brevity. On 1 Jul 2013 20:58, "Gary Trakhman" <gary.trakh...@gmail.com> wrote:
> Whenever I make a macro like this, I pass the symbol to be captured as an > argument to the macro. Any collisions would be more explicit and less > surprising. > > > On Mon, Jul 1, 2013 at 11:21 AM, Baishampayan Ghose <b.gh...@gmail.com>wrote: > >> Hi, >> >> What you need is called an anaphoric macro (via symbol capture). >> Clojure makes it hard to do a symbol capture in macros since it >> qualifies all symbols automatically unless you use the auto gensym >> feature. This is done for hygiene purposes and to avoid any unintended >> side-effects. >> >> However indeed, it's possible to create such a macro in Clojure. >> >> Behold - >> >> (defmacro test-it [addend & body] `(let [~'foo (+ ~addend 1)] ~@body)) >> ;; notice the ~' in front of foo. >> >> (macroexpand-1 '(test-it 1 (println foo))) >> ;=> (clojure.core/let [foo (clojure.core/+ 1 1)] (println foo)) >> >> Please apply caution before using this feature; document the symbol to >> be captured sufficiently. >> >> Hope that helps. >> >> Regards, >> BG >> >> On Mon, Jul 1, 2013 at 7:44 PM, Sean Johnson <belu...@acm.org> wrote: >> > Hello, >> > >> > If a macro calculates something and wants to make that result available >> to >> > the body, what's the right way to do that? So far I've tried these ways >> > which don't work (this is not really my macro, it's just a simplified >> > example to demonstrate the issue): >> > >> > 1) lexical scope doesn't work, the symbol gets qualified >> > >> > (defmacro test-it [addend & body] `(let [foo (+ ~addend 1)] ~@body)) >> > >> > => (test-it 3 (println foo)) >> > CompilerException java.lang.RuntimeException: Can't let qualified name: >> > fcms.app/foo, compiling:(NO_SOURCE_PATH:1:1) >> > >> > => (macroexpand '(test-it 3 (println foo))) >> > (let* [fcms.app/foo (clojure.core/+ 3 1)] (println foo)) >> > >> > >> > 2) autogen'ed lexical scope doesn't work, the body doesn't know what the >> > autogen'ed symbol will be called >> > >> > (defmacro test-it [addend & body] `(let [foo# (+ ~addend 1)] ~@body)) >> > >> > => (test-it 1 (println foo)) >> > CompilerException java.lang.RuntimeException: Unable to resolve symbol: >> foo >> > in this context, compiling:(NO_SOURCE_PATH:1:12) >> > >> > => (macroexpand '(test-it 3 (println foo))) >> > (let* [foo__5755__auto__ (clojure.core/+ 3 1)] (println foo)) >> > >> > >> > 3) an interned var partially works, if the macro and its use are all in >> one >> > namespace, but not if not, it fails to intern the var >> > >> > (defmacro test-it [addend & body] `(do (def foo (+ ~addend 1)) ~@body)) >> > >> > => (test-it 3 (println foo)) >> > 4 >> > nil >> > >> > => (macroexpand '(test-it 3 (println foo))) >> > (do (def fcms.app/foo (clojure.core/+ 3 1)) (println foo)) >> > >> > => (ns test) >> > nil >> > test=> (fcms.app/test-it 3 (println foo)) >> > CompilerException java.lang.RuntimeException: Can't create defs outside >> of >> > current ns, compiling:(NO_SOURCE_PATH:1:1) >> > >> > >> > I don't think my scenario is too unusual, but so far I haven't been >> able to >> > find anything about how to best do this. >> > >> > It case it helps, the real macro is with-collection and it takes a >> > collection name, does a lookup and some validation in a DB, then needs >> to >> > share the collection's id with the body. >> > >> > Cheers, >> > Sean >> > >> > >> > >> > -- >> > -- >> > You received this message because you are subscribed to the Google >> > Groups "Clojure" group. >> > To post to this group, send email to clojure@googlegroups.com >> > Note that posts from new members are moderated - please be patient with >> your >> > first post. >> > To unsubscribe from this group, send email to >> > clojure+unsubscr...@googlegroups.com >> > For more options, visit this group at >> > http://groups.google.com/group/clojure?hl=en >> > --- >> > You received this message because you are subscribed to the Google >> Groups >> > "Clojure" group. >> > To unsubscribe from this group and stop receiving emails from it, send >> an >> > email to clojure+unsubscr...@googlegroups.com. >> > For more options, visit https://groups.google.com/groups/opt_out. >> > >> > >> >> >> >> -- >> Baishampayan Ghose >> b.ghose at gmail.com >> >> -- >> -- >> You received this message because you are subscribed to the Google >> Groups "Clojure" group. >> To post to this group, send email to clojure@googlegroups.com >> Note that posts from new members are moderated - please be patient with >> your first post. >> To unsubscribe from this group, send email to >> clojure+unsubscr...@googlegroups.com >> For more options, visit this group at >> http://groups.google.com/group/clojure?hl=en >> --- >> You received this message because you are subscribed to the Google Groups >> "Clojure" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to clojure+unsubscr...@googlegroups.com. >> For more options, visit https://groups.google.com/groups/opt_out. >> >> >> > -- > -- > You received this message because you are subscribed to the Google > Groups "Clojure" group. > To post to this group, send email to clojure@googlegroups.com > Note that posts from new members are moderated - please be patient with > your first post. > To unsubscribe from this group, send email to > clojure+unsubscr...@googlegroups.com > For more options, visit this group at > http://groups.google.com/group/clojure?hl=en > --- > You received this message because you are subscribed to the Google Groups > "Clojure" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to clojure+unsubscr...@googlegroups.com. > For more options, visit https://groups.google.com/groups/opt_out. > > > -- -- You received this message because you are subscribed to the Google Groups "Clojure" group. 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