Thank you, Marc! I will try to use your example to develop more general "instance" function.
Sincerely, Ru воскресенье, 20 октября 2019 г., 15:35:48 UTC+3 пользователь mlimotte написал: > > Hey Ru, > > I'd also add that `new` is a special form, not a function. It is > evaluated at compile-time, which is why the macro doesn't work. You can > also try macro-expansion to see what's going on: > > > (defn itest [x] (macroexpand-1 '(instance x))) > => #'user/itest > (itest java.lang.String) > => (new x) > > > So, after macro-expansion, this would be like typing the following code: > > (defn itest [x] (new x)) > Syntax error (IllegalArgumentException) compiling new at [...] > Unable to resolve classname: x > > > This doesn't work as a straight up function, so it won't work as a macro. > Macros are "shortcuts". They won't let you do anything you couldn't do > with "regular" code. > > Finally, as Chris suggested above, reflection is probably what you want. > Here's a sample to get you started: > > (defn itest [x & args] > (let [cnstr (.getConstructor x (into-array java.lang.Class [String]))] > (.newInstance cnstr (into-array java.lang.Object args)))) > => #'user/itest > (itest java.lang.String "Hello") > => "Hello" > > > This is only a start though, because as written (line 2 in particular), it > only works for constructors that take exactly one String argument. To do > this for real, you would need to build the [String] array dynamically by > introspecting the args list. > > Marc > > > > > On Sat, Oct 19, 2019 at 3:02 PM <ri...@chartbeat.com <javascript:>> wrote: > >> Macros also allow you to create a dsl to make your code easier to use or >> understand by obfuscating away bits. For example a macro that times a >> function call by proxying the call or wrapping a call to defn. >> >> That said, I consider many uses of macros that I’ve seen to be >> antipatterns that lead to hard to understand code. They should almost >> always be considered a last resort. >> >> >> On Oct 19, 2019, at 12:12 PM, Matching Socks <phil...@gmail.com >> <javascript:>> wrote: >> >> Macros manipulate program symbols. Macros fill the role that is filled >> by Perl scripts in the Java world, when they pre-process stuff (a database >> schema, or a gui model, for example) into actual Java code before you >> compile it. If a task could not be solved by pre-processing the source >> code before the program starts to run, then it is not a task for a macro. >> >> On the bright side, using Java reflection is less tedious in Clojure than >> it is in Java. >> >> -- >> You received this message because you are subscribed to the Google >> Groups "Clojure" group. >> To post to this group, send email to clo...@googlegroups.com >> <javascript:> >> Note that posts from new members are moderated - please be patient with >> your first post. >> To unsubscribe from this group, send email to >> clo...@googlegroups.com <javascript:> >> 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 clo...@googlegroups.com <javascript:>. >> To view this discussion on the web visit >> https://groups.google.com/d/msgid/clojure/2c56ec62-9908-40b8-aacc-aecaf157049c%40googlegroups.com >> >> <https://groups.google.com/d/msgid/clojure/2c56ec62-9908-40b8-aacc-aecaf157049c%40googlegroups.com?utm_medium=email&utm_source=footer> >> . >> >> -- >> You received this message because you are subscribed to the Google >> Groups "Clojure" group. >> To post to this group, send email to clo...@googlegroups.com >> <javascript:> >> Note that posts from new members are moderated - please be patient with >> your first post. >> To unsubscribe from this group, send email to >> clo...@googlegroups.com <javascript:> >> 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 clo...@googlegroups.com <javascript:>. >> To view this discussion on the web visit >> https://groups.google.com/d/msgid/clojure/E0B8D5A1-AC56-4FD6-BAC9-220E628FC510%40chartbeat.com >> >> <https://groups.google.com/d/msgid/clojure/E0B8D5A1-AC56-4FD6-BAC9-220E628FC510%40chartbeat.com?utm_medium=email&utm_source=footer> >> . >> > -- 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. To view this discussion on the web visit https://groups.google.com/d/msgid/clojure/da73be2b-305b-4032-828f-a417c7ba52de%40googlegroups.com.
