Hi, Why are there multiple "Logging str" output. Also in (apply str-orig args), I don't see any args passed at all!
Thanks sun On Jan 5, 2:14 pm, Meikel Brandmeyer <m...@kotka.de> wrote: > Hi, > > Am 05.01.2009 um 18:35 schrieb wubbie: > > > This example is from clojure site. > > My question is on line 5 and line 6: > > The confusion is str is a function and here looks like used as a > > regular variable. > > > Thanks in advance. > > binding can be used to temporarily assign > a different value to a global Var. > > Let's understand the example line by line. > > > (defn loves [x y] > > (str x " loves " y)) > > First we define a function "loves" which uses > the standard str function to concate it's > arguments. So far easy to understand. > > > (defn test-rebind [] > > (println (loves "ricky" "lucy")) > > Here we do a proof of concept and show > that our function "loves" works as expected. > > Output: > > > ricky loves lucy > > Now to nitty-gritty: > > > (let [str-orig str] ---> line 5: > > First we save the original str function in > a local. > > > (binding [str (fn [& args] --> line 6: > > (println "Logging str") > > (apply str-orig args))] -->line 8: > > Now we use binding to replace the global > str function with another one, which we > define using the original str function. We > log, that we called our modified function > and the just delegate all arguments to the > original str function, which we saved in > str-orig. > > This modified str is only available inside > the body of the binding form. We leave > it by reaching the end of the body or throwing > an exception, the original str is again > restored. Also other threads also don't see > the modified str. This is really restricted > to the body of the binding. > > Now we call again our initial function "loves". > Please note: we did not modify our function > in any way. > > > (println (loves "fred" "ethel"))))) > > See the output: > > > Logging str > > Logging str > > Logging str > > Logging str > > Logging str > > fred loves ethel > > Huh? So although we did not modify our "loves" > function we get a different output? Why is that. > The solution is binding. The loves function is > called *inside* the binding body and although > it is defined *outside* the binding, it will use the > modified str function. > > So you are right the binding looks similar to > a let. > > (binding [str ...] > ...) > > vs. > > (let [str ...] > ...) > > However the let introduces a new "str" which > *shadows* the global str function and is only > visible for the code in the let. In particular > functions called inside the let will see the > str which was in effect when they were defined. > > The binding however *replaces* the str which > was in effect, when the function was defined. > Of course this works only with global Vars and > not with locals as introduced by let. > > (Note: the number of "Logging"s depends on > the implementation of the str function.) > > Hope this helps. > > Sincerely > Meikel > > smime.p7s > 5KViewDownload --~--~---------~--~----~------------~-------~--~----~ 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 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 -~----------~----~----~----~------~----~------~--~---
