OK, I think I'm starting to get it now - the idea is a function that accepts x & xs , which are functions, and returns a function which has a var-args arity [& args], which is the composition of the xs.
Thanks to all for the replies - I still haven't looked at the source to comp but it will be fun to see. On Sat, Aug 25, 2012 at 2:46 PM, John Holland <jbholl...@gmail.com> wrote: > Thanks to a for the replies..... I will study them later when I am free. > > On Aug 25, 2012 2:37 PM, "nicolas.o...@gmail.com" <nicolas.o...@gmail.com> > wrote: >> >> Here's my take: >> >> We want to define a function my-comp. It takes n functions and return >> their composition. >> We want to return a function of any number of arguments, so let's >> start by working with a given >> set of argument args, and returning the value of the composition >> applied to those arguments. >> >> - If I compose only one function, you get the value of the function >> itself: >> (value-my-comp f) = (apply f args) >> >> - By definition of composition: >> (value-my-comp f1 f2 .... fn) = (f1 (value-my-comp f2 .... fn)) >> (Take all the arguments, apply it to the composition of function f2 >> .... fn, and then apply the result to f1). >> >> Then you remove the dots: >> >> (value-my-comp f1 & f2-fn) = (f1 (apply value-my-comp f2-fn)) >> >> Then you can just put them all together: >> >> >> (fn [& fs] ;; we take a bunch of functions in >> (fn [ & args ] ;; we take the arguments of the resulting function in. >> (letfn [(value-my-comp ;; the value of the result in term of the >> functions is: >> ([f] (apply f args)) ;; only one function >> ([f1 & f2-fn] (f1 (apply value-my-comp f2-fn))))] ;; >> more than one function >> (apply value-my-comp fs)))) ;; we compute the value of the >> actual function we got. >> >> If you don't like (apply value-my-comp f2-fn), you can make >> value-my-comp actually takes a sequence >> of functions: >> >> (fn [& fs] ;; take the functions >> (fn [ & args ] ;; the arguments of the resulting functions >> (letfn [(value-my-comp >> [[f1 & f2-fn]] ;; a list of functions >> (if (empty? f2-fn) ;; only one function? >> (apply f1 args) ;; then we apply the args to it >> (f1 (value-my-comp f2-fn))))] ;; else we compose >> (value-my-comp fs)))) >> This one is another way of writing the one you posted. >> (Using a let instead of applying it directly, and not using a >> non-varying argument a.) >> >> If you want to see how this function can be written to be very >> performant look at >> comp source code, it is very interesting. >> >> Best, >> >> Nicolas. >> >> -- >> 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 -- ______________________________________ Note new email address jbholl...@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
