Re: loop/recur and tail postion
Thanks everyone. Makes perfect sense. -- 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
Re: loop/recur and tail postion
On Wed, Jun 15, 2011 at 10:08 AM, Kevin Sookocheff wrote: > Hi, I'm going through some Scheme code to learn Clojure. I defined a > function > > rember > > as > > > (defn rember [atom l] > (loop [a atom lat l] > (cond > (empty? lat) `() > (= (first lat) a) (rest lat) > :else (cons (first lat) (recur a (rest lat)) > > > To which the REPL responds: > > Can only recur from tail position > [Thrown class java.lang.UnsupportedOperationException] > > Is the call to recur not in tail position here? Nope. The return from recur has to be, directly, the return from the enclosing loop form for it to be a tail position. Here you're consing something onto it in between. Generally that means you need to add a result accumulator to the loop, something like: (defn rember [atom l] (loop [a atom lat l res []] (cond (empty? lat) res (= (first lat) a) (rest lat) :else (recur a (rest lat) (conj res (first lat)) though I can think of further improvements, like hoisting a out of the loop (it doesn't change) and using seq/next like so: (defn rember [atom l] (loop [lat (seq l) res []] (if lat (let [f (first lat)] (if (= f atom) (rest lat) (recur (next lat) (conj res f res))) -- Protege: What is this seething mass of parentheses?! Master: Your father's Lisp REPL. This is the language of a true hacker. Not as clumsy or random as C++; a language for a more civilized age. -- 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
Aw: loop/recur and tail postion
Hi, no, it's not. The cons is in the tail position. Here a working version. (defn rember [a l] (loop [ret [] lat (seq l)] (cond (not lat) ret (= (first lat) a) (recur ret (next lat)) :else (recur (conj ret (first lat)) (next lat) Note, how recur is now in the tail position and how an accumulator is used to collect the results. Hope that helps. Sincerely Meikel BTW: you can also use direct recursion in your original function instead of recur by simply calling the function (rember) itself again. -- 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
Re: loop/recur and tail postion
> Is the call to recur not in tail position here? > > No, because of (cons (first lat) (recur a (rest lat))). You cons (first lat) after you call (recur ...). That is why (recur ...) is not in tail position. -- 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
loop/recur and tail postion
Hi, I'm going through some Scheme code to learn Clojure. I defined a function rember as (defn rember [atom l] (loop [a atom lat l] (cond (empty? lat) `() (= (first lat) a) (rest lat) :else (cons (first lat) (recur a (rest lat)) To which the REPL responds: Can only recur from tail position [Thrown class java.lang.UnsupportedOperationException] Is the call to recur not in tail position here? Thank you, Kevin -- 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
