Depends who is doing the expecting as to whether that behaviour is correct. Formal logicians, mathematicians, computer scientists etc. would respond "sure, it is vacously true". For almost everybody else it "feels" wrong but is then true when you think about it a bit.
I would suggest the question you are trying to ask is (and (not (empty? nil)) (every? #(= 77 %) nil)). For more info check out http://en.wikipedia.org/wiki/Vacuous_truth. On Tuesday, 8 April 2014 07:08:56 UTC+1, Jeff Mad wrote: > > Hi, > I am new to Clojure, so please forgive me if this does not make sense. > > I was surprised to find out in the REPL that every? returns true if you > pass in an empty or nil collection. > > user=> (every? #(= 77 %) nil) > > true > > user=> (every? #(= 77 %) '()) > > true > > > I looked at the source for every? and it made sense to me why this > happens given that every? is recursive and the termination condition is > when coll runs out of items to process. > > Would it make more sense to define every? with a loop, or is the caller > expected to know better than to call it with nil? > > Thanks, > > --jeff > > > (defn every2? > > "Returns true if (pred x) is logical true for every x in coll, else > > false." > > {:tag Boolean > > :added "1.0" > > :static true} > > [pred coll] > > (if (empty? coll) > > false > > (loop [c coll] > > (cond > > (nil? (seq c)) true > > (pred (first c)) (recur (next c)) > > :else false)))) > > > -- 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/d/optout.
