Thanks to all for the nice explanations. I understand the reasoning. On Tuesday, April 8, 2014 1:53:44 AM UTC-7, Colin Yates wrote: > > 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)))) >> >> >>
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