On Jan 15, 2010, at 3:26 AM, Sean Devlin wrote: > user=> (seq []) > nil > > Why is nil returned, instead of an empty sequence?
It's fundamental to Clojure's seq abstraction that every seq has a first. There is no such thing as an empty seq. If you call the seq function on an empty collection, it can't return a seq because that (hypothetical) seq would have no first. Instead seq returns nil meaning "no seq". (seq x) (used as a predicate) is the canonical way in Clojure to ensure that x contains at least one item. Rich's video about Clojure sequences at iTunes and on blip.tv provides more details and rationale. --Steve
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