seq
- (seq coll)
Returns a seq on the collection. If the collection is empty, returns nil. (seq nil) returns nil. seq also works on Strings, native Java arrays (of reference types) and any objects that implement Iterable. Note that seqs cache values, thus seq should not be used on any Iterable whose iterator repeatedly returns the same mutable object.
© Rich Hickey。保留所有权利。Eclipse 公共许可证 1.0
5 Examples
(seq '(1)) ;;=> (1) (seq [1 2]) ;;=> (1 2) (seq "abc") ;;=> (\a \b \c) ;; Corner cases (seq nil) ;;=> nil (seq '()) ;;=> nil (seq []) ;;=> nil (seq "") ;;=> nil
;; (seq x) is the recommended idiom for testing if a collection is not empty (every? seq ["1" [1] '(1) {:1 1} #{1}]) ;;=> true
;; 'seq' can be used to turn a map into a list of vectors. ;; Notice how the list is built adding elements to the beginning ;; of the seq (list) not to the end, as with vectors. ;; (Of course, the order that items are ;; taken from a map should not be relied upon ;; unless a deterministic 'sorted-map' is used.) (seq {:key1 "value1" :key2 "value2"}) ;;=> ([:key2 "value2"] [:key1 "value1"])
;; Here is the difference between seq and sequence (seq nil) ;;=> nil (seq ()) ;;=> nil (sequence ()) ;;=> () (sequence nil) ;;=> ()
;; seq is very similar to not-empty: (every? seq ["1" [1] '(1) {:1 1} #{1}]) ;;=> true (every? not-empty ["1" [1] '(1) {:1 1} #{1}]) ;;=> true (seq '(1)) ;;=> (1) (not-empty '(1)) ;;=> (1) (seq [1 2]) ;;=> (1 2) (not-empty [1 2]) ;;=> [1 2] (seq "abc") ;;=> (\a \b \c) (not-empty "abc") ;;=> "abc" (map seq [nil '() [] "" {}]) ;;=> (nil nil nil nil nil) (map not-empty [nil '() [] "" {}]) ;;=> (nil nil nil nil nil)