What is the difference between the functions seq? sequential? and coll?
I found some information scattered throughout the internet, but I think it would be better to centralize that information here.
Asked
Active
Viewed 2,865 times
25
-
3http://www.brainonfire.net/files/seqs-and-colls/main.html – Kevin Mar 16 '14 at 16:29
2 Answers
29
seq? is a predicate that returns true if it's argument implements ISeq interface, which is to say it provides the methods first,rest,cons. See http://clojure.org/sequences.
(seq? [1 2])
false
(seq? (seq [1 2]))
true
sequential? is a predicate that returns true if it's argument implements Sequential interface. Sequential is a marker interface (no methods) and is a promise that the collection can be iterated over in a defined order (e.g. a list, but not a map).
(sequential? [])
true
(sequential? {})
false
coll? is a predicate that returns true if its argument implments IPersistentCollection. So for example the clojure data structures would return true, whereas native java data structures would not:
(coll? {:a 1})
true
(coll? (java.util.HashMap.))
false
Kevin
- 24,871
- 19
- 102
- 158
-
4
-
4@FeifanZ, these(`first`,`rest`,`cons`) functions first call [seq](http://clojuredocs.org/clojure.core/seq) function for its argument. So if you use these functions for vector (`clojure.lang.PersistentVector`) which doesn't implement ISeq interface (seq? function return `false`), it means that you use these function for vector (`clojure.lang.PersistentVector$ChunkedSeq`) which implements ISeq interface(seq? return `true`). – snufkon Apr 01 '16 at 23:46
9
seq?is true for any sequence.sequential?is true for any sequential (not associative) collection.coll?is true for any Clojure collection.
seq? implies sequential? implies coll?
=> ((juxt seq? sequential? coll?) ()) ; [true true true]
=> ((juxt seq? sequential? coll?) []) ; [false true true]
=> ((juxt seq? sequential? coll?) #{}); [false false true]
Inaccurate: sequential? is related to the others purely by convention - see Kevin's answer.
