What is the difference between the functions seq?, sequential? and coll?

Question

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.

Answer 1

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