What's the difference between a sequence and a collection in Clojure

Question

I am a Java programmer and am new to Clojure. From different places, I saw sequence and collection are used in different cases. However, I have no idea what the exact difference is between them.

For some examples:

1) In Clojure's documentation for Sequence:

The Seq interface (first coll)   Returns the first item in the collection.    Calls seq on its argument. If coll is nil, returns nil. (rest coll)   Returns a sequence of the items after the first. Calls seq on its argument.    If there are no more items, returns a logical sequence for which seq returns nil. (cons item seq)   Returns a new seq where item is the first element and seq is the rest. 

As you can see, when describing the Seq interface, the first two functions (first/rest) use coll which seems to indicate this is a collection while the cons function use seq which seems to indicate this is a sequence.

2) There are functions called coll? and seq? that can be used to test if a value is a collection or a sequence. It is clearly collection and sequence are different.

3) In Clojure's documentation about 'Collections', it is said:

Because collections support the seq function, all of the sequence functions can be used with any collection

Does this mean all collections are sequences?

(coll? [1 2 3]) ; => true  (seq? [1 2 3]) ; => false 

The code above tells me it is not such case because [1 2 3] is a collection but is not a sequence.

I think this is a pretty basic question for Clojure but I am not able to find a place explaining this clearly what their difference is and which one should I use in different cases. Any comment is appreciated.

Answer 1

Any object supporting the core first and rest functions is a sequence.

Many objects satisfy this interface and every Clojure collection provides at least one kind of seq object for walking through its contents using the seq function.

So:

user> (seq [1 2 3])     (1 2 3) 

And you can create a sequence object from a map too

user> (seq {:a 1 :b 2})     ([:a 1] [:b 2]) 

That's why you can use filter, map, for, etc. on maps sets and so on.

So you can treat many collection-like objects as sequences.

That's also why many sequence handling functions such as filter call seq on the input:

 (defn filter   "Returns a lazy sequence of the items in coll for which   (pred item) returns true. pred must be free of side-effects."   {:added "1.0"    :static true}   ([pred coll]    (lazy-seq       (when-let [s (seq coll)] 

If you call (filter pred 5)

  Don't know how to create ISeq from: java.lang.Long                   RT.java:505 clojure.lang.RT.seqFrom                   RT.java:486 clojure.lang.RT.seq                  core.clj:133 clojure.core/seq                 core.clj:2523 clojure.core/filter[fn] 

You see that seq call is the is this object a sequence validation.

Most of this stuff is in Joy of Clojure chapter 5 if you want to go deeper.

Answer 2

Here are few points that will help understand the difference between collection and sequence.

1. "Collection" and "Sequence" are abstractions, not a property that can be determined from a given value.

2. Collections are bags of values.

3. Sequence is a data structure (subset of collection) that is expected to be accessed in a sequential (linear) manner.

The figure below best describes the relation between them:

You can read more about it here.