Why does `(count nil)` return 0?

Question

In Clojure, I find this surprising:

> (count nil)
0

I would expect a type error, as in this case:

> (count 77)
java.lang.UnsupportedOperationException: count not supported on this type: Long

since nil is not a list:

> (list? nil)
false

Does nil have a special status as an empty sequence?

Answer 1

From the official documentation:

count:

Returns the number of items in the collection. (count nil) returns 0. Also works on strings, arrays, and Java Collections and Maps

So this is the spec ;)

I imagine that it ensures that any mutating value having a "countable" type can be handled at runtime.
Indeed, any reference, referencing an allowed type (strings, arrays, and Java Collections and Maps) might target nil at some point.