Why does (keys "") return nil in Clojure, whereas (keys "abc") is an error?

Question

I'm new to Clojure. The behaviour of keys strikes me as inconsistent:

user=> (keys "")
nil
user=> (keys "abc")
ClassCastException

Empty collections appear to be treated specially, and the test cases indicate that this is intentional. What's the thinking behind this behaviour?

Answer 1

The reason for this is that when a collection is converted to a sequence using seq function, in case the collection is empty seq will return nil rather than an empty sequence. There is another thread which discuss the reason for that.