why does “(def vowel? (set ”aeiou“))” work?

Question

I\'m taking a look at the excellent Clojure tutorial here. In one of the examples it has Clojure code along the following lines:

(def vowel? (set \"aeiou\"))
         


        

Answer 1

This is perfectly analogous to how maps (the most natural objects in Clojure) work. When the map is called as a function, it works as a mapping:

user=> (def ob {:foo "bar", :bar :baz, :qwerty 42})
#'user/ob
user=> (ob :foo)
"bar"

So it makes sense that a Clojure set can be called as a function, and will work as a membership test. By the way, if you use keywords (those things that start with a colon) as the keys of a mapping, they also work as similar functions, so you can do

user=> (:bar ob)
:baz

and even the same thing with sets of keywords:

user=> (def vowel-keywords (set [:a :e :i :o :u]))
#'user/vowel-keywords
user=> (:a vowel-keywords)
:a
user=> (:b vowel-keywords)
nil

But, again, this latter trick only works with keywords, not anything else that you might use as keys in a mapping or members in a set.