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')) 

This makes vowel return true for vowels and false for consonants:

(vowel? (first 'abc')) ; => true (vowel? (first 'cba')) ; => false 

Why is this? I’m assuming it has something to do with the question mark behind the variable name. Couldn’t find anything right away in the tutorial…

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Edit I just realized vowel? doesn’t return true or false but rather the element itself or nil. See my own answer.

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.