Getting Clojure

Sets

Along with maps, Clojure also sports a built-in set data type. The literal syntax for a set borrows the braces from maps but adds a # to the front:

(def genres #{:sci-fi :romance :mystery})

(def authors #{"Dickens" "Austen" "King"})

Like their mathematical namesakes, Clojure sets are all about membership: a value either is or is not a member of a set. Since a value can only be in a set once, if you repeat a value in a set literal, you’ll get an error. Thus, this:

#{"Dickens" "Austen" "Dickens"}

is one "Dickens" too many:

IllegalArgumentException Duplicate key: Dickens...

Like maps, sets have their own ideas about the order of their elements. The set that you wrote as #{:sci-fi :romance :mystery} is liable to come ...