Getting Clojure

Truthy and Falsy

One notable aspect of Clojure’s handling of Booleans is that the language is willing to treat any value as a Boolean. This, for example is a perfectly good if:

(if 1
  "I like science fiction!"
  "I like mysteries!")

As is this:

(if "hello"
  "I like science fiction!"
  "I like mysteries!")

And even this:

(if [1 2 3]
  "I like science fiction!"
  "I like mysteries!")

Not only are these good if statements, but in all three cases the winner is science fiction. The rule is simple: in an if statement and any other Boolean context, only false and nil get treated as false. Everything else is treated as true. Thus this expression will announce a love of mysteries:

(if false "I like scifi!" "I like mysteries!" ...