What is the meaning of '?' in the Racket docs?

Question

Example:

(number? v) ? boolean?

  v : any/c

I understand the '?' behind 'number' but the second '?', behind 'boolean' irritates me. Does it mean that it maybe returns a boolean, and maybe not?

Edit for clarity

I come from Python, to me this reads as: is_number::function returns is_boolean::function and not is_number::function returns bool::bool.

Solution

As Jérôme explained, it reads as:

is_number(v) returns b where is_boolean(b) == True

Answer 1

boolean? is a predicate. It's a function that has (by convention) a question mark at the end to show it can be used to check for the type of a value and return a boolean, like so:

> (boolean? #f)
#t
> (boolean? "hello")
#f

By default Racket is dynamically typed, so in order to show type information in the documentation, predicates are used as a kind of type annotation.

So basically, (number? v) ? boolean? means "The function number? returns a value which, when passed to the predicate boolean?, returns true".

It becomes useful when you have more complex predicates:

(pick-random-stuff bag?) ? (listof (or/c toy? food? paperclip? aligator?))

This function returns a list containing any amount of those different objects in any order.

Notes

It is considered a good practice to always name your predicates with a ? at the end.

You might have noticed though that in my examples, listof and or/c don't have question marks. It's because they are not predicates themselves, but functions that build predicates.

In most lisp languages, like Scheme, Racket, Clojure, or Common Lisp, a lot of symbols that have specific meanings in other languages are just valid identifiers. ?, =, -, ->, + are all allowed inside variable and function names.