True, False, and Boolean Expressions in Clojure
Truthiness and falsiness**
Nil and false represent logical falsiness. Any value that isn’t falsey is truthy. You can check if a value is nil (or falsey) using the nil? operand:
(nil? 1) returns false because 1 is not nil
(nil? nil) returns true because nil is nil
Boolean expressions will treat values as truthy or falsey. The equality operator (“=”) will evaluate if operands are equal (in other words, use “=” when evaluating truth rather than truthiness).
Boolean operators
- or returns the first truthy value or the last value (if none are truthy) of a list of operands
- and returns the first falsey value or the last truthy value (if none are falsey) of a list of operands
(or false nil 1 2 3)
- does not return the first operand, “false”, because it is falsey
- does not return the second operand, “nil”, because it is also falsey
- it will return the third operand, 1, which evaluates as truthy
(or false nil)
- does not return the first operand, “false” because it is falsey
- does return the second operand, “nil” even though it is falsey, because it is the last value in the list
(and 1 2 nil 3 false)
- does not return the first operand, 1, because it is truthy
- does not return the second operand, 2, because it is truthy as well
- does return the third operand, “nil”, because it is falsey
- will not evaluate further once it finds a value to return
(and 1 2 3)
- will not return the first operand, 1, which is truthy
- will not return the second operand, 2, which is truthy
- will return the third and final operand, 3, even though it is truthy, because it is the last operand in the list.
see also
references
- Higginbotham, Daniel. Clojure for the Brave and True Learn the Ultimate Language and Become a Better Programmer. Daniel Higginbotham, 2015, [https://www.braveclojure.com/clojure-for-the-brave-and-true/].