Booleans in

About Booleans

The Bool type is one of the building blocks of the language. It aims at describing whether something is true or false, with the respective True and False values. The name Bool is short for Boolean, which is a field of mathematics related to logical thinking, named after the work of George Boole.

Boolean operators

There are two functions and two operators to manipulate values of type Bool.

• not : Bool -> Bool This function transforms True into False and False into True.
• (&&) : Bool -> Bool -> Bool This operator called "AND" takes two booleans and returns True if they are both True. For example, True && True returns True but True && False returns False.
• (||) : Bool -> Bool -> Bool This operator called "OR" takes two booleans and returns True if at least one of them is True. For example, False || False returns False but True || False returns True.
• xor : Bool -> Bool -> Bool This function (not operator) called "XOR" takes two booleans and returns True if and only if one is True and the other is False.

The && and || operators do not have the same priority. The priority of && is higher than the one of ||, just like with multiplication * and addition +. As a consequence, the expression True || False && False is equivalent to True || (False && False) which is different from (True || False) && False. While the former returns True the latter returns False.

Conditional branching

Booleans are extremely useful to make branches in your code that only get executed if certain conditions are met. In Elm, this is done with an if-then-else expression, used as follows.

if someCondition || (someOtherCondition && anotherCondition) then
    doSomething
else
    doSomethingElse

The value someCondition can be any expression that returns a Bool. If that boolean value is True, the code will execute doSomething. Otherwise it is False and the code will execute doSomethingElse. The else branch is mandatory, contrary to some other programming languages.

If you need to chain conditions, you can just chain the if-then-else expressions.

if someCondition then
    doSomething
else if someOtherCondition then
    doAnotherThing
else
    doSomethingElse