boolean

A data type with two possible values: true and false.

Boolean logic can be performed with the and, or and not operators, for example:

>>> true and false
false
>>> true or false
true
>>> not true
false
>>> not false
true

In addition, booleans are used in ternary expressions:

>>> if (true) 1 else 2
1
>>> if (false) 1 else 2
2

The in operator returns a boolean:

>>> 1 in list<integer>()
false
>>> 1 in [1, 2]
true

Booleans are used in if- and if/else-statements:

>>> if (false) { print("hello"); } else { print("goodbye"); }
goodbye
>>> if (false) { print("hello"); }
>>> if (true) { print("hello"); }
hello
>>>

Boolean expressions are the basis of zero-argument when-statements (each expression to the left of a '->' symbol has boolean type (and in this context else is equivalent to true)):

when {
    x == 1 -> return 'One';
    x >= 2 and x <= 7 -> return 'Several';
    x == 11, x == 111 -> return 'Magic number';
    some_value 1000 -> return 'Special case';
    else -> return 'Unknown';
}

Boolean conditions are used in while loops:

while (x < 10) {
    print(x);
    x = x + 1;
}

Functions can have boolean return type (as can queries and operations), and indeed many functions and properties in the Rell standard library have boolean type.

function foo(x: integer): boolean {
    return x >= 10;
}