Milestone 4

assert statement

The assert statement throws an exception if a condition is not true, and can narrow the type of attributes in the statements following it.

Usage

The general form of the assert statement is

assert ( /* some conditions */ );

Description

Execution

The condition (or conditions) in the assert statement are evaluated. If they evaluate as true then execution proceeds with the statement following the assert. Otherwise an exception is thrown.

Purpose

assert is usually used to make assertions about a program which the programmer knows are true, but which cannot be proved to be true within the type system.

Here's an example using the parseInteger() method from ceylon.language which returns Integer? so as to force you to handle the possibility that the argument was not String representing a number:

value num = parseInteger("1");
// before the assert statement num is of type Integer?
assert (exists num);
// after the assert statement num is of type Integer
value plusOne = num + 1;

In thise case parseInteger() is being called with a String literal which we know is a valid number. The type checker cannot know this, however, because it can only reason about types, not about what the parseInteger() method does for a particular input.

Boolean conditions

Any Boolean expression can be used as a condition in an assert statement.

'Special' conditions

The assert statement also supports the use of certain special form conditions:

these narrow the type of a reference in the statements following the assert.

Condition lists

The condition in an assert statement can also be a condition list.

The difference between a condition list and a single Boolean condition constructed using the && operator is that the typecasting of conditions in the list take effect for conditions later in the list, allowing you to write:

void m(Object x) {
    assert (is Integer x, x < 10);
}

See also