assert statement

Syntax
AssertStatement :
   assert Expression (, Expression)?

The assert statement is used express invariants in the code. It consists of a boolean expression optionally followed by a comma followed by a string expression.

If the boolean expression evaluates to false, the code reverts with a panic code of 0x01. In the case that the first expression evaluates to false and a second string expression is given, the code reverts with the given string as the error code.

Warning: The current implementation of assert is under active discussion and likely to change.

An example of a assert statement without the optional message:

contract Foo:
    fn bar(val: u256):
        assert val > 5

An example of a assert statement with an error message:

contract Foo:

    fn bar(val: u256):
        assert val > 5, "Must be greater than five"