assert statement

The assert statement validates a given condition, throwing an AssertionError if the condition is not satisfied. An assertion may narrow the type of values as seen by subsequent statements.


The general form of the assert statement is

assert ( /* some conditions */ );


Unlike the assert statement in the Java programming language, assert in Ceylon cannot be disabled at runtime. It's important to know about bugs that occur in running production systems, and assertions help us find out about them.


The condition (or conditions) in the assert statement are evaluated in the order they occur. If they are all satisfied, then execution continues with the statement immediately following the assert. Otherwise, an AssertionError is thrown with information about the condition that was violated.


assert is used to make assertions:

  • regarding program invariants which the programmer knows to be true, but which cannot be proved to be true within the type system, or
  • about preconditions required by an API, which cannot be enforced within the type system.

Failure of an assertion represents a bug in the program or misuse of the API.

Here's an example using the parseInteger() function from ceylon.language which returns Integer?, forcing the caller 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;

Here, 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() function does for a particular input value.

Here's a different example, from the internal implementation of that function:

shared Integer? parseInteger(String string, Integer radix = 10) {
    assert (radix >= minRadix, radix <= maxRadix);

Here, the assertion imposes a constraint upon the argument radix. If a client calls parseInteger() with an illegal value for radix, then the AssertionError will indicate this.


The conditions in an assert statement form a condition list.

Any expression of type Boolean may be occur in the condition list of an assert statement. The assert statement also supports the use of typing conditions:

These conditions narrow the type of a reference in the statements following the assert, and in later conditions in the condition list.

void printSqrt(Object x) {
    assert (is Float x, x >= 0.0);

See also