assert

Built-in statement · Available everywhere, with no import

assert checks that a condition is true. When it is not, the program stops with an AssertionError that shows the condition that failed. Use it to check what must be true at a point in a program, and in tests.

const scores = [90, 72, 85]
assert scores.count == 3
assert scores.all? { score => score >= 0 }, "scores cannot be negative"

assert

Does nothing when condition is true. When it is false, raises an AssertionError whose message is message if one was written, or the condition’s own text otherwise: assertion failed: \scores.count == 3``.

It is written without parentheses, as assert condition or assert condition, message. The condition must be a Bool, and the message a String.

When an == or != fails, the error Emerald shows also gives both sides’ values, so a failed check says what the values actually were:

assertion failed: `total == 10`
Left was 9; right was 10.