Assertions and panic
You already saw let assert earlier. It binds with a partial pattern and crashes if the value does not match. We use it for invariants we consider impossible to violate, and prefer returning a Result in library code so the caller can decide what to do.
panic crashes immediately. It is for code paths that must never run, such as unreachable branches or unimplemented cases. Add a message with as "..." so failures are clear:
import gleam/io
import gleam/int
pub fn describe(score: Int) {
case score {
s if s > 1000 -> io.println("High score!")
s if s > 0 -> io.println("Still working on it")
_ -> panic as "Scores should never be negative!"
}
}
assert is another way to cause a panic. It checks that a boolean expression evaluates to True. Like the others, you can add a custom message with as. This replaces the generic message when the assertion fails. Do not use assert in applications or libraries. Keep it in tests:
pub fn add_test() {
assert add(1, 2) == 3
assert add(1, 2) < add(1, 3)
assert add(6, 2) == add(2, 6) as "Addition should be commutative"
assert add(2, 2) == 5
}
fn add(a: Int, b: Int) -> Int {
a + b
}