Whole-Pattern Aliases

This example uses as to inspect a result while keeping the whole value available. The positive function checks the number inside Ok, returns the original result if it is positive, and passes existing errors through unchanged.

positive : Try(I64, [BadNumber, NotPositive]) -> Try(I64, [BadNumber, NotPositive])
positive = |input| match input {
	Ok(n) as result => if n > 0 {
		result
	} else {
		Err(NotPositive)
	}
	Err(_) as result => result
}

expect positive(Ok(3)) == Ok(3)
expect positive(Ok(0)) == Err(NotPositive)
expect positive(Err(BadNumber)) == Err(BadNumber)

In Ok(n) as result, n is the number and result is the complete Ok(n) value. This lets us examine part of a value without having to reconstruct the whole thing when returning it.

Output

Run this from the directory that has main.roc in it:

$ roc test main.roc

All (3) tests passed in 9.2 ms.

Validated with Roc nightly-2026-10-29-a0a9fc2.