match 按模式分派。编译器检查穷尽性：漏了分支会报错，并告诉你漏了哪个。
fn sign(x: Int) -> String =
match x {
0 -> "zero"
n if n > 0 -> "positive"
_ -> "negative"
}
pub fn main() -> Unit !io = {
println(sign(0))
println(sign(7))
println(sign(-2))
}zero
positive
negative另见：spec.md §5