// Ackermann function

`export ack : Fn[m; n] $ (
    ? !=(`type m; `type n) {
        `exit "m and n must be the same type and above zero"
    }
    ? {
        m = 0 { n + 1 }
        &(m > 0; n = 0) { ack `call_sync (m - 1; 1) }
        &(m > 0; n > 0) { ack `call_sync (m - 1; ack `call_sync (m; n - 1)) }
        { 0 }
    }
)

`export ack_string : Fn[m; n] $ (
    String $ ("ack("; m; " "; n; ") = "; ack `call_sync (m; n); "\n")
)

`is_main Fn $ (
    ( args ) : `use "sys"
    ? 4 != `length args {
         `return `error String $ (
            "Got: "; " " `join args; "\n"
            "Usage: "; args `get 0; " "; args `get 1; " <m> <n>\n"
         )
    }
    `print ack_string `call_sync (I64 $ args `get -2; I64 $ args `get -1)
)