Range pattern

// A literal pattern matches one value. A range pattern matches a whole run of them, written with
// the same two operators as a range in a `for` loop:
//
//     200..=299   from 200 up to and including 299
//     0..10       from 0 up to, but not including, 10
//
// That turns "is the value between these two numbers?" into a single arm instead of a pair of
// comparisons, and a ladder of ranges reads like the table it came from.
//
// Both ends must be literal numbers. A named constant is not accepted as an end, because a bare
// name in a pattern means "bind a new variable". With `const Low = 100;`, the arm `Low..=199 =>`
// stops with
//     error: pattern 'Low' cannot bind a new variable because 'Low' already names a constant
// Ranges work on integers only; characters are matched one at a time (see the CharacterPattern
// lesson).
import Io::PrintLine;

// Inclusive ranges, `..=`, suit a table whose rows name their first and last value.
func Status(code: int32) -> char8[..] {
    return match code {
        100..=199 => "informational",
        200..=299 => "success",
        300..=399 => "redirection",
        400..=499 => "client error",
        500..=599 => "server error",
        else => "not a status code"
    };
}

// Exclusive ranges, `..`, suit boundaries: each range stops where the next one starts, so no value
// falls between two arms and none is claimed twice. Ranges mix freely with literal arms, and a
// negative end is written with its minus sign.
func Size(count: int32) -> char8[..] {
    return match count {
        0 => "none",
        1..10 => "a few",
        10..100 => "dozens",
        100..1000 => "hundreds",
        -1000..0 => "a debt",
        else => "a lot"
    };
}

// A range takes a guard like any other pattern. The arm is chosen only when the age is in the range
// *and* the condition holds; otherwise the match moves on, so a non-student of 20 pays full fare.
func Fare(age: int32, student: bool) -> int32 {
    return match age {
        0..=5 => 0,
        6..=17 => 5,
        18..=25 if student => 5,
        65..=120 => 6,
        else => 10
    };
}

func Main() -> int {
    let codes: int32[4] = [204, 302, 404, 99];
    for index in 0..4 {
        PrintLine("{} {}", codes[index], Status(codes[index]));
    }

    let counts: int32[5] = [0, 9, 10, 250, -3];
    for index in 0..5 {
        PrintLine("{} is {}", counts[index], Size(counts[index]));
    }

    PrintLine("fares: age 4 pays {}, 12 pays {}, 70 pays {}", Fare(4, false), Fare(12, false),
        Fare(70, false));
    PrintLine("age 20 pays {} as a student, {} otherwise", Fare(20, true), Fare(20, false));
    return 0;
}