fun dummy() {}
Func = type(dummy) 
     | type(print);

Scanner = {
    src: String, 
    i: int,
    hint: Func,
    current: Func,
    consume: Func,
    consumeSpaces: Func
};

fun hint(scan: Scanner, n: int = 1) {
    k = scan.i + n;
    if k < count(scan.src):
        return scan.src[k];
    return none;
}

fun current(scan: Scanner)
    return hint(scan, 0);

fun consume(scan: Scanner, n: int = 1) {
    assert(n > 0);
    limit = count(scan.src);
    scan.i = min(scan.i + n, limit);
    return current(scan);
}

fun consumeSpaces(scan: Scanner) {
    char = current(scan);
    while isSpace(char):
        char = consume(scan);
    return char;
}

fun newScanner(src: String) {
    scan = {
        src: src, 
        i: 0,
        hint: hint,
        consume: consume,
        current: current,
        consumeSpaces: consumeSpaces
    };
    assert(istypeof(Scanner, scan));
    return scan;
}

fun isDigit(char: ?String) {
    if char == none:
        return false;
    return string.ord(char) >= string.ord('0')
       and string.ord(char) <= string.ord('9');
}

fun isSpace(c: String)
    return c == ' '
        or c == '\t'
        or c == '\n';

fun min(x, y) {
    if x < y:
        return x;
    return y;
}