fun is_space(c)
    return c == ' ' 
        or c == '\t' 
        or c == '\n';

fun is_digit(c)
    return unicode(c) <= unicode('9')
       and unicode(c) >= unicode('0');

fun skip(ctx, test)
    while not ended(ctx): {
        if not test(current(ctx)):
            break;
        next(ctx);
    }

fun ended(ctx)
    return ctx.i == count(ctx.str);

fun next(ctx) {
    assert(ctx.i < count(ctx.str));
    ctx.i = ctx.i + 1;
}

fun current(ctx) {
    assert(ctx.i < count(ctx.str));
    return ctx.str[ctx.i];
}

fun parse(str) {

    if str == none: 
        str = '';

    ctx = {str: str, i: 0};

    skip(ctx, is_space);

    if ended(ctx): {
        print('Source only contains whitespace');
        return none;
    }

    return parse_any_value(ctx);
}

fun parse_any_value(ctx) {

    if is_digit(current(ctx)): {
        # Number (int or float)
        parsed = 0;
        while not ended(ctx) and is_digit(current(ctx)): {
            parsed = parsed * 10 + unicode(current(ctx)) - unicode('0');
            next(ctx);
        }

        if current(ctx) == '.': {

            next(ctx);

            if ended(ctx):
                # Source string ended unexpectedly after dot.
                return none;

            if not is_digit(current(ctx)):
                # Got something other than a digit after dot.
                return none;

            fact = 1;
            while not ended(ctx) and is_digit(current(ctx)): {
                fact = fact / 10.0;
                parsed = parsed + fact * (unicode(current(ctx)) - unicode('0'));
                next(ctx);
            }
        }
        
        return parsed;
    }

    assert(false);
}


tests = [
    '',
    '1'
];

i = 0;
while i < count(tests): {
    parse(tests[i]);
    i = i + 1;
}