func double(value: i32) -> i32:
    return value * 2
func apply<t>(callback: func(t) -> t, value: t) -> t:
    return callback(value)
func entry(value: i32) -> i32:
    var callback: (func(i32) -> i32)?
    if callback is Ok(var absent):
        return absent(value)
    callback = double
    return match callback:
        Ok(var present): apply(present, value)
        null: -1