effect StringBuilder {
    fun append(String) -> Unit;
}

pub fun main() ! Io -> Unit {
    var result = "";

    try {
        StringBuilder.append!("hello");
        StringBuilder.append!(", ");
        StringBuilder.append!("world");
        StringBuilder.append!("\n");
    } handle StringBuilder {
        fun append(value) {
            set result = result + value;

            resume;
        }
    }

    print!(result);
}