代數資料型態：子型態多型+模式比對

AlgebraicType.java

package cc.openhome;

import java.util.Arrays;

sealed interface List permits Nil, Cons {
    default Integer head() { return null; }
    default List tail() { return null; }
    
	default Integer sum() {
    	return switch(this) {
	        case Nil nil -> 0;
	        case Cons cons -> cons.head() + cons.tail().sum();
	    };
	}
}

final class Nil implements List {
	public static final Nil INSTANCE = new Nil();
	private Nil() {}
	public String toString() { return "[]"; }
}

record Cons(Integer head, List tail) implements List {
	public String toString() { return head() + ":" + tail(); }
}

public class AlgebraicType {
    public static List list(Integer... elems) {
        if(elems.length == 0) return Nil.INSTANCE;
        var remain = Arrays.copyOfRange(elems, 1, elems.length);
        return new Cons(elems[0], list(remain));
    }
    
    public static void main(String[] args) {
    	var lt = list(1, 2, 3);
    	System.out.println(lt); // 1:2:3:[]
    	
    	var n = lt.sum();
    	System.out.println(n);  // 6
    }
}