代數資料型態：Java 17

AlgebraicType.java

package cc.openhome;

import java.util.Arrays;

sealed interface List<T> permits Nil, Cons<T> {
    default T head() { return null; }
    default List<T> tail() { return null; }
}

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

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

public class AlgebraicType {
    @SafeVarargs
    @SuppressWarnings("unchecked")
    public static <T> List<T> list(T... elems) {
        if(elems.length == 0) return Nil.INSTANCE;
        var remain = Arrays.copyOfRange(elems, 1, elems.length);
        return new Cons<T>(elems[0], list(remain));
    }
    
    // Java 17 使用預覽的 switch pattern matching
    public static Integer sum(List<Integer> lt) {
    	return switch(lt) {
    	    case Nil nil -> 0;
    	    case Cons<Integer> cons -> cons.head() + sum(cons.tail());
    	};
    }
    
      /* switch pattern matching 結合未來版本的  deconstruction pattern 可以這樣？ */
    
//    public static Integer sum(List<Integer> lt) {
//    	return switch(lt) {
//    	    case Nil nil -> 0;
//    	    case Cons(head, tail) -> head + sum(tail);
//    	};
//    }
    
    public static void main(String[] args) {
    	var lt = list(1, 2, 3);
    	System.out.println(lt); // 1:2:3:[]
    	
    	var n = sum(lt);
    	System.out.println(n);  // 6
    }
}