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  1. /* package whatever; // don't place package name! */
  2.  
  3. import java.util.function.Supplier;
  4.  
  5. /* Name of the class has to be "Main" only if the class is public. */
  6. class Ideone {
  7.  
  8. 	public static void main (String[] args) throws java.lang.Exception {
  9. 		var ll = Lazy.list( () -> print(1), () -> print(2), () -> print(3) );
  10.         ll.tail().head();
  11. 	}
  12.  
  13. 	private static Unit print(Object o) {
  14.         System.out.println(o);
  15.         return Unit.UNIT;
  16.     }
  17. }
  18.  
  19. final class Lazy {
  20.  
  21.     public static <T> Promise<T> delay(Supplier<T> expr) {
  22.         return new Promise<>(expr);
  23.     }
  24.  
  25.     public static <T> List<T> cons(Supplier<T> headExpr, Supplier<List<T>> tailExpr) {
  26.         return new List<>(new Promise<>(headExpr), new Promise<>(tailExpr));
  27.     }
  28.  
  29.     public static <T> List<T> nil() {
  30.         @SuppressWarnings("unchecked")
  31.         var nil = (List<T>) List.NIL;
  32.         return nil;
  33.     }
  34.  
  35.     @SafeVarargs
  36.     public static <T> List<T> list(Supplier<T>... values) {
  37.         if (values == null || values.length == 0) {
  38.             return nil();
  39.         }
  40.         return list(values, 0);
  41.     }
  42.  
  43.     private static <T> List<T> list(Supplier<T>[] values, int headCursor) {
  44.         if (headCursor == values.length) {
  45.             return nil();
  46.         }
  47.         return new List<>(new Promise<>(values[headCursor]), new Promise<>(() -> list(values, headCursor+1)));
  48.     }
  49.  
  50.     private Lazy() {
  51.         throw new UnsupportedOperationException("utility class");
  52.     }
  53. }
  54.  
  55. final class List<T> {
  56.  
  57.     static final List<?> NIL = new List<>(null, null);
  58.  
  59.     private final Promise<T> head;
  60.     private final Promise<List<T>> tail;
  61.  
  62.     List(Promise<T> head, Promise<List<T>>tail) {
  63.         this.head = head;
  64.         this.tail = tail;
  65.     }
  66.  
  67.     public T head() {
  68.         return head.force();
  69.     }
  70.  
  71.     public List<T> tail() {
  72.         return tail.force();
  73.     }
  74. }
  75.  
  76. final class Promise<T> {
  77.  
  78.     private static final Object undefined = new Object();
  79.  
  80.     private final Supplier<T> delayedExpression;
  81.  
  82.     @SuppressWarnings("unchecked")
  83.     private T value = (T) undefined;
  84.  
  85.     Promise(Supplier<T> expr) {
  86.         this.delayedExpression = expr;
  87.     }
  88.  
  89.     public T force() {
  90.         if (value == undefined) {
  91.             value = delayedExpression.get();
  92.         }
  93.         return value;
  94.     }
  95. }
  96.  
  97. final class Unit {
  98.  
  99.     public static final Unit UNIT = new Unit();
  100.  
  101.     private Unit() {}
  102. }
  103.  
Success #stdin #stdout 0.1s 52148KB
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https://ideone.com/r0ttxo
language:
Java (HotSpot 12)
created:
3 years ago
visibility:
public

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