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  1. /* package whatever; // don't place package name! */
  2.  
  3. import java.util.ArrayList;
  4. import java.util.function.BiFunction;
  5. import java.util.function.Supplier;
  6.  
  7. class Ideone {
  8.  
  9.     public static void main(String[] args) {
  10.         System.out.println(Lazy.forceList(Lazy.take(10, fibs())));
  11.     }
  12.  
  13.     private static List<Integer> fibs() {
  14.         return Lazy.cons(
  15.                 () -> 0,
  16.                 () -> Lazy.cons(
  17.                         () -> 1,
  18.                         () -> Lazy.zip(Integer::sum, fibs(), fibs().tail())));
  19.     }
  20. }
  21.  
  22. final class Promise<T> {
  23.  
  24.     private static final Object undefined = new Object();
  25.  
  26.     private final Supplier<T> delayedExpression;
  27.  
  28.     @SuppressWarnings("unchecked")
  29.     private T value = (T) undefined;
  30.  
  31.     Promise(Supplier<T> expr) {
  32.         this.delayedExpression = expr;
  33.     }
  34.  
  35.     public T force() {
  36.         if (value == undefined) {
  37.             value = delayedExpression.get();
  38.         }
  39.         return value;
  40.     }
  41. }
  42.  
  43. final class List<T> {
  44.  
  45.     static final List<?> NIL = new List<>(null, null);
  46.  
  47.     private final Promise<T> head;
  48.     private final Promise<List<T>> tail;
  49.  
  50.     List(Promise<T> head, Promise<List<T>> tail) {
  51.         this.head = head;
  52.         this.tail = tail;
  53.     }
  54.  
  55.     public T head() {
  56.         return head.force();
  57.     }
  58.  
  59.     public List<T> tail() {
  60.         return tail.force();
  61.     }
  62. }
  63.  
  64. final class Lazy {
  65.  
  66.     public static <T> Promise<T> delay(Supplier<T> expr) {
  67.         return new Promise<>(expr);
  68.     }
  69.  
  70.     public static <T> List<T> cons(Supplier<T> headExpr, Supplier<List<T>> tailExpr) {
  71.         return new List<>(new Promise<>(headExpr), new Promise<>(tailExpr));
  72.     }
  73.  
  74.     public static <T> List<T> nil() {
  75.         @SuppressWarnings("unchecked")
  76.         var nil = (List<T>) List.NIL;
  77.         return nil;
  78.     }
  79.  
  80.     public static <T> List<T> take(int n, List<T> xs) {
  81.         if (n <= 0) {
  82.             return nil();
  83.         }
  84.         if (xs == nil()) {
  85.             return nil();
  86.         }
  87.         return cons(xs::head, () -> take(n - 1, xs.tail()));
  88.     }
  89.  
  90.     public static <A, B, R> List<R> zip(BiFunction<A, B, R> f, List<A> xs, List<B> ys) {
  91.         if (xs == nil()) {
  92.             return nil();
  93.         }
  94.         if (ys == nil()) {
  95.             return nil();
  96.         }
  97.         return cons(
  98.                 () -> f.apply(xs.head(), ys.head()),
  99.                 () -> zip(f, xs.tail(), ys.tail()));
  100.     }
  101.  
  102.     public static <T> ArrayList<T> forceList(List<T> xs) {
  103.         final var list = new ArrayList<T>();
  104.         for (var it = xs; it != nil(); it = it.tail()) {
  105.             list.add(it.head());
  106.         }
  107.         return list;
  108.     }
  109.  
  110.     @SafeVarargs
  111.     public static <T> List<T> list(Supplier<T>... values) {
  112.         if (values == null || values.length == 0) {
  113.             return nil();
  114.         }
  115.         return list(values, 0);
  116.     }
  117.  
  118.     private static <T> List<T> list(Supplier<T>[] values, int headCursor) {
  119.         if (headCursor == values.length) {
  120.             return nil();
  121.         }
  122.         return new List<>(new Promise<>(values[headCursor]), new Promise<>(() -> list(values, headCursor+1)));
  123.     }
  124.  
  125.     private Lazy() {
  126.         throw new UnsupportedOperationException("utility class");
  127.     }
  128. }
  129.  
Success #stdin #stdout 0.1s 47820KB
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[0, 1, 1, 2, 3, 5, 8, 13, 21, 34]
https://ideone.com/DDjz1i
language:
Java (HotSpot 12)
created:
3 years ago
visibility:
public

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