Data commit

Task/Church-numerals/Java/church-numerals.java

@@ -0,0 +1,66 @@
+package lvijay;
+
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.function.Function;
+
+public class Church {
+    public static interface ChurchNum extends Function<ChurchNum, ChurchNum> {
+    }
+
+    public static ChurchNum zero() {
+        return f -> x -> x;
+    }
+
+    public static ChurchNum next(ChurchNum n) {
+        return f -> x -> f.apply(n.apply(f).apply(x));
+    }
+
+    public static ChurchNum plus(ChurchNum a) {
+        return b -> f -> x -> b.apply(f).apply(a.apply(f).apply(x));
+    }
+
+    public static ChurchNum pow(ChurchNum m) {
+        return n -> m.apply(n);
+    }
+
+    public static ChurchNum mult(ChurchNum a) {
+        return b -> f -> x -> b.apply(a.apply(f)).apply(x);
+    }
+
+    public static ChurchNum toChurchNum(int n) {
+        if (n <= 0) {
+            return zero();
+        }
+        return next(toChurchNum(n - 1));
+    }
+
+    public static int toInt(ChurchNum c) {
+        AtomicInteger counter = new AtomicInteger(0);
+        ChurchNum funCounter = f -> {
+            counter.incrementAndGet();
+            return f;
+        };
+
+        plus(zero()).apply(c).apply(funCounter).apply(x -> x);
+
+        return counter.get();
+    }
+
+    public static void main(String[] args) {
+        ChurchNum zero  = zero();
+        ChurchNum three = next(next(next(zero)));
+        ChurchNum four  = next(next(next(next(zero))));
+
+        System.out.println("3+4=" + toInt(plus(three).apply(four))); // prints 7
+        System.out.println("4+3=" + toInt(plus(four).apply(three))); // prints 7
+
+        System.out.println("3*4=" + toInt(mult(three).apply(four))); // prints 12
+        System.out.println("4*3=" + toInt(mult(four).apply(three))); // prints 12
+
+        // exponentiation.  note the reversed order!
+        System.out.println("3^4=" + toInt(pow(four).apply(three))); // prints 81
+        System.out.println("4^3=" + toInt(pow(three).apply(four))); // prints 64
+
+        System.out.println("  8=" + toInt(toChurchNum(8))); // prints 8
+    }
+}