54 lines
2.0 KiB
Java
54 lines
2.0 KiB
Java
interface Function<A, B> {
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public B call(A x);
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}
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public class YCombinator {
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interface RecursiveFunc<F> extends Function<RecursiveFunc<F>, F> { }
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public static <A,B> Function<A,B> fix(final Function<Function<A,B>, Function<A,B>> f) {
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RecursiveFunc<Function<A,B>> r =
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new RecursiveFunc<Function<A,B>>() {
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public Function<A,B> call(final RecursiveFunc<Function<A,B>> w) {
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return f.call(new Function<A,B>() {
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public B call(A x) {
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return w.call(w).call(x);
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}
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});
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}
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};
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return r.call(r);
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}
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public static void main(String[] args) {
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Function<Function<Integer,Integer>, Function<Integer,Integer>> almost_fib =
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new Function<Function<Integer,Integer>, Function<Integer,Integer>>() {
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public Function<Integer,Integer> call(final Function<Integer,Integer> f) {
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return new Function<Integer,Integer>() {
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public Integer call(Integer n) {
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if (n <= 2) return 1;
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return f.call(n - 1) + f.call(n - 2);
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}
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};
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}
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};
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Function<Function<Integer,Integer>, Function<Integer,Integer>> almost_fac =
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new Function<Function<Integer,Integer>, Function<Integer,Integer>>() {
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public Function<Integer,Integer> call(final Function<Integer,Integer> f) {
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return new Function<Integer,Integer>() {
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public Integer call(Integer n) {
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if (n <= 1) return 1;
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return n * f.call(n - 1);
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}
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};
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}
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};
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Function<Integer,Integer> fib = fix(almost_fib);
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Function<Integer,Integer> fac = fix(almost_fac);
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System.out.println("fib(10) = " + fib.call(10));
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System.out.println("fac(10) = " + fac.call(10));
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}
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}
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