/** * Y Combinator Factorial * Copyright 2009 Ken Shirriff * http://arcfn.com */ class YFact { // Integer function returning an integer // int -> int interface IntFunc { int apply(int n); } // Function on int function returning an int function // (int -> int) -> (int -> int) interface IntFuncToIntFunc { IntFunc apply(IntFunc f); }; // Higher-order function returning an int function // F: F -> (int -> int) interface FuncToIntFunc { IntFunc apply(FuncToIntFunc x); } // Function from IntFuntToIntFunc to IntFunc // ((int -> int) -> (int -> int)) -> (int -> int) interface IntFuncToIntFuncToIntFunc { IntFunc apply(IntFuncToIntFunc r);}; public static void main(String args[]) { System.out.println( // Y combinator (new IntFuncToIntFuncToIntFunc() { public IntFunc apply(final IntFuncToIntFunc r) { return (new FuncToIntFunc() {public IntFunc apply(final FuncToIntFunc f) { return f.apply(f); }}) .apply( new FuncToIntFunc() { public IntFunc apply(final FuncToIntFunc f) { return r.apply( new IntFunc() { public int apply(int x) { return f.apply(f).apply(x); }});}});}} ).apply( // Recursive function generator new IntFuncToIntFunc() { public IntFunc apply(final IntFunc f) { return new IntFunc() { public int apply(int n) { if (n == 0) return 1; else return n * f.apply(n-1); }};}} ).apply( // Argument Integer.parseInt(args[0]))); } }