kau - 6 months ago 25

Java Question

I recently came across this piece of code in Java. It involves Function and printing fibonacci numbers and it works.

`public class AppLambdaSubstitution {`

public static Function<Integer, Integer> Y(Function<Function<Integer, Integer>, Function<Integer, Integer>> f) {

return x -> f.apply(Y(f)).apply(x);

}

public static void main(String[] args) {

Function<Integer, Integer> fib = Y(

func -> x -> {

if (x < 2)

return x;

else

return func.apply(x - 1) + func.apply(x - 2);

});

IntStream.range(1,11).

mapToObj(Integer::valueOf).

map(fib).forEach(System.out::println);

}

}

The part that has me confused is

`return x -> f.apply(Y(f)).apply(x);`

`Y(f)`

`Y`

`f`

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Answer Source

Fundamentally you are missing the point that `x -> f.apply(Y(f)).apply(x);`

will not call `apply`

, it will `return`

a `Function`

.

That's just a very complicated (and non-intuitive?) way of showing currying and recursive function IMO. Things would be much simpler if you would replace a couple of things and make it a bit more readable.

This construction:

```
Function<Function<Integer, Integer>, Function<Integer, Integer>>
```

is not needed at all, since the left parameter is not *used* at all. It's simply needed to get a hold of the right one. As such the `left`

parameter could *be anything* at all (I will later replace it with `Supplier`

- that is not needed either, but just to prove a point).

Actually all you care about here is this `Function`

that does the actual computation for each element of the `Stream`

:

```
public static Function<Integer, Integer> right() {
return new Function<Integer, Integer>() {
@Override
public Integer apply(Integer x) {
if (x < 2) {
return x;
} else {
return apply(x - 1) + apply(x - 2);
}
}
};
}
```

Now you could write that entire construct with:

```
Supplier<Function<Integer, Integer>> toUse = () -> right();
Function<Integer, Integer> fib = curry(toUse);
IntStream.range(1, 11)
.mapToObj(Integer::valueOf)
.map(fib)
.forEach(System.out::println);
```

This `Supplier<Function<Integer, Integer>> toUse = () -> right();`

should make you understand why in the previous example (`Function<Function, Function>`

) the *left part* was needed - just to get a hold of the `right`

one.

If you look even closer, you might notice that the `Supplier`

is **entirely not needed**, thus you could even further simplify it with:

```
IntStream.range(1, 11)
.mapToObj(Integer::valueOf)
.map(right())
.forEach(System.out::println);
```

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