Make Functions.constant covariant in argument type.

Presently, a constant function is provided in the Functions class with a type signature as follows

public static <E> Function<Object, E> constant(@Nullable E value) 

Personally, I think that a type signature as follows would be superior

public static <Q, E> Function<Q, E> constant(@Nullable E value) 

the reason being that while functions in the “mathematical” sense are co-variant with regard to their argument types, generics in java are not (for well-known reasons).

It’s probably not trivial to introduce such a change without triggering a compilation error here or there. Still I think it might be worth considering.

[vincentk]

There are no doubt many plausible workarounds (as it were, I am quite content with mine). It is also a very minor issue. I simply felt it noteworthy that a workaround would seem to be required in the first place. Please do feel free to close it if it isn’t worth your time.

As “solutions” go though, my proposal would be to make ConstantFunction generic in the parameter type as follows:

  private static class ConstantFunction<D,E> implements Function<D, E>, Serializable {
    private final E value;

    public ConstantFunction(@Nullable E value) {
      this.value = value;
    }

    @Override
    public E apply(@Nullable D from) {
      return value;
    }

[snip]

and either change or provide an alternative to constant(T val):

    public static <D, E> Function<D, E> constantD(E value) {
        return new ConstantFunction<>(value);
    }

    public static <E> Function<Object, E> constant(E value) {
        return new ConstantFunction<>(value);
    }

Though again, I do agree that this is generics cosmetics, and perhaps not worth the possible API change/extension.

[lowasser]

Generics aren’t, but specific uses of generic types that happen to be covariant should be – e.g. most places that accept a Function<F, T> should accept a Function<? super F, ? extends E> where appropriate.

Is there a place you cannot make this change to the user of the Function?