Add directive @void to unify the definition of fields with no return value

[spawnia]

• Added or updated tests
• Documented user facing changes
• Updated CHANGELOG.md

See graphql/graphql-spec#906

Changes

• Add directive @void to unify the definition of fields with no return value

Breaking changes

None, since the service provider is optional. When added by default, it will reserve the directive name @void and the type name Unit.

[stayallive]

I like it, on a technical basis!

I just don't care for the choice for Unit, it seems like a random name that easily clashes with userland (I now learned that this is a type theory thing and makes sense but only if you know the sauce). Why not use enum Void { VOID } or enum Empty { EMPTY } or enum Accepted { ACCEPTED } (close to a HTTP 202 response) or even enum Null { NULL } or something a little closer to what it is actually doing?

Regardless, this seems like a nice addition!

[pyrou]

This PR becomes a bit old.
But for what it worth, here is what I use in my project :

scalar Void @scalar(class: "App\\GraphQL\\Scalars\\VoidType")

With following dummy implementation (a class simply named "Void" isn't allowed in PHP) :

class VoidType extends ScalarType
{
    public function serialize($value): mixed {
        return null;
    }

    public function parseValue($value): mixed {
        return null;
    }

    public function parseLiteral(Node $valueNode, ?array $variables = null): mixed {
        return null;
    }
}

By making Void a type rather than a directive, it becomes much more clear when defining the schema :

        type Query {
            foo: Void
        }

The approach proposed with @void directive ends with very unnatural definition in schema, like the one seen in unit test :

        type Query {
            foo: Int @void
        }

Is it void or Int ?