Input Coercion for Non-Null states that field(arg: null) is illegal, but it's not

[swolchok]

From http://facebook.github.io/graphql/#sec-Non-Null :

NOTE that null is not a valid keyword in GraphQL, so a query cannot look like:
{
field(arg: null)
}
to indicate that the argument is null.

However, this example should actually parse, according to the given grammar. It is a valid Arguments with one Argument whose Value is an EnumValue with the Name "null". This is a consequence of the keywordless approach to grammar definition. I'm not sure how to succinctly explain that this example may be legal, but not because null is a pre-defined keyword that necessarily means the same thing as "unset variable", and that there isn't a way to get it to typecheck for all calls generally given the rest of the system.

[leebyron]

Whoops indeed. We were writing spec as we were deciding on whether null should be a keyword or not.

The reason we decided not to have null as a keyword was that field(arg: null) and field would be isomorphic and it felt a little weird to have two ways of specifying the same thing.

I think we have a few possible paths forward and I'm interested in your opinions on which we might follow:

1. Fix up the spec, leave things as they are, carry on.

2. Introduce a null keyword which represents the value null. Introduce this slightly odd isomorphism but allow null to be used as expected.

3. Introduce a reserved blacklist which the Name grammar cannot represent (or more conservatively the EnumValue cannot represent?), and include null on this list. This means there's no way to represent the input value null as is currently the case, but just makes field(arg: null) not even parse rather than do an unexpected thing.

[clentfort]

Strong vote for fixing the example. With arguments being unordered and having optional default values I don't see a reason for having an explicit null-type in GraphQL. A list of reserved words that are not going to be used in the future only because they have a meaning something in a different popular language makes no sense.

[leebyron]

I added some clarification to the example itself, but also implemented the conservative form of option "3" I described above in a12f6df. An EnumValue cannot be null to avoid this confusion.