OneOf inhabitability

[jbellenger]

OneOf types can form graphs for which finite values cannot be created.

The simplest example of this is a self-recursing OneOf type:

input A @oneOf { a:A }

While this is the simplest form of the issue, we can build more complex examples from trees where every type is a OneOf:

input A @oneOf { b:B c:C }
input B @oneOf { c:C a:A }
input C @oneOf { a:A b:B }

These kinds of types are considered valid on two fronts today:

1. They are valid as OneOf types because all field types are nullable and there are no default values
2. They are valid as a circular input object because there are no unbreakable chains of non-null non-list fields

While this follows the letter of the Circular References part of the spec, it violates the spirit of the spec in that there is no way to create a legal value for these types.

I propose that in keeping with the spirit of the Circular References section, that these topologies be declared invalid. I took a stab at clarifying the spec around these kinds of types. With a provisional thumbs-up I'll plan on adding validation to graphql-js and graphql-java.

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[benjie]

Excellent catch! I think we can and should fast track this. Once you have the GraphQL.js implementation in place either add it to the next GraphQL WG agenda or let me know if you’d rather I represent it on your behalf.

[benjie]

Let's change the named type to an assertion for clarity

[benjie]

Suggested change

	- Let {namedFieldType} be the underlying named type of {fieldType}.
	- If {namedFieldType} is not an Input Object type:
	- Return {true}.
	- If {namedFieldType} is not a _OneOf Input Object_:
	- Return {true}.
	- If {OneOfInputObjectCanBeProvidedAFiniteValue(namedFieldType, nextVisited)}:
	- Return {true}.
	- Assert: {fieldType} is a named type.
	- If {fieldType} is not an Input Object type:
	- Return {true}.
	- If {fieldType} is not a _OneOf Input Object_ type:
	- Return {true}.
	- If {OneOfInputObjectCanBeProvidedAFiniteValue(fieldType, nextVisited)}:
	- Return {true}.