Allow singular variables in list locations

At a glance

• Identifier: #509
• Stage: RFC 1 / Proposal
• Champion: @taion
• Latest activity: Advanced to RFC 1 on 2018-10-02
• Spec PR: https://github.com/graphql/graphql-spec/pull/509

Spec PR description

This is a follow-up to discussion in https://github.com/graphql/graphql-relay-js/issues/20#issuecomment-416809871.

The current coercion rules, for convenience, allow passing in singular literals in positions expecting lists. However, they do not allow passing in singular variables.

This proposal would allow passing in singular variables as well. The coercion rules already ensure reasonable behavior when this happens. Enabling this allows easier evolution of APIs, as it would enable converting singular filtering/ordering fields per the linked issues to plural ones without breaking clients.

cc @dschafer