Doing discovery before going hard with graphcool. How to handle the use case of wanting to report to the user on the client that there is an error with an email field used in a mutation? Say, it might not be a valid email. Or, it might not be unique in the DB.

You can't check for valid emails right now, but uniqueness is checked if you mark a field as registered. GC will return an error if user doesn't register with a unique email