I am sure I will be corrected if I am wrong, but from what I understand:
Version 20.07.0 (which is still in beta) is suppose to have a solid support for the @custom directive. The use case examples tend to not have resolvers per se, but more along the lines of remote endpoints. You could write a lambda script and then use a @custom directive to point to the lambda script to get back the data that you want given the variables provided.
I asked recently about a way to do it without a external script: Custom Field concatenation without needing external script and was referred back to the docs and github examples in source.
What would be nice is a way to use some simple functions inline like math or concatenation functions in the schema to make simple custom resolvers.
I am working out my use case for how to call back to the Dgraph instance to enable one field to be a container of multiple field parts. For instance name = {firstName, middleName, lastName} with a schema like:
type Person {
id: ID!
firstName: String
middleName: String
lastName: String
name: String @custom(...)
}