Support null variables

[zth]

Closes #348

This introduces a directive @rescriptRelayNullableVariables on operations that will allow you to leverage Js.Nullable.t to send null explicitly to your APIs.

You can test this in your project by running yarn add rescript-relay@support-nullable-variables.

How it works in a nutshell:

module Mutation = %relay(`
    mutation TestNullableVariablesMutation($avatarUrl: String) @rescriptRelayNullableVariables {
      updateUserAvatar(avatarUrl: $avatarUrl) {
        user {
          avatarUrl
        }
      }
    }
`)

// All optional variables in this operation can now be `null`, which will be preserved and sent to your server.
Mutation.commitMutation(~environment=RelayEnvironment.environment, ~variables={avatarUrl: Js.Nullable.null}, ())

// Or if you want to send a value:
Mutation.commitMutation(~environment=RelayEnvironment.environment, ~variables={avatarUrl: Js.Nullable.return("some-avatar-url")}, ())

This works for:

• Variables in queries/mutations/subscriptions.
• Input objects (they'll automatically be nullable when you use this directive)

The reason for this being an explicit directive and not applied globally is that this isn't the default you want, working with options is still so much easier.

Notes:

• This is in a sense a "temporary" solution. I'll want to do a retake on this and a bunch of related things eventually. But that's more for a "RescriptRelay 2.0", and this will be the way to go for the foreseeable future.
• Use this only if you really need it.
• It doesn't work for refetch variables. That likely won't be covered until a "RescriptRelay 2.0".

[mununki]

type Input {
  a: Int!
  b: Int
  c: int
}

module M = %relay(`
  mutation Test($input: Input!) @rescriptRelayNullableVariables {
    test(input: $input) {
      result
    }
  }
`)

M.commitMutation(~variables=M.makeVarialbes(~input=M.makeVariables_input(~a=1, ~b=Js.Nullable.null, ())))

The variables on the network request would be something like{a: 1, b: null}?

[zth]

type Input {
  a: Int!
  b: Int
  c: int
}

module M = %relay(`
  mutation Test($input: Input!) @rescriptRelayNullableVariables {
    test(input: $input) {
      result
    }
  }
`)

M.commitMutation(~variables=M.makeVarialbes(~input=M.makeVariables_input(~a=1, ~b=Js.Nullable.null, ())))

The variables on the network request would be something like{a: 1, b: null}?

Not 100%, you'll need to wrap everything with Js.Nullable. I haven't added support for object makers (most likely won't since they might be going away in the future), so it'd need to be:

M.commitMutation(~variables=(input: Js.Nullable.return({a: Js.Nullable.return(1), b: Js.Nullable.null})}, ())

But then yes, the network request would be: {input: {a: 1, b: null}}

[zth]

Has anyone tried this for real yet? Cc @lozlow

[zth]

Will merge this and we can iterate on it more if needed.