Future Added Values

graphql-ppx will add the polymorphic variant \FutureAddedValue(value)` by default to both enum fields & union variants. This is in accordance to the graphql specification, in order to build robust clients against potentially changing server schemas.

Lee Byron, the co-creator of graphql, says the following about this topic:

These are generated as a reminder that GraphQL services often expand in capabilities and may return new enum values. To be future-proof, clients should account for this possibility and do something reasonable to avoid a broken product.

Adding this variant is intentional default behaviour of the ppx, to avoid unintentional production bugs. You have however the option, to specifically opt-out of this behaviour and disable the generation of this additional variant. This could be useful, if you have absolute control over both the client and the server schema and are confident, that they may never be out of sync.

To opt-out, you can specify the option future_added_value: false, either in your bsconfig.json (see config), or directly on your query.

Example:

module ByConfig = [%graphql
  {|
    {
      someQuery {
        enumField
      }
    }
|};
  {future_added_value: false}
];

The second way is to use the directive @ppxOmitFutureValue directly on your queried field.

module ByDirective = [%graphql
  {|
    {
      someQuery {
        enumField @ppxOmitFutureValue
      }
    }
|}
];

// t_someQuery_enumField without config / directive
type t_someQuery_enumField = [
    | `FutureAddedValue(string)
    | `FIRST
    | `SECOND
    | `THIRD
  ];
// t_someQuery_enumField with config / directive
type t_someQuery_enumField = [
    | `FIRST
    | `SECOND
    | `THIRD
  ];

Please note: Decoding the raw query result while having the future value variant disabled, can lead to a Not_found exception being thrown if an unexpected result is received.