code: function convertFetch(fn: FetchFunction): ExecuteFunction {
    return function fetch(request, variables, cacheConfig, uploadables) {
      const result = fn(request, variables, cacheConfig, uploadables);
      // Note: We allow FetchFunction to directly return Error to indicate
      // a failure to fetch. To avoid handling this special case throughout the
      // Relay codebase, it is explicitly handled here.
      if (result instanceof Error) {
        return RelayObservable.create(sink => sink.error(result));
      }
      return RelayObservable.from(result);
    };
  }
docs: Converts a FetchFunction into an ExecuteFunction for use by RelayNetwork.