default @NonNull V fallbackOrMapPrimary(final @NonNull Function<U, V> mapPrimary) {
        return this.fallback().orElseGet(() -> mapPrimary.apply(this.primary().get()));
    }
Extract {@link #fallback()}, or if it's not present, map {@link #primary()} to {@code V}.

@param mapPrimary function mapping the primary type to {@code V}
@return extracted or mapped value
cloud-core/src/main/java/org/incendo/cloud/type/Either.java