Lookup API

Two main questions arise from the discussion around message translation:

  1. What does the concrete interface to the [Lookup] service look like in the programmer's language of choice? And,
  2. How do translations make their way to my program so that they are available to use?

We will answer these in turn.

Lookup API

The Lookup API library is used to obtain translated strings. A simplified view of the Lookup API in C++ is as follows:

class Lookup {
  enum class Status {
    // No error.
    OK = 0,
    // The resource was unavailable as requested.
  static fpromise::result<std::unique_ptr<Lookup>, Lookup::Status>
    New(const std::vector<std::string>& locale_ids);
  fpromise::result<std::string_view, Lookup::Status> String(uint64_t message_id);

For the time being, the Lookup API is a C++ API only. There are no conceptual issues with extending it to support other languages.

The actual API can be seen in the file lookup.h, and is essentially the same as shown above, except that it contains documentation, construction and testing overhead. At the time of this writing, only a high level C++ API is available for use. We will be adding high level APIs in other languages as need arises. A low-level C API is available as a basis for writing FFI bindings to this functionality in other languages. As a special case, rust does not need the FFI bindings since the low-level implementation is in rust and can be interfaced with directly; but an actual rust API has not been formulated yet.

A basic usage of the Lookup API looks like this:

std::vector<std::string> locale_ids = {"nl-NL"};
auto result = Lookup::New(locale_ids);
if (result.is_error()) {
auto lookup = result.value();
auto lookup_result = lookup.string(42);
if (lookup_result.is_error()) {
  // handle error
std::string_view message = lookup_result.value();
// Use `message`.

The example is taken from the lookup.h documentation. Knowing the API, this example is fairly straightforward, save for one thing: the call lookup.string(...) uses a magic number 42 to look up a message. It is fair of you as a programmer to ask where this number comes from. The next section addresses this question.