Incommensurate E types - Boost.Outcome documentation

# Incommensurate E types

Back in the essential tutorial section on `result`, we studied a likely very common initial choice of `E` type: a strongly typed enum. We saw how by marking up strongly typed enums to tell the C++ standard library what they are, they gain implicit convertibility into `std::error_code`, and we then pointed out that you might as well now always set `E = std::error_code`, as that comes with the enormous advantage that you can use the boilerplate saving `BOOST_OUTCOME_TRY` macro when the `E` type is always the same.

We thus strongly recommend to users that for any given piece of code, always using the same `E` type across the codebase is very wise, except where you explicitly want to prevent implicit propagation of failure up a call stack e.g. local failures in some domain specific piece of code.

However it is unreasonable to expect that any non-trivial codebase can make do with `E = std::error_code`. This is why Outcome allows you to use custom `E` types which carry payload in addition to an error code, yet still have that custom type treated as if a `std::error_code`, including lazy custom exception throw synthesis.

All this is good, but if library A uses `result<T, libraryA::failure_info>`, and library B uses `result<T, libraryB::error_info>` and so on, there becomes a problem for the application writer who is bringing in these third party dependencies and tying them together into an application. As a general rule, each third party library author will not have built in explicit interoperation support for unknown other third party libraries. The problem therefore lands with the application writer.

The application writer has one of three choices:

1. In the application, the form of result used is `result<T, std::variant<E1, E2, ...>>` where `E1`, `E2` … are the failure types for every third party library in use in the application. This has the advantage of preserving the original information exactly, but comes with a certain amount of use inconvenience and maybe excessive coupling between high level layers and implementation detail.

2. One can translate/map the third party’s failure type into the application’s failure type at the point of the failure exiting the third party library and entering the application. One might do this, say, with a C preprocessor macro wrapping every invocation of the third party API from the application. This approach may lose the original failure detail, or mis-map under certain circumstances if the mapping between the two systems is not one-one.

3. One can type erase the third party’s failure type into some application failure type, which can later be reconstituted if necessary. This is the cleanest solution with the least coupling issues and no problems with mis-mapping, but it almost certainly requires the use of `malloc`, which the previous two did not.

Things get even more complicated in the presence of callbacks. If in the callback you supply to library A, you call library B, you may need to insert switch statement maps or other mechanisms to convert library B’s failures into something library A can understand, and then somehow extract that out – preferably without loss of original information – into the application’s failure handling mechanism if library A subsequently returns failure as well. This implies transmitting state by which to track these interrelated pieces of failure data.

Let us see what Outcome can do to help the application writer address some of these issues, next.

Last revised: February 09, 2019 at 15:18:26 UTC