Use symbolic error representations

[Keryan-dev]

Essentially remove failures on error from everything but the executable.

The main culprit is Errors.raise_error

[Ninjapouet]

What's the objective beyond that rewriting ?

[Keryan-dev]

To allow a more refine handling of failure. For better error messages, better diagnosis for the source of error (e.g. having to point out several code locations to explain an issue), etc. Also not having to rely on a catch-all when using the library.

Note that the logs mechanism already addresses part of it.

[Ninjapouet]

That's the point of my question : there is globally 3 strategies for error reporting with traces :

• using a monadic style that agglomerates errors through composition
• using exceptions with traditionnal try .. with .. to propagate errors (and agglomerating them if needed)
• using a log system.

Symbolic errors (in the general OCaml sense) are mostly used for the monadic solution but as the compiler is not written in a monadic style, it would require a lot of rewriting which is usually a bad idea. We can use an exception system but the current log system allows to specify rust-like errors which seems sufficient in practice if we don't specially need a fullstack trace (and in this case, we can combine with it the exception stack).

[Keryan-dev]

I think the log system is fine as an endpoint. Some pieces of the compiler with require some work to have more precise failure reporting before raising actual errors to the user. This could be achieved through richer return types or exceptions, and this is what I meant by symbolic, sorry for the confusion.

We will need a proper way to carry error/diagnostic information internally, as I feel logging is more fitted to the compiler interface.