Trapping LaTeX error/warning

Question

Is there a way to trap an error/warning in LaTeX during compile time? I'm thinking of something similar to the VB script On Error Goto <blah>, but this time for LaTeX in something like \OnErrorExecute{<command>} and \OnWarningExecute{<command>}.

Sadly enough there are no standard error/warning-codes provided (AFAIK), since packages report warnings/error via the commands

\PackageError{<message>}
\PackageWarning{<message>}

By codes I mean, as an example,

• Warning 1: Underfull \hbox...;
• Warning 2: Overfull \hbox...;
• ...
• Error 1: No \begin{document};
• Error 2: Perhaps a missing \item...;
• Error 3: File ended while scanning..., etc.

since one would ideally want to condition on the type of error/warning that is produced. Understandably a new error/warning reporting mechanism would be required, since package authors are allowed to issue warnings/errors as they please. So, there warnings/errors could be made package-specific with some number prefix (say amsmath.warning.1 for warning 1 using the amsmath package).

If not in this version (probably), what about LaTeX3 (hopefully)?

Answer 1

(Partly taking the comments to make an answer.)

There are various different types of errors, warnings and messages that come from a LaTeX run. At the TeX level, you can get a warning like Underfull \hbox... or and error such as File ended while scanning.... These cannot be altered at the LaTeX end.*

At the LaTeX level, most messages are generated using \PackageError and similar macros. You can redefine these, but an easier way would be to use the silence package. It provides a pre-built set of macros to do this redefinition in a selective way, thus allowing 'filtering out' of unwanted messages.

Turing to LaTeX3, the approach taken in the code there is to separate definition of messages (of all types) from their use. This means that each message has a 'name', which can be used to alter the behaviour of the message when it is given. Thus we might have

\msg_new:nnnn { module } { my-message } { Some~text } { Some~more~text }

to define a message, with

\msg_error:nn { module } { my-message } 

when it is used. With no filtering, this will raise an error. However, we could alter the behaviour with

\msg_redirect_class:nn { error } { warning }

to turn all errors into warnings, or with

\msg_redirect_module:nnn { module } { error } { warning }

to alter just those messages for module, or even

\msg_redirect_name:nnn { module } { my-message } { warning }

to target just one message. As Bruno notes, the filtering behaviour may not currently be ideal, but I think that the separation idea is worth having. There is still a need to write a 'user level' interface for filtering in this way. (Note. Redirection can be applied before modules are loaded: useful to get rid of load-time messages. The mechanism used keeps the message text definition and and redirection separate.)

[*] Altering how the engine behaves is possible with LuaTeX. I'm not sure if there are appropriate hooks at the moment for the messages mentioned, but I'd imagine that this is possible. I'm assuming in the rest of my answer that we are talking about a cross-engine solution.