"index out of bounds" error shouldn't exit REPL

[brianhuffman]

For example:

sawscript> print {{ [True]@2 }}
saw: at: index out of bounds

The REPL then exits to the command prompt. It should instead return to the REPL prompt.

[brianhuffman]

Part of the problem is that some of our primitives have a single implementation that runs in a different monad depending on the backend (Identity for concrete evaluation, IO for SBV and ABC). If a primitive uses fail to report an error, this turns into an IOException in IO, but it turns into a call to error in Identity (which raises an asynchronous exception). This makes it more complicated to catch the errors in each case, since we can't use the same method for both.

[atomb]

Can we use some sort of error-tracking monad instead of Identity for concrete evaluation?

[eddywestbrook]

A simpler idea, that would require less change to the simulator(s), would be to have errors in the simulator throw a new type of Exception, maybe a SimulatorException or something like that. The idea would be that error should really be reserved for unrecoverable failures, but things that are recoverable at the REPL, like having an index that is too large, should use a different mechanism.

Thoughts?

[atomb]

I would take that one step farther and say that even totally unrecoverable things could use some custom exception so that we can give good error messages, even if we can't otherwise recover. Also, @kwf has been giving some thought to this topic, and I'd be interested in their input.

[atomb]

This specific case no longer causes the REPL to exit (and I think many other failures should be fixed along with that). I think #870 captures the remaining work to be done.