Revert "lastError" for now

[d-torrance]

This reverts #3654, which appears to be responsible for a considerable slowdown.

[d-torrance]

@mikestillman - I think this fixed the slowdown. The cmake-macos build finished in 4.5 hours

[mahrud]

Did you revert specific commits, or did you manually undo some changes?

[d-torrance]

Specific commits (in particular, the final 3 commits of the original PR that actually dealt with lastError)

[pzinn]

I don't think this is a good idea. Instead one should focus on the source of the slow down.
It's most likely due to try being overused, cf the wise advice in the documentation:

users are recommended to use this function sparingly, if at all.

For example, I'm removing try in promote in #3903. Unfortunately this is more complicated than I thought due to issue #3910.

[d-torrance]

Good point.

I did a little experimenting and I think the big slowdown is the try in package(Symbol). I'll push an alternate PR and see if refactoring that helps

[d-torrance]

It's more than just package(Symbol)... There are quite a few things that use try and ??= that are slowing things down.

[mahrud]

1. I'm not opposed to solutions that don't use try, for instance in promote, but try is a useful and sometimes irreplaceable feature, and that recommendation you cite is from almost 28 years ago (647f753), so I wouldn't call try "overused".
2. lastError isn't being used anywhere yet, so reverting this PR until it can be correctly implemented is reasonable. In particular, as I pointed out in my last comment in "try" should suppress error processing #3848 (comment), preparing the error message should not happen until we actually want to print it. I gave an example where producing the error computes a Groebner basis, which definitely should not happen inside try if the error message won't even be printed (a basic principle in functional programming).

[mahrud]

For future reference, I think this feature could be re-implemented as a function printLastError() which only upon being called processes the error, along with a variable lastErrorLocation which produces the location of the error.

[pzinn]

I thought ??= didn't cause updating lastError? if so it's a valid alternative in many circumstances to try.

[d-torrance]

It does. try and ?? both use tryEval under the hood, which evaluates the code and suppresses errors.