try fails to catch alarm

[pzinn]

There are already existing open issues with alarm, this I think is a new one:

i1 : try (alarm 1; sleep 15;) else <<"should end up here (and does)"<<endl;
should end up here (and does)

i2 : try (alarm 1; sleep 15) else <<"should end up here (and doesn't)"<<endl;
stdio:2:51:(3): error: alarm occurred

This issue is presumably related to tail call optimisation (?).
Note that I already submitted a PR a while ago about tail call, namely #3144, but it does not impact the issue described above.

I traced the issue to two different places that alarm is triggered, both in evaluate.d, namely evalraw in the first case, evalexcept in the second. I could try to fix something but first I would need to understand better the difference between these two functions. Note that evalraw has this comment

	  if test(exceptionFlag) && !steppingFurther(c) then (    -- compare this code to the code in evalexcept() below

presumably suggesting I should notice something that I don't...

[mahrud]

To make sure I understand, the problem is that try isn't catching the errors occurred during TCO, right? e.g. things look fine without try:

i27 : f = t -> (alarm 1; sleep t;); f 10
stdio:23:38:(3): error: alarm occurred

i29 : f = t -> (alarm 1; sleep t); f 10
stdio:24:37:(3): error: alarm occurred

I'm sure you've seen it, but the solution might be related to #2596 also.

[pzinn]

I don't think this is the same bug as #2596, though experience fixing one might help with the other.

[pzinn]

@DanGrayson (or anyone else) can you explain why eval(f:Frame,c:Code) calls evalexcept:

M2/M2/Macaulay2/d/evaluate.d

Lines 1616 to 1621 in ec9e9ac

	export eval(f:Frame,c:Code):Expr := (
	saveLocalFrame := localFrame;
	localFrame = f;
	ret := evalexcept(c);
	localFrame = saveLocalFrame;
	ret);

and not just eval(c:Code)? I guess I'm asking the same question as above, what is the purpose of evalexcept?

[pzinn]

considering #3144 has been reverted, this can hopefully be fixed at the same time as the issue #3546 underlying the latter.