exceptions

[Stephen Weeks]

> Are we eventually planning on moving to the point where the SSA IL doesn't
> need HandlerPush and HandlerPop, and implement handlers works entirely
> from the information in  Transfer.Call {return, ...}?

Yes.  The only reason HandlerPush and Pop are there is for the
exceptions paper, where we will want to compare our fine new
strategies for installing handlers against a strategy based on the
handle expressions in the source program.

> But, we can get into situations like
>  
>  fun f () = ... raise Overflow ...
>  fun g () = ... raise Overflow ...
>  fun h () = ... ( ... f () ... g () ...) handle _ => exp1 ... 
>  fun i () = ... ( ... f () ... ) handle Overflow => exp2 ...
>  fun main () = ... h () ... i () ...

The right fix for your problems is to use equality constraints instead
of less-than constraints in the analysis.  That way, we determine that
f must raise because of i, and that therefore the handle in h must
handle, and therefore that g must raise.

It's clearly worse, but I think we can survive until Push and Pop go
away.