[MLton-devel] cvs commit: -exn-history reimplemted

[Matthew Fluet]

> > I suspect we could get away with always having implement-exceptions.fun
> > rewrite as if exn-history were true.  Then -exn-history would only affect
> > the behaviour of MLton.Exn.history.  When it was false, nothing could
> > access the history component and it would be eliminated.
> 
> I wouldn't want to do this because people might want to use
> MLton.Exn.history in their programs, but not always incur the cost.

No, I meant leave the implementation of MLton.Exn.history the same:
      val history: t -> string list =
	 if keepHistory
	    then (setInitExtra ([]: extra)
		  ; setExtendExtra (op ::)
		  ; extra)
	 else fn _ => []
where it is predicated on the keepHistory constant.  When -exn-history is
false, then MLton.Exn.history would simplify to fn _ => [] (as it does
now).  Then nothing could access the history component and it would be
eliminated.  When -exn-history is true, then it would acces the history
component and it would be kept so long as the program did something
"useful" with it.  (In practice, the top-level handler would always do
something useful with it by printing it, so it would be kept around.)




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