On Thu, Mar 26, 2009 at 2:40 AM, Duncan Coutts
I was not being precise when I said "tests for _|_". As you point out, the semantics of imprecise exceptions distinguishes exceptions from bottom, however pure code cannot make that distinction and so that's why I was lumping them together and saying that tryArith tests for certain kinds of _|_ value.
Right... I had this mental picture of exceptions being values denotationally smaller than any "real" value of the domain before I read the imprecise exceptions paper, so I interpreted your imprecise description that way =)
Anyway, I hope this is enough to dissuade people from using unsafePerformIO to catch exceptions.
Yes, unfortunately you can't even use it to return Nothing on errors (ie without returning the specific exception) because of the (overflow + loop) issue where you have nondeterminism between returning Nothing and looping forever. I have my own history of wondering "why isn't there in Control.Exception a function that..." for a while before figuring out why there isn't :-) - Benja