Extensive Notes on can_fail and has_side_effects
In fixing Trac #9390 I discovered that I *still* didn't really understand what the can_fail and has_side_effects properties of a PrimOp mean, precisely. The big new things I learned are * has_side_effects needs to be true only of *write* effects, Reads (which are, strictly speaking, effects) don't matter here. * has_side_effects must be true of primops that can throw a synchronous Haskell exception (eg raiseIO#) * can_fail is true only of primops that can cause an *unchecked* (not Haskell) system exception, like divide by zero, or accessing memory out of range through an array read or write. I've documented all this now. The changes in this patch are only in comments.
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