On Wed, Aug 12, 2009 at 2:39 PM, Derek Elkins
(To Alberto as well.)
Unsurprisingly, Lennart stated the real issue, but I'll re-emphasize it. As much trouble as such a profound violation of purity would cause, it's not the biggest problem. If that were all, you could easily write a C/assembly function that would do what was desired. The real problem is that there isn't necessarily any "data chunk" at all, i.e. there may not be anything to mutate.
Lennart is right of course, but wouldn't his example just be a specific case of my argument? That is, the compiler decided to evaluate the data at compile time by replacing it with the primitive value and inlining it? It seems to me that in the absence of putting some scope or sequencing upon the mutating function, there's no way for such an unsafeMutate* to have any defined meaning in Haskell. And once you have provided either scope or evaluation order, it would be equivalent to just using a monad (although not necessarily IO). Which leads to the point I really want to make. Would a monad other than IO be acceptable? You could provide this functionality with a Reader's 'local' function for example, or with State. Cheers, John