Can you implement your own Data.Array.Diff in Haskell or it's just a standardized fallback to C? I'm on chapter 6 of 'Real World Haskell' and I honestly don't know yet.
It appears, from skimming the source code, that they use "unsafePerformIO", which is Haskell's "I want to trick the compiler into thinking that this imperative code is pure" function. (It's not part of standard Haskell but I think all implementations provide it.)
You can use unsafePerformIO in your own code if you really need it--it's basically a giant loophole in the type system, so if you don't know what you're doing, you can screw yourself, but if you really need that power, it's there.
