Ian Stark wrote (snipped):
Way back in this thread, Koen Claessen mentioned the idea of a commutative version of the IO monad for handling things with identity. That doesn't quite do it, but I have a refinement that might. The thing is to focus on IO computations that are:
a) central -- their effect commutes with every other IO action b) affine -- their effect is not directly observable, and can be discarded.
Unfortunately I have a number of examples where I use global variables with initialisation actions which cannot conceivably be proven to be central & affine by the compiler. For example, where I want to call up an external program (such as wish) which I will later use for doing graphics. The Haskell libraries would run into a similar problem when they tried to open stdin/stdout/stderr. Or indeed when they tried to implement RandomGen, which I presume is going to want to get at the system clock to seed the random number generator. My guess is that if we were to have top-level <- actions where the actions were restricted to those certified to be ACIO, it would not be very long before GHC would implement an unsafeForceACIO function ...