[Haskell-cafe] Re: Yet another IO initializer: Effectful
declarations and an ACIO monad
ger at informatik.uni-bremen.de
Fri Nov 26 02:20:43 EST 2004
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 ...
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