On Wed, 2008-11-12 at 18:47 -0500, Dan Doel wrote:
On Wednesday 12 November 2008 6:18:38 pm David MacIver wrote:
To put it a different way, in the absence of unsafeInterleaveIO the IO monad has the property that if f and g are observably equal up to termination then x >>= f and x >>= g are equivalent in the IO monad (actually this may not be true due to exception handling. Our three main weapons...). In the presence of unsafeInterleaveIO this property is lost even though referential transparency is retained.
I'm not sure what exactly you mean by this, but, for instance:
randomIO >>= (print :: Int -> IO ())
obviously isn't going to give the same results as:
randomIO >>= (print :: Int -> IO ())
every time. Your weirdTuple is semantically similar, in that it selects between returning (1,1) and (2,1), but instead of being random, it operationally selects depending on how you subsequently evaluate the value.
I think the point is that randomIO is non-deterministic (technically, pseudo-random) but causal --- the result is completely determined by events that precede its completion. unsafeInterleaveIO, by contrast, is arguably (sometimes) deterministic --- but is, regardless, definitely not causal. jcc