Jonathan Cast wrote:
On Fri, 2009-02-06 at 00:51 +0100, Peter Verswyvelen wrote:
On Thu, Feb 5, 2009 at 8:20 PM, ChrisK <[email protected]>
wrote:
Since this is strict there is no laziness and the code must
evaluate the input and output "State RealWorld" to ensure they
are not bottom or error.
Interesting. I also thought it was the passing of the RealWorld that
caused the sequencing, I never realized that the strictness played an
important role here.
So what would happen if it would be lazy instead of strict? What kind
of craziness would occur?
The order of side effects would be demand-driven, rather than
order-of-statement driven. So if I said:
[snip]
Essentially, the program would act as if every statement was wrapped up
in an unsafeInterleaveIO.
jcc
I do not think so. Consider
http://darcs.haskell.org/packages/base/GHC/IOBase.lhs to see unsafeInterleaveIO:
unsafeInterleaveIO m = unsafeDupableInterleaveIO (noDuplicate >> m)
unsafeDupableInterleaveIO (IO m)
= IO ( \ s -> let
r = case m s of (# _, res #) -> res
in
(# s, r #))
And Control.Monad.State.Lazy which is the non-strict State monad:
newtype State s a = State { runState :: s -> (a, s) }
instance Monad (State s) where
return a = State $ \s -> (a, s)
m >>= k = State $ \s -> let
(a, s') = runState m s
in runState (k a) s'
And you can see that the data dependence is broken in unsafeInterleaveIO by _,
and not broken in State.Lazy. But neither s nor s' are not forced to WHNF.
What you can do State.Lazy is put an bottom into the state. Or you can put an
unevaluated thunk into the state, and if it gets replaced later then you never
paid to evaluate it.
But the sequencing is retained. One could make an unsafeInterleaveIO for state:
interleaveState :: State s a -> State s a
interleaveState (State m) = State ( \ s -> let (a,_) = runState m s
in (a,s) )
Now the dependency chain is broken in the above, due to the _ being ignored.
Cheers,
Chris
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