[Haskell-cafe] MonadRandom-computation that does not terminate
nccb2 at kent.ac.uk
Wed Jan 12 14:59:58 CET 2011
On 11/01/11 23:19, Tim Baumgartner wrote:
> I'm having difficulties with this function I wrote:
> iterateR :: (MonadRandom m) => (a -> m a) -> a -> m [a]
> iterateR g s = do
> s' <- g s
> return (s:) `ap` iterateR g s'
> I'm running the computation with evalRandIO and surprisingly the first
> call of main in ghci succeeds, but the second does not terminate.
> Any clues what I'm doing wrong here?
If we unfold ap we get:
iterateR g s = do
s' <- g s
f <- return (s:)
x <- iterateR g s'
return (f x)
What happens here depends on exactly how the monad is defined, but for
many monads that will form an infinite loop that prevents a value being
returned. In the case of RandT from MonadRandom, it is not possible to
execute the action after the iterateR call finishes without knowing the
final state from the call, which requires evaluating the infinite loop
of monadic actions. Does that help?
More information about the Haskell-Cafe