[Haskell-cafe] MonadRandom-computation that does not terminate

[Neil Brown]

On 11/01/11 23:19, Tim Baumgartner wrote:
> Hi,
>
> 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. 
> Reproducible.
> 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?

Thanks,

Neil.