2011/1/12 Neil Brown <nc...@kent.ac.uk> > 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? >
Yes, this helps definitely. So if I understand you right, the infinite loop was not entered immediately because of lazyness? That's funny somehow. Thanks a lot
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