[Haskell-cafe] WriterT [w] IO is not lazy in reading [w]

[Paolino]

How do I read "IO is not lazy" ?
 Is IO (>>=) forcing the evaluation of its arguments, causing the unwanted
neverending loop?
And, this happens even in (MonadTrans t => t IO)  (>>=) ?

Thanks

paolino

2008/12/31 Ryan Ingram <ryani.spam at gmail.com>

> IO is not lazy; you never make it to "print".
>
> Consider this program:
>
> > k = f 0 where
> >    f n = do
> >        lift (print n)
> >        tell [n]
> >        f (n+1)
>
> > weird :: IO [Int]
> > weird = do
> >     (_, ns) <- runWriterT k
> >     return (take 20 ns)
>
> What should "weird" print?  According to "k", it prints every Int from
> 0 up.  Aside from the extra printing, it has the same behavior as your
> writer.
>
> For the result of a WriterT to be lazy readable, you need both the
> monoid to be lazy readable, and the transformed monad to be lazy,
> which IO isn't.
>
>  -- ryan
>
> 2008/12/31 Paolino <paolo.veronelli at gmail.com>:
> > As someone suggested me, I can read the logs from Writer and WriterT as
> > computation goes by,
> > if the monoid for the Writer  is lazy readable.
> > This has been true until I tried to put the IO inside WriterT
> >
> >
> >> {-# LANGUAGE FlexibleContexts #-}
> >> import Control.Monad.Writer
> >
> >
> >> k :: (MonadWriter [Int] m) => m [Int]
> >
> >> k = let f x = tell [x] >> f (x + 1) in f 0
> >
> >
> >> works :: [Int]
> >> works = snd $ runWriter k
> >
> >
> >> hangs :: IO [Int]
> >> hangs = snd `liftM` runWriterT k
> >
> >
> >> main = take 20 `liftM` hangs >>= print
> >
> >
> >
> > The main hangs both interpreted and compiled on ghc 6.10.1.
> >
> > The issue is not exposing with IO alone as
> >
> > main = print "test" >> main
> >
> > is a working program.
> >
> > Thanks for explanations.
> >
> >
> > paolino
> >
> >
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> >
> >
>
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