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

[Ryan Ingram]

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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