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

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

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 :

...

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]

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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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