[Haskell-cafe] Lazy IO with recursive reads?

[Fabian Roth]

Hi Stephen

Thank you very much, this indeed does the trick!
Using UnsafeIO, however, leaves a creepy unsafe feeling...
I don't fully understand though why it is unsafe. Doesn't hGetContents do
the exact same thing (i.e. reading from IO returning a lazy string) but does
not require UnsafeIO.

Fabian

On Wed, Feb 24, 2010 at 4:38 PM, Stephen Tetley <stephen.tetley at gmail.com>wrote:

> Hi Fabian
>
> You need to yield with unsafeInterleaveIO to allow some of the list to
> be be consumed.
>
> Something like this (which never terminates of course, but do produce
> output):
>
>
> import System.IO.Unsafe
> import Control.Monad
>
> main = do messages <- readLazy
>          mapM_ (\x -> putStr $ show x ++ "\n") $ messages
>          return ()
>          where
>            readLazy :: IO [String]
>             readLazy = unsafeInterleaveIO $ do
>                          { c <- fancyIORead
>                         ; liftM2 (++) (return c) readLazy
>                         }
>            fancyIORead :: IO [String]
>            fancyIORead = return ["aa","bb"]
>
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