Re: [Haskell-cafe] Lazy IO with recursive reads?

24 Feb 2010

      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 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"]

Re: [Haskell-cafe] Lazy IO with recursive reads?

Fabian Roth