[Haskell-cafe] Tracking characters and a timestamp ?

[Marc Weber]

Jump to question 1 and question 2

most simple streaming example:
=============  =======================================================
module Main where
import System.IO
import Control.Monad

main = do
  lines <- liftM lines getContents
  mapM_ print lines -- *
===========  =======================================================

Matthiew:

>The sequencing imposed by the IO monad means that the first mapM must
>complete before the second can start.
This does not apply to the example above, does it?
This example behaves like a pipe output is written before all input has been read.
But we have 2 IO actions, don't we?:
	lines <- liftM lines getContents
and 
	mapM_ print lines

question 1:
So this example should "hang", as well, shouldn't it?

Bulat:

Its getting interesting: This works as expected.
=============  =======================================================
module Main where
import Data.Time.Clock
import Data.Time
import Control.Monad
import System.Exit
import System.IO
import System.IO.Unsafe

handleChar :: Show a => (Char, a) -> IO ()
handleChar ('s', _) = exitWith (ExitFailure 1)
handleChar tuple = print tuple

addTimeCode a = liftM ( (,) a) getCurrentTime

main = do
  hSetBuffering stdin NoBuffering
  liftM (take 4) (hGetContents stdin) >>= unsafeInterleavedMapM addTimeCode 
      >>= mapM print

unsafeInterleavedMapM f (x:xs) = do a <- f x
				    as <- unsafeInterleaveIO (unsafeInterleavedMapM f xs)
				    return (a:as)
=============  =======================================================

When also using unsafeInterleavedMapM for the second mapM the program will stop
after processing the first list item. 
question 2
I can't see why this is the case.


Continuation does work as well:
============= continuation example ===================================
module Main where
import Control.Monad.Cont
import System.Exit
import System.IO
import Data.Time

takeChar [] = exitWith ExitSuccess
takeChar (c:cs) = do
  print c
  print =<< getCurrentTime
  when (c =='s') $ exitWith $ ExitFailure 1
  takeChar cs

main = do
  hSetBuffering stdin NoBuffering
  getContents >>= takeChar
=============  =======================================================

Marc