Re: [Haskell-cafe] Simple but interesting (for me) problem

2009/10/21 Tim Wawrzynczak :

True...here we go then:

import Data.IORef import System.IO.Unsafe

mkNext :: (Num a) => IO (IO a) mkNext = do ref <- newIORef 0 return (do modifyIORef ref (+1) readIORef ref)

next :: IO () next = do foo <- mkNext a <- sequence [foo,foo,foo] putStrLn $ show a

running next will print [1,2,3] which is the result of calling 'foo' 3 times.

But technically then, mkNext is just an IO action which returns an IO action ;) and not a function which will return the next value each time it is called, hence the need to extract the value from mkNext, then use it...

That why it is called mkNext: do next <- mkNext sequence [next, next, next]

Cheers, Tim

On Wed, Oct 21, 2009 at 1:30 PM, minh thu wrote:

...

2009/10/21 Tim Wawrzynczak

...

Here's an example in the IO monad:

import Data.IORef import System.IO.Unsafe

counter = unsafePerformIO $ newIORef 0

next = do modifyIORef counter (+1) readIORef counter

Naturally, this uses unsafePerformIO, which as you know, is not kosher...

But you don't close around the Ref like in your schemy example.

mkNext = do ref <- newIORef 0 return (do modifyIORef ref succ readIORef ref)

mimic your other code better.

Cheers, Thu