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

[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...

Cheers,
Tim


On Wed, Oct 21, 2009 at 1:30 PM, minh thu <noteed at gmail.com> wrote:

> 2009/10/21 Tim Wawrzynczak <inforichland at gmail.com>
> >
> > 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
>
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