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

21 Oct 2009

      And just because this has not been explicitly stated:  it's not just  
for aesthetic reasons that you couldn't do this with a pure function,  
but because it violates the semantics and gets you the wrong result.   
So for example, if you modified Tim's code to be

import Data.IORef
import System.IO.Unsafe

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

main :: IO ()
main = do
   foo <- mkNext
   print foo
   print foo
   print foo

Then the output that you will see (with GHC at least) is

1
1
1

because the compiler assumes that it only needs to evaluate foo once,  
after which it can cache the result due to assumed referential  
transparency.

- Greg

On Oct 21, 2009, at 11:40 AM, Tim Wawrzynczak wrote:
...
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  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
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Re: [Haskell-cafe] Simple but interesting (for me) problem

Gregory Crosswhite