[Haskell-cafe] Re: Haskell code for this example of flow control

[Robert Dockins]

I think you're looking for IORef http://www.haskell.org/ghc/docs/ 
latest/html/libraries/base/Data-IORef.html

Something like this (untested) should do what you want:

example :: IO ()
example = do { ref <- newIORef 1000; loop ref }
  where loop ref = do
        x <- readIORef ref
        print x
        when (x>1) (writeIORef ref (x/2) >> loop ref)


On Feb 2, 2006, at 10:57 AM, Maurício wrote:

> Donald Bruce Stewart wrote:
>> briqueabraque:
>>>  Hi,
>>>
>>>  I would like to know what options I have in Haskell to do  
>>> something similar to this C++ code:
>>>
>>> double a = 1000;
>>> while (a>1) a/=2;
>>>
>>>  I'm able to do that with lists, but I would like to know how to  
>>> do that with monads and variables with state.
>> You'll get good code using a normal recusive loop:
>>     main = print (loop 1000)
>>         where
>>             loop a | a <= 1    = a                    | otherwise  
>> = loop (a/2)
>> All such control structures may be implemented using recursion.
>> -- Don
>
>   I understand those examples, but I really would like to know how  
> to do that with monads. I would like to ask the same question, but  
> now with this code:
>
> double a = 1000;
> double b = 0;
> while (a != b) {
>     a /= 2;
>     cout << a; // Prints a
>     cin << b; // User gives a number, stored in b
> };
>
>   Best,
>   Maurício
>
>
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe



Speak softly and drive a Sherman tank.
Laugh hard; it's a long way to the bank.
           -- TMBG