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

[Robert Dockins]

On Feb 3, 2006, at 11:28 AM, Maurício wrote:
> Kurt Hutchinson wrote:
>> On 2/2/06, Maurício <briqueabraque at yahoo.com> wrote:
>>>   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
>>> };
>> An idiomatic approach:
>> example :: Double -> Double -> IO ()
>> example a b
>>     | a == b    = return ()
>>     | otherwise = do
>>         let a' = a / 2
>>         print a'
>>         b' <- readLn
>>         example a' b'
>> main = example 1000 0
>
>   Thanks! Robert's, Chris' and yours examples solved many of my  
> questions. I understand I can insert modifications in IORefs (as  
> used by Robert and Chris) inside the loop above:
>
> | otherwise = do
>      let a' = a / 2
>      ...
>      modifyIORef some_ioref some_function
>      ...
>      example a' b'
>
>   I wonder if I could write a generic while based on your example:
>
> while :: (a -> IO a) -> (a -> Bool) -> IO ()
>
>   I'll probably learn something trying that.

FYI, here's a thread from a few months back about monad control  
structures; it may also provide some enlightenment.


http://www.haskell.org/pipermail/haskell-cafe/2005-October/011890.html



Rob Dockins

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