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

3 Feb 2006

      Kurt Hutchinson wrote:
...
On 2/2/06, Maurício  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.

   Best,
   Maurício