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

3 Feb 2006


      On Feb 3, 2006, at 11:28 AM, Maurício wrote:
...
Kurt Hutchinson 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'
On 2/2/06, Maurício  wrote:
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

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

Robert Dockins