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

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 };

A close to line-for-line translation: import Data.IORef import Control.Monad(liftM2,when) main = example 1000 example :: Double -> IO () example originalA = do refA <- newIORef originalA -- allocate local variable a refB <- newIORef 0 -- allocate local variable b let loop = do -- loop in scope of refA, refB flag <- liftM2 (/=) (readIORef refA) (readIORef refB) when flag $ do modifyIORef refA (/2) print =<< readIORef refA -- This will give an error if not a number: writeIORef refB =<< readIO =<< getLine loop loop -- start executing loop

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

Hello Maurício, Friday, February 03, 2006, 7:28:16 PM, you wrote: M> I wonder if I could write a generic while based on your example: while :: (a ->> IO a) -> (a -> Bool) -> IO () M> I'll probably learn something trying that. i have about 5-10 imperative control structures defined in my common lib, including while, until, forever, doInChunks -- Best regards, Bulat mailto:bulatz@HotPOP.com