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
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