Kaveh Shahbazian wrote:
Haskell is the most powerfull and interesting "thing" I'v ever
encountered in IT world. But with an imparative background and lack of
understanding (because of any thing include that maybe I am not that
smart) has brought me problems. I know this is an old issue. But
please help it.
Question : Could anyone show me a sample of using a monad as a
statefull variable?
That question is a bit ill-posed. A monad is a type of interface. A stateful
variable would probably be an IORef or a STRef which can be created and used in
the IO and ST monads, respectively.
For example see this code in C# :
//
public class Test
{
int var;
static void Fun1() { var = 0; Console.Write(var); }
static void Fun2() { var = var + 4; Console.Write(var); }
static void Main() { Fun1(); Fun2(); var = 10; Console.Write("var
= " + var.ToString()); }
}
//
I want to see this code in haskell.
Thankyou
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Here is one translation:
module Imp where
import Data.IORef
data Test = Test {var :: IORef Int
,fun1 :: IO ()
,fun2 :: IO ()
,testMain :: IO ()
}
newTest :: IO Test
newTest = do var <- newIORef 0
let fun1 = do writeIORef var 0
print =<< readIORef var
fun2 = do modifyIORef var (+4)
print =<< readIORef var
main = do fun1
fun2
writeIORef var 10
value <- readIORef var
print ("var = "++show value)
return Test {var = var
,fun1 = fun1
,fun2 = fun2
,testMain = main}
main :: IO ()
main = do
test <- newTest
fun1 test
fun2 test
testMain test
print =<< readIORef (var test)
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