This is a simple example how to use Hasim.
First, import the correct module.

> import Control.Hasim

Import some monad libraries

> import Control.Monad.State
> import Control.Monad 

Now, define a simulation with actions. This will be a very simple example.

> simdef = do
>   p1 <- mkProcess "sender" 0
>   p2 <- mkProcess "receiver" ()
>   p1 `setAction` p1act p2
>   p2 `setAction` p2act

Define the actions that belong to these processes.

The receiver, p2, will just receive packets and print
the contents on the screen,

> p2act = do
>   p <- receive
>   t <- getTime
>   liftIO $ putStrLn $ "received on t = " ++ show t ++ ", " ++ show p
>   p2act

The sender, will keep sending packets with increasing integers,
until 37 is reached.

> p1act p2 = do
>   i <- get
>   put $ i + 1
>   wait 4
>   unless (i >= 37) $ do
>     sendBlock i p2 
>     p1act p2

Now define the main function:

> main = createSimulation simdef >>= runSimulation