Date: Mon, 24 Dec 2007 01:44:16 -0500 To: "Galchin Vasili" , haskell-cafe@haskell.org From: Dean Herington Subject: Re: [Haskell-cafe] more on the "Beautiful Concurrency" function "withdraw" application ... Cc: Bcc: X-Attachments:

At 12:19 AM -0600 12/24/07, Galchin Vasili wrote:

...

module Main where import Control.Concurrent.STM import Control.Concurrent import System.Random

type Account = TVar Int

transfer :: Account -> Account -> Int -> IO () transfer from to amount = atomically (do {deposit to amount; withdraw from amount})

deposit :: Account -> Int -> STM () deposit acc amount = withdraw acc (- amount)

withdraw :: Account -> Int -> STM () withdraw acc amount = do { bal <- readTVar acc; writeTVar acc (bal - amount) }

When I try

withdraw (TVar 10) ..

I get "Not in scope: data constructor 'TVar'. OK .. "Tvar" is a class and not a Haskell data type ... so now what?

You first need to create an account:

...

acct <- newTVar 100

Sorry, that would be:

acct <- atomically (newTVar 100)

Then you can withdraw from it:

...

atomically (withdraw acct 10)

Dean