Fwd: Re: [Haskell-cafe] more on the "Beautiful Concurrency" function "withdraw" application ...

[Dean Herington]

>Date: Mon, 24 Dec 2007 01:44:16 -0500
>To: "Galchin Vasili" <vigalchin at gmail.com>, haskell-cafe at haskell.org
>From: Dean Herington <heringtonlacey at mindspring.com>
>Subject: Re: [Haskell-cafe] more on the "Beautiful Concurrency" 
>function	"withdraw" application ...
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>
>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