Date: Mon, 24 Dec 2007 01:44:16 -0500
To: "Galchin Vasili" <[EMAIL PROTECTED]>, [email protected]
From: Dean Herington <[EMAIL PROTECTED]>
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
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