Beginning Haskell by Alejandro Serrano Mena

Author:Alejandro Serrano Mena
Language: eng
Format: epub, mobi, pdf
ISBN: 9781430262503
Publisher: Apress

payByCash :: Eq a => a -> Integer -> TVar Integer -> TVar [(a,Integer)] -> STM ()

payByCash = ... -- code that asks for cash omitted

In EXERCISE 8-2 you can use your knowledge of transactions to build a Time Machine system.

EXERCISE 8-2. TRAVELING THROUGH TIME

The Time Machine Store also provides the service of time traveling. However, there are some restrictions that customers must abide by: at most n people can be traveling at the same moment (because the company only owns n time machines) and by no means should two people be on the same year at the same time.

Develop a small application where customers are simulated by different threads, and the restrictions are always satisfied via a careful use of the stm library. Hint: use a shared TVar for saving the years that people are traveling to, and retry to block customers for traveling without satisfying the rules.

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