Parallel and Concurrent Programming in Haskell by Simon Marlow

Author:Simon Marlow
Language: eng
Format: epub, mobi, pdf
Tags: COMPUTERS / Programming Languages / General
ISBN: 9781449335892
Publisher: O’Reilly Media
Published: 2013-07-11T16:00:00+00:00

writeChan :: Chan a -> a -> IO () writeChan (Chan _ writeVar) val = do newHole <- newEmptyMVar oldHole <- takeMVar writeVar putMVar oldHole (Item val newHole) putMVar writeVar newHole

To make the code exception-safe, our first thought might be to try this:

wrongWriteChan :: Chan a -> a -> IO () wrongWriteChan (Chan _ writeVar) val = do newHole <- newEmptyMVar modifyMVar_ writeVar $ \oldHole -> do putMVar oldHole (Item val newHole) -- return newHole --

But that doesn’t work because an asynchronous exception could strike between and . This would leave the old_hole full and writeVar pointing to it, which violates the invariants of the data structure. Hence we need to prevent that possibility too, and the simplest way is just to mask_ the whole sequence:

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