[Haskell-cafe] MVar which can not be null ?

[Tim Docker]

On 18/03/13 19:07, s9gf4ult wrote:
>
> nor STM, becuase IO is not acceptable inside STM transaction.
>
> I just need some thread-safe blocking variable like MVar
>
> modifyMVar :: MVar 
> <http://hackage.haskell.org/packages/archive/base/4.6.0.1/doc/html/Control-Concurrent-MVar.html#t:MVar> 
> a -> (a -> IO 
> <http://hackage.haskell.org/packages/archive/base/4.6.0.1/doc/html/System-IO.html#t:IO> 
> (a, b)) -> IO 
> <http://hackage.haskell.org/packages/archive/base/4.6.0.1/doc/html/System-IO.html#t:IO> 
> b
>

Whilst it's true that IO cannot be performed within an STM action, a 
common pattern is to return the necessary IO action from the STM action, 
and then run it once the STM transaction has completed successfully.

Tim
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