Applicative and Alternative instances for Async #90

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1. cohei commented on Oct 25, 2018

   

   cohei

   on Oct 25, 2018

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   Concurrently is expected to use for those instances, but I'm not sure.
2. vwwv commented on Oct 25, 2018

   

   vwwv

   on Oct 25, 2018

   Last edited by vwwv

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   I think it might be useful for some people to have both of them fit those instances, as some people (me included) prefer to use Async sometimes and Concurrent other times.

   I was thinking about a potential way to implement those instances for Async, without modifying much of the previous code, could be:

   • Move the current definition of Async and related functions to an Internal package, then I think it could be done, keeping the same semantics, with something like:

   module Control.Concurrent.Async  
        ( ...
        ) where
   
   import Control.Concurrent.Async.Internal as Internal
   
   data Async :: * -> * where
     BasicAsync       :: Internal.Async a          -> Async a
     ConcurrentAsync  :: Async (a -> b) -> Async a -> Async b 
     AlternativeAsync :: Async a        -> Async a -> Async a
     EmptyAsync       ::                           -> Async a 
     PureAsync        :: a                         -> Async a
   
   
   instance Applicative Async where
     (<*>) = ConcurrentAsync
     pure  = PureAsync
   
   instance Alternative Async where
     (<|>) = AlternativeAsync
     empty = EmptyAsync
   
   async :: IO a -> IO (Async a)
   async = fmap BasicAsync . Internal.async
   
   cancel :: Async a -> IO ()
   cancel = \case
               BasicAsync        x   -> Internal.cancel x
               ConcurrentAsync   x y -> Internal.cancel x >> Internal.cancel y
               AlternativeAsync  x y -> Internal.cancel x >> Internal.cancel y
               EmptyAsync            -> return ()
               PureAsync         _   -> return ()
   
   
   
   .... and so on with the rest of the methods
   
   

   What do you think?
3. vwwv commented on Oct 25, 2018

   

   vwwv

   on Oct 25, 2018

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   I'll send an actual PR so you can check if it is ok.
4. simonmar commented on Oct 26, 2018

   

   simonmar

   on Oct 26, 2018

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   I'm not sure I understand the motivation for this, and it adds a lot of complexity. Is there somem reason you can't use Concurrently?
5. LukaJCB commented on Nov 6, 2018

   

   LukaJCB

   on Nov 6, 2018

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   One thing I like about an Alternative instance for Async over an Alternative instance for Concurrently, is the fact that with Async <*> distributes over <|>. IMO nice to have both.