[GHC] #4076: Add Applicative instance for STM

[GHC]

#4076: Add Applicative instance for STM
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    Reporter:  basvandijk        |       Owner:                
        Type:  proposal          |      Status:  new           
    Priority:  normal            |   Component:  libraries/base
     Version:  6.12.2            |    Keywords:                
          Os:  Unknown/Multiple  |    Testcase:                
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown  
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 I propose adding an `Applicative` instance for `STM`.

 The first attached patch adds an `Applicative` instance for `STM` to
 `base/GHC/Cons.lhs`.

 Note I had to create a `base/Control/Applicative.hs-boot` file and import
 `Applicative` using `{-# SOURCE #-}` because I got a "Module imports form
 a cycle for modules" error.

 The second attached patch adds an `Applicative` instance for the
 sequential `STM` to: `stm/Control/Sequential/STM.hs`

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4076>
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