[GHC] #13511: ApplicativeDo incorrectly requiring Monad

2 Apr 2017

      #13511: ApplicativeDo incorrectly requiring Monad
-------------------------------------+-------------------------------------
           Reporter:  mnislaih       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 We ran into this bug in production, this is a simplified reproduction. The
 following program will not type check, requiring an unnecessary Monad
 instance:

 {{{
 {-# LANGUAGE ApplicativeDo, GeneralizedNewtypeDeriving #-}

 import Data.Functor.Identity
 import Data.Monoid

 newtype A x = A (Identity x) deriving (Functor, Applicative)

 shouldWork :: A ()
 shouldWork = do
   a <- pure ()
   b <- pure ()
   let ab = a <> b
   return ab
 }}}

 {{{
 pepe:~/code/snippets$ ghci ApplicativeDoBug.hs
 GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( ApplicativeDoBug.hs, interpreted )

 ApplicativeDoBug.hs:10:14: error:
     • No instance for (Monad A) arising from a do statement
     • In the expression:
         do { a <- pure ();
              b <- pure ();
              let ab = a <> b;
              return ab }
       In an equation for ‘shouldWork’:
           shouldWork
             = do { a <- pure ();
                    b <- pure ();
                    let ab = ...;
                    return ab }
 Failed, modules loaded: none.
 }}}

 There is a simple workaround, which worked for us in production:
 {{{
 workaround :: A ()
 workaround = do
   a <- pure ()
   b <- pure ()
   return $
     let ab = a <> b
     in ab
 }}}

 I asked in #ghc and it seems this is not yet fixed in HEAD.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13511
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