[GHC] #15681: Take {-# COMPLETE #-} pragma into consideration when using MonadFailDesugaring

27 Sep 2018

      #15681: Take {-# COMPLETE #-} pragma into consideration when using
MonadFailDesugaring
-------------------------------------+-------------------------------------
           Reporter:  chshersh       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.6.1
           Keywords:  pattern-       |  Operating System:  Unknown/Multiple
  matching,monadfail,desugaring      |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Consider the following code:

 {{{#!hs
 import Data.List.NonEmpty (NonEmpty (..))

 foo :: Monad m => m (NonEmpty a) -> m a
 foo m = do
     (x :| _) <- m
     pure x
 }}}

 It works completely fine on GHC 8.6.1 and doesn't require `MonadFail`
 constraint because `NonEmpty` has only single constructor so there're no
 other cases in pattern-matching. Howewer, if I rewrite this code using
 `-XPatternSynonyms` with `{-# COMPLETE #-}` pragma, it doesn't work
 anymore.

 {{{#!hs
 {-# LANGUAGE PatternSynonyms #-}

 import Data.List.NonEmpty (NonEmpty (..))

 newtype Foo a = Foo (NonEmpty a)

 pattern (:||) :: a -> [a] -> Foo a
 pattern x :|| xs <- Foo (x :| xs)
 {-# COMPLETE (:||) #-}

 foo :: Monad m => m (Foo a) -> m a
 foo m = do
     (x :|| _) <- m
     pure x
 }}}

 And I see the following error:

 {{{
     • Could not deduce (Control.Monad.Fail.MonadFail m)
         arising from a do statement
         with the failable pattern ‘(x :|| _)’
       from the context: MonadFoo m
         bound by the type signature for:
                    foo :: forall (m :: * -> *) a. MonadFoo m => m (Foo a)
 -> m a
         at /Users/fenx/haskell/sandbox/Fail.hs:13:1-37
       Possible fix:
         add (Control.Monad.Fail.MonadFail m) to the context of
           the type signature for:
             foo :: forall (m :: * -> *) a. MonadFoo m => m (Foo a) -> m a
     • In a stmt of a 'do' block: (x :|| _) <- m
       In the expression:
         do (x :|| _) <- m
            pure x
       In an equation for ‘foo’:
           foo m
             = do (x :|| _) <- m
                  pure x
    |
 15 |     (x :|| _) <- m
    |     ^^^^^^^^^^^^^^
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15681
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler