[GHC] #14228: PatternSynonyms Non-exhaustive with UnboxedSums

13 Sep 2017

      #14228: PatternSynonyms Non-exhaustive with UnboxedSums
-------------------------------------+-------------------------------------
           Reporter:  guibou         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
           Keywords:  UnboxedSum,    |  Operating System:  Unknown/Multiple
  PatternSynonyms                    |
       Architecture:                 |   Type of failure:  Incorrect result
  Unknown/Multiple                   |  at runtime
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 The following implementation of `Maybe` using UnboxedSums results in `Non-
 exhaustive patterns in case`:

 (`Failure.hs` file)

 {{{#!haskell
 {-# LANGUAGE UnboxedSums #-}
 {-# LANGUAGE PatternSynonyms #-}

 type Maybe' t = (# t | () #)

 pattern Just' :: a -> Maybe' a
 pattern Just' x = (# x | #)

 pattern Nothing' :: Maybe' a
 pattern Nothing' = (# | () #)

 foo x = case x of
   Nothing' -> putStrLn "nothing"
   Just' _ -> putStrLn "just"

 main = do
   putStrLn "Nothing'"
   foo Nothing'

   putStrLn "Just'"
   foo (Just' "hello")
 }}}

 When executed, it prints:

 {{{
 Nothing'
 nothing
 Just'
 Failure: Failure.hs:10:20-29: Non-exhaustive patterns in case
 }}}

 Compiled with `ghc Failure.hs`.

 Please note that by removing the `pattern`s, and writting `foo` as
 following works as expected:

 {{{#!haskell
 foo x = case x of
   (# | () #) -> putStrLn "nothing"
   (# _ | #) -> putStrLn "just"
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14228
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