[GHC] #13752: Odd pattern synonym type errors

[GHC]

#13752: Odd pattern synonym type errors
-------------------------------------+-------------------------------------
           Reporter:  dfeuer         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.4.1
          Component:  Compiler       |           Version:  8.2.1-rc2
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
  PatternSynonyms                    |
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Suppose we have

 {{{#!hs
 newtype Arrange = Arrange {getArrange :: [Int] -> [Int]}
 }}}

 If I write

 {{{#!hs
 pattern Heh :: (c ~ ((~) Int)) => (forall a. c a => [a] -> [a]) -> Arrange
 pattern Heh f <- Arrange f
 }}}

 I get

 {{{
     • Couldn't match type ‘[Int] -> [Int]’
                      with ‘forall a. Int ~ a => [a] -> [a]’
       Expected type: forall a. c a => [a] -> [a]
         Actual type: [Int] -> [Int]
     • In the declaration for pattern synonym ‘Heh’
 }}}

 It surprises me that these don't match. I can hack around that a bit:

 {{{#!hs
 newtype Help = Help {getHelp :: forall a. Int ~ a => [a] -> [a]}

 pattern Heh :: (c ~ ((~) Int)) => (forall a. c a => [a] -> [a]) -> Arrange
 pattern Heh f <- ((\x -> Help (getArrange x)) -> Help f)
 }}}

 Now this compiles, and I can use it in a variety of ways, but not quite
 all. If I write

 {{{#!hs
 pattern Heh' :: ([Int] -> [Int]) -> Arrange
 pattern Heh' f <- Heh f
 }}}

 I get

 {{{
 Haha.hs:78:23: error:
     • Couldn't match expected type ‘[Int] -> [Int]’
                   with actual type ‘forall a. Int ~ a => [a] -> [a]’
     • In the declaration for pattern synonym ‘Heh'’
    |
 78 | pattern Heh' f <- Heh f
    |                       ^
 }}}

 This strikes me as perhaps even more surprising. I can hack around that
 too, of course, but yuck!

 {{{#!hs
 pattern Heh' :: ([Int] -> [Int]) -> Arrange
 pattern Heh' f <- ((\(Heh g) -> g @ Int) -> f)
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13752>
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