[GHC] #13735: RankNTypes don't work with PatternSynonyms

[GHC]

#13735: RankNTypes don't work with PatternSynonyms
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
                                     |  PatternSynonyms
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 Another infelicity of the interaction between `PatternSynonyms` and
 `RankNTypes` is that this unidirectional version typechecks:

 {{{#!hs
 {-# LANGUAGE PatternSynonyms #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 module Bug where

 data PLambda a = Var a | Int Int | Bool Bool | If (PLambda a) (PLambda a)
 (PLambda a)
   | Add (PLambda a) (PLambda a) | Mult (PLambda a) (PLambda a)
   | Eq  (PLambda a) (PLambda a) | Lam (a -> PLambda a)
   | App (PLambda a) (PLambda a)

 newtype Lam = L { un :: forall a. PLambda a }

 pattern LLam a <- L (Lam a)
 }}}

 And this is what GHCi says about the inferred type of `LLam`:

 {{{
 GHCi, version 8.3.20170516: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Bug              ( Bug.hs, interpreted )
 Ok, modules loaded: Bug.
 λ> :i LLam
 pattern LLam :: (a -> PLambda a) -> Lam         -- Defined at Bug.hs:13:1
 }}}

 But this does not work in an expression context:

 {{{#!hs
 {-# LANGUAGE PatternSynonyms #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 module Bug where

 data PLambda a = Var a | Int Int | Bool Bool | If (PLambda a) (PLambda a)
 (PLambda a)
   | Add (PLambda a) (PLambda a) | Mult (PLambda a) (PLambda a)
   | Eq  (PLambda a) (PLambda a) | Lam (a -> PLambda a)
   | App (PLambda a) (PLambda a)

 newtype Lam = L { un :: forall a. PLambda a }

 llam f = L (Lam f)
 }}}

 {{{
 GHCi, version 8.3.20170516: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Bug              ( Bug.hs, interpreted )

 Bug.hs:13:17: error:
     • Couldn't match expected type ‘a -> PLambda a’
                   with actual type ‘p’
         because type variable ‘a’ would escape its scope
       This (rigid, skolem) type variable is bound by
         a type expected by the context:
           forall a. PLambda a
         at Bug.hs:13:10-18
     • In the first argument of ‘Lam’, namely ‘f’
       In the first argument of ‘L’, namely ‘(Lam f)’
       In the expression: L (Lam f)
     • Relevant bindings include
         f :: p (bound at Bug.hs:13:6)
         llam :: p -> Lam (bound at Bug.hs:13:1)
    |
 13 | llam f = L (Lam f)
    |                 ^
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13735#comment:1>
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