[GHC] #12643: class declaration works in ghci, but not in a file

30 Sep 2016

      #12643: class declaration works in ghci, but not in a file
-------------------------------------+-------------------------------------
           Reporter:  dmwit          |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           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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 The following ghci session prints no errors:

 {{{
...
:set -XStandaloneDeriving -XDeriveGeneric
import GHC.Generics
import Control.Monad.Except
deriving instance Generic (ExceptT e m a)
class F a where f :: Rep (Except String a) x
 }}}
However, when I transfer this to a file:

 {{{
 {-# LANGUAGE StandaloneDeriving #-}
 {-# LANGUAGE DeriveGeneric #-}
 import GHC.Generics
 import Control.Monad.Except
 deriving instance Generic (ExceptT e m a)
 class F a where f :: Rep (Except String a) x
 }}}

 I get a mysterious error:

 {{{
 test.hs:6:17: error:
     • Couldn't match type ‘Rep (Except String a0)’
                      with ‘Rep (Except String a)’
       Expected type: Rep (Except String a) x
         Actual type: Rep (Except String a0) x
       NB: ‘Rep’ is a type function, and may not be injective
       The type variable ‘a0’ is ambiguous
     • In the ambiguity check for ‘f’
       To defer the ambiguity check to use sites, enable
 AllowAmbiguousTypes
       When checking the class method:
         f :: forall a. F a => forall x. Rep (Except String a) x
       In the class declaration for ‘F’
 }}}

 If I turn on -fdefer-type-errors, I can verify that the type family indeed
 reduces far enough for a and x to be arguments to injective types, so I
 believe GHC should not consider this an error:

 {{{
...
:set -XRankNTypes
:kind! forall a x. Rep (Except String a) x
 forall a x. Rep (Except String a) x :: *
 = D1
     ('MetaData
        "ExceptT"
        "Control.Monad.Trans.Except"
        "transformers-0.5.2.0"
        'True)
     (C1
        ('MetaCons "ExceptT" 'PrefixI 'False)
        (S1
           ('MetaSel
              'Nothing 'NoSourceUnpackedness 'NoSourceStrictness
 'DecidedLazy)
           (Rec0 (Data.Functor.Identity.Identity (Either [Char] a)))))
     x
 }}}
--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12643
GHC http://www.haskell.org/ghc/
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