[GHC] #14303: HasField ambiguity error

1 Oct 2017

      #14303: HasField ambiguity error
-------------------------------------+-------------------------------------
           Reporter:  cloudhead      |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
           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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 Both of these functions fail to compile with the error below:

 {{{#!haskell
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeApplications #-}

 import Control.Monad.Reader
 import GHC.Records
 import Data.Proxy

 askField :: forall x a m r. (HasField x r a, MonadReader r m) => m a
 askField =
     asks (getField @x)

 askField' :: forall x a m r. (HasField x r a, MonadReader r m) => m a
 askField' =
     asks (getFieldWithProxy (Proxy :: Proxy x))
   where
     getFieldWithProxy :: forall proxy. proxy x -> r -> a
     getFieldWithProxy = const getField

 }}}

 {{{
 Test.hs:12:14: error:
     • Could not deduce (HasField x0 r a)
       from the context: (HasField x r a, MonadReader r m)
         bound by the type signature for:
                    askField' :: forall x a (m :: * -> *) r.
                                 (HasField x r a, MonadReader r m) =>
                                 m a
         at Test.hs:12:14-69
       The type variable ‘x0’ is ambiguous
     • In the ambiguity check for ‘askField'’
       To defer the ambiguity check to use sites, enable
 AllowAmbiguousTypes
       In the type signature:
         askField' :: forall x a m r.
                      (HasField x r a, MonadReader r m) => m a
    |
 12 | askField' :: forall x a m r. (HasField x r a, MonadReader r m) => m a
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

 }}}

 I hope I'm not missing something, but one of these should compile.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14303
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