[GHC] #11343: Unable to infer type when using DuplicateRecordFields

3 Jan 2016


      #11343: Unable to infer type when using DuplicateRecordFields
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           Reporter:  mpickering     |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.3
           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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 It seems to me that GHC should be able to easily infer the types for the
 record updates in this simple example. Is there a reason that it is unable
 to infer the type currently?

 {{{
 {-# LANGUAGE OverloadedLabels, DuplicateRecordFields #-}
 module C where

 main = do
   print aThing
   print bThing
   print (aThing { a = 5 } )
   print (bThing { a = 5 } )

 data B = B { a :: Int} deriving Show

 bThing = B 10

 data A = A { a :: Int } deriving Show

 aThing = A 10

 {-

 [1 of 1] Compiling C                ( C.hs, C.o )

 C.hs:7:10: error:
     • Record update is ambiguous, and requires a type signature
     • In the first argument of ‘print’, namely ‘(aThing {a = 5})’
       In a stmt of a 'do' block: print (aThing {a = 5})
       In the expression:
         do { print aThing;
              print bThing;
              print (aThing {a = 5});
              print (bThing {a = 5}) }
 -}
 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11343
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