[GHC] #13610: Unhelpful error messages about lifted and unlifted types

[GHC]

#13610: Unhelpful error messages about lifted and unlifted types
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           Reporter:  nomeata        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.1
  (Type checker)                     |
           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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 I wrote this code:
 {{{
 {-# LANGUAGE MagicHash #-}
 import GHC.Prim
 import GHC.Types

 main = do
     let primDouble = 0.42## :: Double#
     let double = 0.42 :: Double
     IO (\s -> mkWeakNoFinalizer# double () s)
 }}}
 and I get this error message:
 {{{
 WeakDouble.hs:8:15: error:
     • Couldn't match a lifted type with an unlifted type
       Expected type: (# State# RealWorld, Weak# () #)
         Actual type: (# State# RealWorld, Weak# () #)
     • In the expression: mkWeakNoFinalizer# double () s
       In the first argument of ‘IO’, namely
         ‘(\ s -> mkWeakNoFinalizer# double () s)’
       In a stmt of a 'do' block:
         IO (\ s -> mkWeakNoFinalizer# double () s)
 }}}
 with `-fprint-explicit-kinds`. (Without the flag, it looks the same, but
 tells me to use this flag…).

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13610>
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