Re: [GHC] #11197: Overeager deferred type errors

18 May 2018

      #11197: Overeager deferred type errors
-------------------------------------+-------------------------------------
        Reporter:  goldfire          |                Owner:  goldfire
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  7.11
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Here's what is happening.  In the former case we get
 {{{
 main
   = let {
       $dNum_a1d8 :: Num String
       [LclId]
       $dNum_a1d8 = Control.Exception.Base.typeError
                       @ 'GHC.Types.LiftedRep @ (Num String) "blah blah
 blah
     } in
     let {
       $dMonad_axl :: Monad IO
       [LclId]
       $dMonad_axl = GHC.Base.$fMonadIO } in
     letrec {
       main_a1da :: IO ()
       [LclId]
       main_a1da
         = >>
             @ IO
             $dMonad_axl
             @ ()
             @ ()
             (putStrLn (GHC.CString.unpackCString# "Hi there."#))
             (putStrLn (fromInteger @ String $dNum_a1d8 1)); } in
     main_a1da
 }}}
 and the binding for `dNum_a1d8` can float inwards/be inlined.  In the
 latter case we get
 {{{
 main
   = case Control.Exception.Base.typeError
            @ ('GHC.Types.TupleRep '[])
            @ ((Bool :: *) GHC.Prim.~# ([Char] :: *))
            "blah blah blah"
     of co_a11b
     { __DEFAULT ->
     >>
       @ IO
       GHC.Base.$fMonadIO
       @ ()
       @ ()
       (putStrLn (GHC.CString.unpackCString# "Hi there."#))
       (putStrLn
          (GHC.Types.True
           `cast` (Sub co_a11b :: (Bool :: *) ~R# ([Char] :: *))))
     }
 }}}
 and the simplifier does not float in arbitrary case-expressions, for fear
 of changing
 error/termination behaviour.

 What do to?  All I can think of is to make a special case for coercions,
 and be willing
 to float them in, on the grounds that evidence bindings are added by the
 compiler and
 should have as narrow scope as possible.  Any objections?

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11197#comment:14
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