[GHC] #14608: Different GHCi error messages for similar scenarios

23 Dec 2017

      #14608: Different GHCi error messages for similar scenarios
-------------------------------------+-------------------------------------
           Reporter:  mb64           |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  GHCi           |           Version:  8.2.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Poor/confusing
  Unknown/Multiple                   |  error message
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 With the following code:

 {{{#!hs
 {-# LANGUAGE UnboxedTuples #-}
 module Test where

 data UnboxedTupleData = MkUTD (# (),() #)

 doThings :: UnboxedTupleData -> ()
 doThings (MkUTD t) = ()
 }}}

 This is accepted and compiled with a simple `ghc --make Test.hs`. However,
 with `ghci Test.hs`, you get an ugly error message:

 {{{#!hs
 GHCi, version 8.2.2: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Test             ( Test.hs, interpreted )
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.2.2 for x86_64-unknown-linux):
         bcIdPrimRep
   t_s1ro :: (# (), () #)
   Call stack:
       CallStack (from HasCallStack):
         prettyCurrentCallStack, called at
 compiler/utils/Outputable.hs:1133:58 in ghc:Outputable
         callStackDoc, called at compiler/utils/Outputable.hs:1137:37 in
 ghc:Outputable
         pprPanic, called at compiler/ghci/ByteCodeGen.hs:1582:5 in
 ghc:ByteCodeGen

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

 Replacing `(MkUTD t)` with just `t` obtains a different error, with a
 nicer message:

 {{{#!hs
 GHCi, version 8.2.2: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Test             ( Test.hs, interpreted )
 Error: bytecode compiler can't handle unboxed tuples and sums.
   Possibly due to foreign import/export decls in source.
   Workaround: use -fobject-code, or compile this module to .o separately.
 }}}

 True to its word, GHCi accepts it with `-fobject-code`, but it also
 accepts the unwrapping one, with the worse error message, with `-fobject-
 code`.

 It only happens with unboxed tuples in GHCi: if you replace the unboxed
 tuple with an unboxed int, GHCi will happily accept it.  GHC cheerfully
 compiles everything.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14608
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