[GHC] #8567: Poor error message when GeneralizedNewtypeDeriving fails

26 Nov 2013

      #8567: Poor error message when GeneralizedNewtypeDeriving fails
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       Reporter:  goldfire          |             Owner:
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.7
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 Say I try to compile this silly module:

 {{{
 {-# LANGUAGE GeneralizedNewtypeDeriving, TypeFamilies #-}

 module Bug where

 class C a where
   meth :: a -> F a

 type family F a
 type instance F Int = Bool
 type instance F Age = Char

 instance C Int where
   meth = (> 0)

 newtype Age = MkAge Int
   deriving C
 }}}

 Then, I get this output:

 {{{
 Bug.hs:16:12:
     No instance for (GHC.Types.Coercible Bool Char)
       because ‛Bool’ and ‛Char’ are different types.
       arising from a use of ‛GHC.Prim.coerce’
     In the expression:
         GHC.Prim.coerce (meth :: Int -> F Int) :: Age -> F Age
     In an equation for ‛meth’:
         meth = GHC.Prim.coerce (meth :: Int -> F Int) :: Age -> F Age
     In the instance declaration for ‛C Age’
 }}}

 What a confusing error message! Of the 9 lines of error, only 3 make
 sense: the line number (which is correct, at the line number of my
 `deriving` statement), saying that `Bool` and `Char` are different, and
 the conclusion that we're in an instance declaration.

 This might be better:
 {{{
 Bug.hs:16:12:
     Cannot coerce from ‛Bool’ to ‛Char’
       because ‛Bool’ and ‛Char’ are different types
       arising from the need to coerce ‛Int -> F Int’ to ‛Age -> F Age’
     In the instance declaration for ‛C Age’
       generated automatically through GeneralizedNewtypeDeriving
       To see the code I am typechecking, use -ddump-deriv
 }}}

 I actually went ahead and tried to fix this myself, but it's unclear to me
 what the best implementation plan for such an error message is. The code
 in !TcDeriv generates `HsSyn RdrName` and then renames it to `HsSyn Name`,
 but that code then gets sent through the normal type-checking pipeline.
 That pipeline builds up the error-reporting context without much heed to
 the fact that GHC itself generated the code. So, while I'm sure I could
 hack away long enough to get the error message to come out right, I'm
 equally sure that the resulting code would make people want to cry. Any
 advice? Or does someone with a better perspective want to take a stab?

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8567
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