[GHC] #4099: Slighlty confusing error message with data families

[GHC]

#4099: Slighlty confusing error message with data families
---------------------------------+------------------------------------------
    Reporter:  rl                |       Owner:              
        Type:  bug               |      Status:  new         
    Priority:  normal            |   Component:  Compiler    
     Version:  6.13              |    Keywords:              
          Os:  Unknown/Multiple  |    Testcase:              
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown
---------------------------------+------------------------------------------
 Example code:

 {{{
 data family T a

 foo :: T a -> a
 foo (Just x) = x
 }}}

 Error message:

 {{{
     Couldn't match expected type `T a' against inferred type `Maybe a1'
       NB: `T' is a type function
     In the pattern: Just x
     In the definition of `foo': foo (Just x) = x
 }}}

 Calling `T` a type function is confusing, I had to double check that I
 hadn't accidentally declared it as a type family. I would

   * call these things type families and data families in messages and

   * omit the message for data families altogether because it doesn't
 matter that `T` is one in this case.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4099>
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