[GHC] #8506: misleading error message for duplicate type class instances

[GHC]

#8506: misleading error message for duplicate type class instances
--------------------------------------------+------------------------------
        Reporter:  carter                   |            Owner:
            Type:  bug                      |           Status:  new
        Priority:  normal                   |        Milestone:
       Component:  Compiler (Type checker)  |          Version:  7.6.3
      Resolution:                           |         Keywords:
Operating System:  Unknown/Multiple         |     Architecture:
 Type of failure:  None/Unknown             |  Unknown/Multiple
       Test Case:                           |       Difficulty:  Unknown
        Blocking:                           |       Blocked By:
                                            |  Related Tickets:
--------------------------------------------+------------------------------

Comment (by carter):

 that'd be better. It brings attention to the whole "you wrote a class dec
 rather than an instance like you meant to"


  Also, the paste above actually gives the following Error  (assuming N1 is
 used rather than One ). Mixed up my examples when sharing
 {{{
 src/Numerical/Types/Shape.hs:88:16:
     Unexpected type `Z' where type variable expected
     In the declaration of `Shapable Z'

 }}}
 which is a bit inscrutable because it doesn't mention "you're declaring a
 class"


 to clarify more, here's the snippet that triggered the original error (i
 gave a further reduced example above)

 {{{

 {-# LANGUAGE PolyKinds   #-}
 {-# LANGUAGE BangPatterns #-}
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE DeriveDataTypeable #-}
 {-# LANGUAGE TypeOperators #-}
 {-# LANGUAGE TemplateHaskell #-}

 module Numerical.Types.Nat(Nat(..),nat)  where
 import Data.Typeable
 import Data.Data
 import Language.Haskell.TH hiding (reify)

 data Nat = S !Nat  | Z
     deriving (Eq,Show,Read,Typeable,Data)

 nat :: Int -> TypeQ
 nat n
     | n >= 0 = localNat n
     | otherwise = error "nat: negative"
     where   localNat 0 =  conT 'Z
             localNat n = conT 'S `appT` localNat (n-1)



 ----------------------
 ----------------------

 {-# LANGUAGE PolyKinds   #-}
 {-# LANGUAGE BangPatterns #-}
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE DeriveDataTypeable #-}
 {-# LANGUAGE TypeOperators #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE FunctionalDependencies #-}
 {-# LANGUAGE UndecidableInstances #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE StandaloneDeriving #-}
 {-# LANGUAGE CPP #-}
 {-# LANGUAGE TemplateHaskell #-}
 {-# LANGUAGE NoImplicitPrelude #-}

 module Numerical.Types.Shape where


 import Numerical.Types.Nat
 import Data.Data


 type N0 = Z

 type N1= S N0

 type N2 = S N1

 type N3 = S N2

 type N4 = S N3

 type N5 = S N4

 type N6 = S N5

 type N7 = S N6

 type N8 = S N7

 type N9 = S N8

 type N10 = S N9


 class Shapable (n :: Nat)   where
     data (Shape n ):: *


 -- zero rank is boring but lets include it for completeness
 class Shapable Z where
     data Shape  Z = Shape0

 class Shapable  (S Z)  where
     data Shape (S Z)  = Shape1  {-# UNPACK #-}  !Int


 {-
 I get the following error

 src/Numerical/Types/Shape.hs:97:17:
     Unexpected type `Z' where type variable expected
     In the declaration of `Shape Z'

 -}

 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8506#comment:3>
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