GitLab

In the following code I accidentally declared a type class more than once, and the error message had absolutely nothing to do with that!

{-# 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 



-- zero rank is boring but lets include it for completeness
class Shapable Z where


class Shapable  One  where



{-
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'

-}

Trac metadata

Trac field	Value
Version	7.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture