Can't make a derived instance of Num

Question

I am using ghci, this code section

newtype Gold = Gold Int
    deriving (Eq, Ord, Show, Num)

is showing the error as

Can't make a derived instance of 'Num Gold':
  'Num' is not a derivable class
  Try GeneralizedNewTypeDeriving for GHC's newtype-deriving extension in the newtype declaration for 'Gold'

Please suggest the solution.

As said by GHC's error message: add `{-# LANGUAGE GeneralizedNewtypeDeriving #-}` to the top of your file. — leftaroundabout, Sep 01 '14 at 10:29
@BartekBanachewicz I think the OP didn't know _how_ to turn on extensions, hence the question. — AndrewC, Sep 01 '14 at 11:01

score 25 · Accepted Answer · edited Mar 05 '15 at 12:16

25

You can only derive from Eq, Ord, Enum, Bounded, Show and Read automatically. In order to derive other instances, you need to enable the GeneralizedNewtypeDeriving extension as GHCi suggests:

{-# LANGUAGE GeneralizedNewtypeDeriving #-}

newtype Gold = Gold Int
  deriving (Eq, Ord, Show, Num)

Note that the {-# ... #-} isn't a comment, but a compiler pragma, in this case enabling the given language extension.

edited Mar 05 '15 at 12:16

freshtop

706
8
17

answered Sep 01 '14 at 10:30

MathematicalOrchid

61,854
19
123
220

Error Message shows: Can't make a derived instance of 'Num Gold': Num is not a derivable class Try GeneralizedNewtypeDeriving for GHC's newtype-deriving extension In the newtype declaration for 'Gold' – Ammlan Ghosh Sep 01 '14 at 10:45
Thank you @MathematicalOrchid. I am very new to Haskell. Actually I didn't include the line {-# LANGUAGE GeneralizedNewTypeDeriving #-}. – Ammlan Ghosh Sep 01 '14 at 10:54
10

@milleniumbug No need to be rude. The OP could be forgiven for thinking it was a comment, given the syntax. – AndrewC Sep 01 '14 at 11:04

Can't make a derived instance of Num

1 Answers1