[Haskell-beginners] warning in making instances of Functor class!

Abhijit Patel abhisandhyasp.ap at gmail.com
Fri Dec 25 18:32:19 UTC 2015


why am i getting warning in the following code?? though it works
perfectly!!!

Prelude> newtype Pair1  c b a=Pair1 {getPair1 :: (a,b,c)}

Prelude> :{
Prelude| instance Functor (Pair1 m n) where
Prelude| fmap f (Pair1 (x,y,z))=Pair1 (f x,y,z)
Prelude| :}

<interactive>:55:10: Warning:
    No explicit implementation for
      ‘Prelude.fmap’
    In the instance declaration for ‘Functor (Pair1 m n)’

-- The below shows it is working fine!
Prelude> getPair1 $ fmap (*100) (Pair1 (2,3,1))
(200,3,1)
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