Problems with instance list in Haskell

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I have a job at the facu and I have this mistake:

'decide' is not a (Visible) method of class `Char'

to the code below: 

data Paridade = Par | Impar deriving Show

class ParImpar a where
    decide :: a -> Paridade

instance ParImpar a => (Char [a]) where
    decide [a]
        | length [a] mod 2 == 0 = Par
        | otherwise = Impar
haskell

1 answer

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The problem is that you are trying to make an instance of Char(?!) for [a]. What you want is an instance of ParImpar for [a]:

instance ParImpar [a] where
    decide a
        | mod (length a) 2 == 0 = Par
        | otherwise             = Impar

Char is a type, not a typeclass.

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