Given the type class

class Dictionary w where
  insert :: String -> String -> w -> w
  remove :: String -> w -> w
  lookUp :: String -> w ->  String

I can't write 

instance Dictionary [(String,String)] where
  insert key value dic = (key,value) : remove key dic
  remove key dic = filter (\entry -> (fst entry) /= key) dic
  lookUp key [] = "not found"
  lookUp key ((k,v):xs) | k == key = v
                        | otherwise = lookUp key xs

because of 

Illegal instance declaration for `Dictionary[(String, String)]'
    (All instance types must be of the form (T a1 ... an)
     where a1 ... an are type *variables*,
     and each type variable appears at most once in the instance head.
     Use -XFlexibleInstances if you want to disable this.)
In the instance declaration for `Dictionary[(String, String)]'

... which I don't quite understand. Something like this works:

newtype Dic = Dic [(String,String)]

instance Dictionary Dic where
  insert key value (Dic dic) = Dic $ (key,value) : filter (\entry -> (fst entry) /= key) dic
  remove key (Dic dic) = Dic $ filter (\entry -> (fst entry) /= key) dic
  lookUp key (Dic []) = "not found"
  lookUp key (Dic ((k,v):xs)) | k == key = v
                              | otherwise = lookUp key (Dic xs)

Is there a better way? Or should I use the suggested compiler directive?

haskell typeclass
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