[Haskell-beginners] Hutton Ex 8.9.7

[trent shipley]

I couldn't get close on my own.

From: https://github.com/pankajgodbole/hutton/blob/master/exercises.hs

{-
7. Complete the following instance declarations:
     instance Eq a => Eq (Maybe a) where
     ...

     instance Eq a => Eq [a] where
     ...
-}

-- suggested answer

instance Eq a => Eq (Maybe a) where
  -- Defines the (==) operation.
  Nothing == Nothing = True
  Just    == Just    = True
    -- why isn't this Just a == Just a ?
    -- My guess is that a and Just a are different types and can't be ==
in            Haskell
  _       == _       = False

instance Eq a => Eq [a] where
  -- Defines the (==) operation.
  [] == []         = True
  [x] == [y]       = x == y
  (x:xs) == (y:ys) = x==y && xs==ys -- I assume this is implicitly
recursive.
  _  == _          = False
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