How can I define Pair as a Monoid?

I have my data type Pair defined as

data Pair a b = Pair a b

I would like to make it a monoid. Here is my definition.

instance (Monoid a,Monoid b) => Monoid (Pair a b) where
    mempty = Pair mempty mempty
    mappend (Pair x1 y1) (Pair x2 y2) = Pair (mappend x1 x2) (mappend y1 y2)

However, I got the following error.

foldable.hs:3:10: error:
    • Could not deduce (Semigroup (Pair a b))
        arising from the superclasses of an instance declaration
      from the context: (Monoid a, Monoid b)
        bound by the instance declaration at foldable.hs:3:10-49
    • In the instance declaration for ‘Monoid (Pair a b)’
  |
3 | instance (Monoid a,Monoid b) => Monoid (Pair a b) where
  |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

What went wrong?

Solution

mappend is not really a method of Monoid anymore. You can implement it, but it should be just a synonym for (<>), which is in Semigroup, a superclass of Monoid. To implement Monoid, you must also implement Semigroup, and put your definition of mappend there, named (<>).