Can you help me understand this code on interfaces? If you were calling (CartStore.GetById()) where does (*cart.Cart) come from? I don't see any logical connection between these two.

  type Cart interface {
      GetById(ID string) (*cart.Cart, error)
      Put(cart *cart.Cart) (*cart.Cart, error)
  }

  // A type that implements the interface :

  type CartStore struct{}

  func (c *CartStore) GetById(ID string) (*cart.Cart, error) {
      // implement me
  }

  func (c *CartStore) Put(cart *cart.Cart) (\*cart.Cart, error){
      // implement me
  }