Smart constructor vs. Generic Lens

NegatePos.hs

module NegatePos where

import Control.Lens
import Data.Generics.Product

import Pos

negatePos :: Pos -> Pos
negatePos = position @1 %~ negate

-- (-1) == getPos (negatePos (fromJust (mkPos 1)))

Pos.hs

module Pos (Pos, mkPos, getPos) where

import Prelude
import GHC.Generics

newtype Pos = MkPos Int deriving (Eq, Show, Generic)

mkPos :: Int -> Maybe Pos
mkPos n
  | n > 0     = Just (MkPos n)
  | otherwise = Nothing

getPos :: Pos -> Int
getPos (MkPos n) = n