Customizable per page smart constructor using type-level naturals

PerPage.hs

module PerPage
    ( PerPage -- don't export the constructor, only constructable via smart constructor prism
    , perPage
    ) where

import qualified Control.Lens as L
import qualified GHC.TypeLits as TL
import Data.Proxy (Proxy(..))

-- Represents a per page parameter you may accept on a web server, API, etc for pagination.
newtype PerPage (max :: TL.Nat) = PerPage Int deriving (Show, Eq, Ord)

instance (TL.KnownNat max) => Bounded (PerPage max) where
  minBound = PerPage 1
  maxBound = PerPage (fromInteger (TL.natVal (Proxy :: Proxy max)))
  
perPage :: forall max. (TL.KnownNat max) => L.Prism' Int (PerPage max)
perPage = L.prism' toI fromI
  where
    toI (PerPage i) = i
    fromI i = let pp = PerPage i
              in if pp >= minBound && pp <= maxBound
                    then Just pp
                    else Nothing
                    
-- examples
type UsersPerPage = PerPage 50

parseUsersPerPage :: Int -> Maybe UsersPerPage
parseUsersPerPage = L.preview perPage