On 10/09/2021 4:14 AM Branimir Maksimovic <branimir.maksimovic@gmail.com> wrote:

{-# LANGUAGE KindSignatures,FlexibleInstances #-}

import Data.IntSet (IntSet)

import qualified Data.IntSet as IntSet

import Data.Set (Set)

import qualified Data.Set as Set

import Maybes

-- Start with this definition:

data Var s x

  = Var {

    xs :: s x

  , getX :: s x -> Maybe x

  }

x :: Var Set Double

x = Var (Set.fromList [1,2,3]) Set.lookupMax

data Person m

  = Person {

    name :: m String

  , age :: m Int

  }

person0 :: Person (Var Set)

person0 = Person {

    name = Var (Set.fromList ["alice", "bob"]) Set.lookupMin

  , age = Var (Set.fromList [20,30]) Set.lookupMin

  }

varMay :: Person (Var Set) -> Person Maybe

varMay (Person nm ag) =

  Person (getX nm (xs nm)) (getX ag (xs ag))

instance Show (Var' IntSet Int)

  where

    show (Var' a b) = show a

data Var' a b = Var'{

  xs' :: a

  ,getX' :: a -> Maybe b

}

y :: Var' IntSet Int

y = Var' (IntSet.fromList [1,2,3]) (fmap fst . IntSet.minView)

main = print y

On 09.10.2021., at 01:56, Ttt Mmm via Haskell-Cafe <haskell-cafe@haskell.org> wrote:

{-# LANGUAGE KindSignatures #-}

import Data.IntSet (IntSet)
import qualified Data.IntSet as IntSet
import Data.Set (Set)
import qualified Data.Set as Set

-- Start with this definition:
data Var s x
  = Var {
    xs :: s x
  , getX :: s x -> Maybe x
  }

x :: Var Set Double
x = Var (Set.fromList [1,2,3]) Set.lookupMax

data Person m
  = Person {
    name :: m String
  , age :: m Int
  }

person0 :: Person (Var Set)
person0 = Person {
    name = Var (Set.fromList ["alice", "bob"]) Set.lookupMin
  , age = Var (Set.fromList [20,30]) Set.lookupMin
  }

varMay :: Person (Var Set) -> Person Maybe
varMay (Person nm ag) =
  Person (getX nm (xs nm)) (getX ag (xs ag))

-- So far so good. But what if you want to define a version of 'Var' that uses 'IntSet' internally?

-- An attempt would be to comment out the definition of 'Var' above and instead say:
{-
data Var' sx x
  = Var {
    xs :: sx
  , getX :: sx -> Maybe x
  }

type Var s x = Var' (s x) x

y :: Var' IntSet Int
y = Var (IntSet.fromList [1,2,3]) (fmap fst . IntSet.minView)
-}