On 09.10.2021., at 07:04, Ttt Mmm <amindfv@mailbox.org> wrote:

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)

Thanks for this suggestion, but here Var' is defined as a totally separate type than Var, whereas I want/need Var to be defined in terms of Var'. 

Cheers, 

Tom 

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) 
-}