Re: [Haskell-cafe] Partially applied type synonyms

{-# 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 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) -}