Re: [Haskell-cafe] Partially applied type synonyms

9 Oct 2021


      
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On 09.10.2021., at 07:04, Ttt Mmm  wrote:
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On 10/09/2021 4:14 AM Branimir Maksimovic  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'.
Why, what do mean by that? they are unrelated types as IntSet has one var less then Set?
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Cheers, 
Tom
Greets, Branimir,
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main = print y
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On 09.10.2021., at 01:56, Ttt Mmm via Haskell-Cafe mailto: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) 
-}