Re: [Haskell-cafe] Partially applied type synonyms

9 Oct 2021


      
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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'.

Cheers,
Tom
...
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)
        -}
>