[Haskell-cafe] Partially applied type synonyms

[Ttt Mmm]

>     On 10/09/2021 4:14 AM Branimir Maksimovic <branimir.maksimovic at 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 at haskell.org mailto:haskell-cafe at 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)
> >         -}
> > 
> >     > 
> 
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