[Haskell-cafe] Partially applied type synonyms

[Branimir Maksimovic]

> On 09.10.2021., at 07:04, Ttt Mmm <amindfv at mailbox.org> wrote:
> 
>> 
>> 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'. 
> 
Why, what do mean by that? they are unrelated types as IntSet has one var less then Set?
> Cheers, 
> Tom 

Greets, Branimir,

> 
>> 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) 
>>> -}

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/haskell-cafe/attachments/20211009/0096ab70/attachment.html>