Hi,

I don’t see any problem there. Why not just a pretty much do a one to one translation?

class Engine e where

  showEng :: e -> String

data Car e = Car

  { getId :: Int

  , getEng :: e

  }

instance Engine e => Show (Car e) where

  show this = "Car with " ++ (showEng $ getEng $ this)

data Steam = Steam { getCar :: Car Steam }

instance Engine Steam where

  showEng this = "id " ++ (show $ getId $ getCar $ this)

car :: Car Steam

car = Car 42 $ Steam car

And then just call „show car“. The should not be a problem with the looped reference.

Greetings

Jos

Von: Daneel Yaitskov
Gesendet: Freitag, 1. Mai 2020 01:33
An: haskell-cafe
Betreff: [Haskell-cafe] [Haskell-Cafe] How to deal with looped referencesinside instances?

{-

Hi,

I am trying to translate a Java sample

with looped references below to Haskell.

I've found 2 solutions, but both of them look ugly,

because they require a helper class.

So I am looking for an advice for best practices on working

with looped references in polymorphic types.

interface Engine {

   String showEng();

}

class Car<E extends Engine> {

   int id;

   E eng;

   String show() {

     return "Car with " + eng.showEng();

   }

   int getId() { return id; }

}

class Steam implements Engine {

   Car<Steam> car;

   Stream(Car<Steam> car) { this.car = car; }

   String showEng() {

     return "id " + car.getId();

   }

}

Car<Steam> car = new Car<>();

car.id = 42;

car.eng = new Stream(car);

car.show();

-}

{-# LANGUAGE UndecidableInstances #-} -- Fix2 shortcoming

class Car c where

    getId :: c -> Int

class Engine e where

    showEng :: e -> String

data SteamEng c = SteamEng c

instance (Car c) => Engine (SteamEng c) where

    showEng (SteamEng c) = "id " ++ show (getId c)

{-

First solution is via ShowEng class.

I don't know how to tell type checker

that the type which is got after application of type function

is instantiating Engine class??

  instance (Engine eng) => Show (Car1 eng) where

      show (Car1 _ eng) = "Car1 with " ++ showEng eng

-}

data Car1 eng = Car1 Int (eng (Car1 eng))

class ShowEng f where

    showEng2 :: (Car a) => f a -> String

instance ShowEng SteamEng where

    showEng2 = showEng

{-

A side note. Intuitive alternative for ShowEng which is not working!

showEng2 = showEng means showEng2 gets Engine, but

class HaveEngine f where

    cast :: (Car a) => f a -> f a

instance HaveEngine SteamEng where

    cast = id

...

instance (HaveEngine eng) => Show (Car1 eng) where

    show (Car1 _ eng) = "Car1 with " ++ (showEng (cast eng))

nor

instance (HaveEngine eng) => Show (Car1 eng) where

    show (Car1 _ eng) = "Car1 with " ++ (showEng eng)

-}

instance Car (Car1 e) where

    getId (Car1 id _) = id

instance (ShowEng eng) => Show (Car1 eng) where

    show (Car1 _ eng) = "Car1 with " ++ (showEng2 eng)

c1 = Car1 42 (SteamEng c1)

{-

Second solution is removing argument from engine type parameter.

-}

data Car0 eng = Car0 Int eng

instance Car (Car0 e) where

    getId (Car0 id _) = id

instance (Engine eng) => Show (Car0 eng) where

    show (Car0 _ eng) = "Car0 with " ++ showEng eng

{-

in this case I have to avoid infinite type and introducing

an extra wrapper and instantiating business logic classes for him.

  λ c0 = Car0 42 (SteamEng c0)

  <interactive>:30:6: error:

     • Occurs check: cannot construct the infinite type:

         car ~ Car0 (SteamEng car)

     • In the expression: Car0 (SteamEng c0)

       In an equation for ‘c0’: c0 = Car0 42 (SteamEng c0)

     • Relevant bindings include c0 :: car (bound at <interactive>:30:1)

-}

newtype Fix2 f g = Fix2 (f (g (Fix2 f g)))

instance (Show (f (g (Fix2 f g)))) => Show (Fix2 f g) where

    show (Fix2 a) = show a

{-

  λ c0 = Car0 42 (SteamEng (Fix2 c0))

  <interactive>:62:1: error:

     • No instance for (Car (Fix2 Car0 SteamEng))

         arising from a use of ‘print’

     • In a stmt of an interactive GHCi command: print it

-}

instance (Car (f (g (Fix2 f g)))) => Car (Fix2 f g) where

    getId (Fix2 a) = getId a

c0 = Car0 42 (SteamEng (Fix2 c0))

{-

Thanks,

Daniil

-}

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