Constrainting types in generics

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I'm trying to implement conversion to YAML for Generic types.

The simple conversion I implemented works well, but I would like to use their Show instance for the types like data Foo = Bar | Biz.

I was thinking of something like bellow. It correctly catches the types mentioned above, but fails on invoking the show as I'm not able to properly constraint it.

instance {-# OVERLAPPING #-} (GToYaml x, GToYaml y) => GToYaml (D1 d (x :+: y)) where
  gToYaml x = string $ pack $ show $ to x

Is it possible to somehow constraint for original type to have Generic and Show instances? Or it should be handled somehow differently?

haskell generics
1 Answers

Presumably, the original typeclass is something like

class ToYaml x where
  toYaml :: x -> Yaml

Suppose we define this auxiliary datatype and typeclass:

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE ScopedTypeVariables #-}
import Data.Proxy
import GHC.Generics

data HowToConvertToYaml = NormalCase | SpecialCase

class ToYaml' (how :: HowToConvertToYaml) x where
  toYaml' :: Proxy how -> x -> Yaml

HowToConvertToYaml is being used as a DataKind, to provide one piece of extra information to ToYaml': whether we are in the special case or not. This allows us to define different instances for each case:

instance Show x => ToYaml' SpecialCase x where
   toYaml' _ = undefined

instance (Generic x, GToYaml (Rep x)) => ToYaml' NormalCase x where
   toYaml' _ = undefined

The problem is that we want to use ToYaml, not ToYaml'. We need a way to automatically compute the HowToConvertToYaml for each type, and then make the ToYaml instance delegate on the ToYaml' one.

We can use a type family:

type family FindHowToConvertToYaml rep :: HowToConvertToYaml where
  FindHowToConvertToYaml (D1 _ (_ :+: _)) = SpecialCase
  FindHowToConvertToYaml _ = NormalCase

And ToYaml in terms of ToYaml' would be:

instance (Generic x, ToYaml' (FindHowToConvertToYaml (Rep x)) x) => ToYaml x where
  toYaml = toYaml' (Proxy @(FindHowToConvertToYaml (Rep x)))
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