Constrainting types in generics-CodePudding

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?

CodePudding user response：

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)))