Generic.Rep.Sum to list

Main.purs

module Main where

import Prelude
import Type.Proxy (Proxy(..))
import Data.Generic.Rep

foreign import kind List
foreign import data Cons :: Type -> List -> List
foreign import data Nil :: List

class AppendList (l :: List) (r :: List) (o :: List) | l -> r o
instance appendCons :: AppendList l r o => AppendList (Cons a l) r (Cons a o)
instance appendNil :: AppendList Nil r r

class SumToList g (l :: List) | g -> l
instance sumCons ::
  ( SumToList ga la
  , SumToList gb lb
  , AppendList la lb l
  ) => SumToList (Sum ga gb) l
instance sumField :: SumToList (Constructor name arg) (Cons (Constructor name arg) Nil)


-------------- test
data LProxy (l :: List) = LProxy

solveSumToList :: forall s r l. Generic s r => SumToList r l => Proxy s -> LProxy l
solveSumToList _ = LProxy

data Maybe a = Nothing | Just a
derive instance genericList :: Generic (Maybe a) _

maybeList = solveSumToList (Proxy :: Proxy (Maybe Int)) :: LProxy
  (Cons (Constructor "Nothing" NoArguments) (Cons (Constructor "Just" (Argument Int)) Nil))