View difference between Paste ID: 1UZddvrJ and T3C4K4xB

 {-# LANGUAGE DataKinds #-}
 module Test where
 
 import Data.WorldPeace (OpenUnion, openUnionLift, relaxOpenUnion)
 import Data.Bifunctor (first)
 
 data ErrorOne = ErrorOne
 data ErrorTwo = ErrorTwo Int
 data ErrorThree = ErrorThree String
 
 checkOne :: Int -> Either (OpenUnion '[ErrorOne]) Int
 checkOne x = case x of
     1 -> Left $ openUnionLift ErrorOne
     otherwise -> Right x
 
 checkTwo :: Int -> Either (OpenUnion '[ErrorOne, ErrorTwo]) Int
 checkTwo x = case x of
     2 -> Left $ openUnionLift ErrorOne
     3 -> Left $ openUnionLift $ ErrorTwo 3
     otherwise -> Right x
 
 checkThree :: Int -> Either (OpenUnion '[ErrorThree]) Int
 checkThree x = case x of
     4 -> Left $ openUnionLift $ ErrorThree "error"
     otherwise -> Right x
 
 checkAll :: Int -> Either (OpenUnion '[ErrorOne, ErrorTwo, ErrorThree]) Int
 checkAll x = do
     a <- first (relaxOpenUnion) $ checkOne x
     b <- first (relaxOpenUnion) $ checkTwo a
     c <- first (relaxOpenUnion) $ checkThree b
     pure b