Recordを剥がして連結しただけ

Extensible0.hs

{-# LANGUAGE TypeFamilies, DataKinds, TypeOperators, PolyKinds, UndecidableInstances, TemplateHaskell, FlexibleContexts, AllowAmbiguousTypes #-}

import Control.Lens
import Data.Extensible
import Data.Extensible.Internal
import GHC.TypeLits

type family GetList r :: [Assoc Symbol *] where
  GetList (h :* xs) = xs

type family UnionRecord xs ys :: * where
  UnionRecord xs ys = Record (GetList xs ++ GetList ys)

mkField "name weight price description featured quantity"

type Stock0 = Record '[
    "name" :> String
  , "weight" :> Float]

type Stock1 c = Record '[
    "price" :> c
  , "featured" :> Bool
  , "description" :> String
  , "quantity" :> Int]

type Stock c = UnionRecord Stock0 (Stock1 c)

s0 :: Num c => Stock c
s0 = name @= "DA-192H"
  <: weight @= 260
  <: price @= 120
  <: featured @= True
  <: description @= "High-quality (24bit 192kHz), lightweight portable DAC"
  <: quantity @= 20
  <: Nil

s1 :: Num c => Stock c
s1 = shrinkAssoc
   $ name @= "HHP-150"
  <: featured @= False
  <: description @= "Premium wooden headphone"
  <: price @= 330
  <: quantity @= 55
  <: weight @= 200
  <: Nil

s2 :: Record '["name" :> Int, "name" :> String]
s2 = name @= 100 <: name @= "hogehoge" <: Nil

printSummary :: (Associate "name" String s, Associate "description" String s) => Record s -> IO ()
printSummary s = putStrLn $ view name s ++ ": " ++ view description s

main = do
  printSummary s0
  printSummary s1