Existensial.hs

{- stack --resolver lts-16.8 --install-ghc exec ghci --package "protolude text binary" -}
{-# LANGUAGE DeriveGeneric, DuplicateRecordFields, ExistentialQuantification, FlexibleContexts, RankNTypes, ScopedTypeVariables, StandaloneDeriving #-}
{- ghcid -c "stack X.hs" -}

module Existenial where

import Data.Text
import Data.Binary
import GHC.Generics

-- Here is my datatype, Token
-- IRL this is a continutation token, passed around between front and backend
-- it **would be nice** if the field "one" is polymorphic, but it must be constrained by the `Binary` class constraint, so we can encode the data type
-- Another way to solve this is with a smart constructor, but I want to use existential types, as an exercise...
--
data Token a =
  forall a. (Binary a, Generic a) => Token {one :: a, two :: Text }

-- However, we will get an error deriving a type class for a generic datatype, so we use standalone deriving
deriving instance Eq a => Eq (Token a)
deriving instance Generic (a) => Generic (Token a)

-- Ultimately, this is the instance we need
instance Binary a => Binary (Token a) where
  get = do
    (oneGet :: a) <- get
    twoGet <- get
    pure $ Token oneGet twoGet
  put (Token a b) = put a <> put b

-- It looks like this is a known issue in ghc...
-- https://gitlab.haskell.org/ghc/ghc/-/issues/10514