Eff computations are coercible to each other

[UlfNorell]

{-# LANGUAGE DataKinds #-}

import Control.Monad.Freer
import Control.Monad.Freer.Reader
import Control.Monad.Freer.State
import Data.Coerce

badness :: Eff '[Reader (Int, Int)] a -> Eff '[State Int] a
badness = coerce

boom :: (Int, Int)
boom = run $ evalState 17 (badness ask)

I believe the fix is to add a role annotation

type role Union nominal nominal

here:

freer-simple/src/Data/OpenUnion/Internal.hs

Lines 39 to 41 in 5304190

	-- | Open union is a strong sum (existential with an evidence).
	data Union (r :: [Type -> Type]) a where
	Union :: {-# UNPACK #-} !Word -> t a -> Union r a

(I can make a PR if welcome)