Variadic operations over an abstract monad

AbstractVariadic.hs

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE IncoherentInstances #-}

import Control.Monad.IO.Class
import GHC.TypeLits

-- All instances are incoherent because of -XIncoherentInstances
class Variadic r where
instance {-# OVERLAPPABLE #-} MonadIO m => Variadic (m b) where
instance Variadic r => Variadic (a -> r) where
instance {-# OVERLAPPABLE #-}
         TypeError (Text "Expected a variadic function but got: " :$$: ShowType x)
         => Variadic x where

call :: Variadic r => r
call = undefined

g :: forall m. MonadIO m => m ()
g = do
  let -- partial :: String -> m ()
      partial = call 'x' (5 :: Int)
  res1 <- partial "z1"
  partial "z2"