Turn any function of type `a -> a -> a` into a variadic function

Variadic.hs

{-# LANGUAGE FlexibleInstances, MultiParamTypeClasses #-}

module Data.Function.Variadic where

class Variadic a r where variadic :: (a -> a -> a) -> a -> r

instance Variadic a a where variadic _ = id

instance Variadic a r => Variadic a (a -> r) where
  variadic f x = variadic f . f x