type coercion with poly-kinded phantom type

Poly.hs

{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE DataKinds #-}

module Poly where

data Mode = Mode1 | Mode2

data Action ctx a = Action

-- | Coerce the phantoms (poly-kinded)
--
coerceAction :: Action ctx a -> Action ctx' a
coerceAction Action = Action

-- 'coerceAction' specialised to restrict input ctx to kind Mode
-- and output ctx to ().  This is not required (see 'x' below);
-- merely a demonstration that 'coerceAction' is poly-kinded.
--
coerceActionModeToUnit :: Action (ctx :: Mode) a -> Action () a
coerceActionModeToUnit = coerceAction

x :: Action () ()
x = coerceAction (Action :: Action 'Mode1 ())