Existential types as a library

exists.purs

module Main where

foreign import data Exists :: (* -> *) -> *

foreign import mkExists
  "function mkExists(fa) {\
  \  return fa;\
  \}" :: forall f a. f a -> Exists f
  
foreign import runExists
  "function runExists(f) {\
  \  return function(fa) {\
  \    return f(fa);\
  \  };\
  \}" :: forall f r. (forall a. f a -> r) -> Exists f -> r
  
data Example a = Example a (a -> String)

runExample :: Exists Example -> String
runExample = runExists (\(Example a f) -> f a)

test = runExample (mkExists (Example 123 show))