phantom.purs

module Test where

foreign import data A :: *
foreign import data C :: *
foreign import data B :: *

newtype Test (a :: #*) = Test String

a :: Test (a :: A)
a = Test "neko"

b :: Test (a :: A, b :: B)
b = Test "neko"

c :: Test (c :: C)
c = Test "nyan"

test :: forall e. Test (a :: A | e) -> String
test (Test a) = a

testA :: String
testA = test a

testB :: String
testB = test b
-- testC = test c -- compile error