Main.purs

module Main (class Foo, foo, BProxy) where

foreign import kind Boolean

foreign import data False :: Boolean
foreign import data True :: Boolean

data BProxy (boolean :: Boolean)
  = BProxy

class Foo (boolean :: Boolean) a | boolean -> a where
  foo :: BProxy boolean -> a

instance fooTrue :: Foo True Int where
  foo _ = 0

instance fooa :: Foo False String where
  foo _ = ""

instance foob :: Foo False Int where
  foo _ = 1
  
foo1 :: String
foo1 = foo (BProxy :: BProxy False)