untagged union types or refinement type support

[lafirest]

Abstract

In Erang, we can often see the following code,
-type text() :: string() | binary().
-type print_text(text()) -> ok.

In Hamler, we can use type class to simulation

class IsText a

instance IsText (List Char)
instance IsText Binary

printText :: forall a. IsText a => a -> IO ()
printText a = do
xxxx

But if we can directly support untagged union, may be better

Design

Untagged Union

data Text = String | Binary

Refinement Type

data Text = forall a.IsText a
IsText a = let type = typeof(a) in
a == typeof(String) || a == typeof(Binary)

or
printText :: (forall a. IsText a) -> IO ()

This is like the following Erlang code:
-spec print_text(A) -> xxx
when A :: string() | binary().