[Haskell-cafe] Type union

[oleg at okmij.org]

Type unions are also necessary for extensible effects. In fact, they
are the foundation of extensible effects. One implementation

        http://okmij.org/ftp/Haskell/extensible/TList.hs

uses only common and uncontroversial extensions (No Typeable). It
could be converted to closed type families. The other, more optimal
implementation, relies on Typeable, so that injection and projection
become constant time, regardless of the size of the union. The
Typeable constraint can be dispensed with; see the HList
implementation of type-indexed coproducts (TIC), which are essentially
open unions.