[Haskell-cafe] RFE: Extensible algebraic user-defined data types?

[Peter Padawitz]

So many words about a simple thing? I wrote to Dimitry that subtyping in 
O'Haskell and Timber provides exactly what he wants:

data Either a b = Left a  | Right b

data NEither > Either = None

Why has this elegant concept not yet been implemented in standard Haskell?

Peter