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

[Simon Peyton-Jones]

| Regarding reusing constructor names across several datatypes: is it
| possible to qualify them with their enclosing datatype name, like
| Maybe.Nothing where there is a name conflict? Then I might reuse
| Nothing in my hypothetical data type, and it would be NEither.Nothing
| if conflicting with Maybe.Nothing

Yes, this'd occasionally be useful regardless of extensible data types,
particularly constructor fields too.  E.g. "T.x" could mean "field x
from type T".

There's a difficulty about parsing the name though.  Does
Neither.Nothing mean
	the constructor imported from module Neither
or	the constructor Nothing from type Neither

In these days of lazy ambiguity resolution, though, it's not so bad.  We
could just say that "Neither.Nothing" names *both* of those things, and
if both exist in the current scope, you need to disambiguate by adding
further module qualifiers.

This is one of those features that's hard to evaluate: modest gain in
expressiveness, and modest increase in complexity.

Simon