[Haskell-cafe] Data.Generics question

[Mark T.B. Carroll]

Hmmm. That's interesting. I have a slightly different problem and I
don't know if it's relevant or not. I have nested algebraic data types
for things like expressions with terms and operators and whatever too,
and some of my functions do a transformation that removes all instances
of particular constructors (by expressing them in other ways) but that
leaves most of the data structure intact. I'd love to use the type
system to enforce the constraint that the result from the transformer
function has eliminated occurrences of certain constructors within the
nested type, but to do that I seem to have to create a whole other
near-copy of the data declarations with slightly different names and the
occasional constructor missing. I wonder if there's a better way.

-- Mark