Variable (VVariable(varName, (Value (Number (NNumber (varValue, varDimension))))))
Here VVariable and NNumber are newtype constructors of tuples, and the entire expression is an "Expression" which, among other things has:
data Expression = Value Value | Variable Variable | ...
and Value has "data Value = Number Number | ..."
Now the newtype constructors seem a bit unnecessary, perhaps, but I guess they increase the type-checking. So I still feel that the above construtor is overly verbose.
Not every embedding has to add constructors. If you look at the types: VVariable :: (String,Value) -> Variable Variable :: Variable -> Expression, you see that you are just using the constructor as a simple way to embed your subtype of variables in the type of expressions. To leave out the intermediate constructor, redefine data Expression = .. | Variable (String,Value) | .. and write your own embedding function, with the same type as the old constructor, that does not preserve the intermediate constructor: variable :: Variable -> Expression variable (VVariable (s,v)) = Variable (s,v) However, my real reason for posting is to recommend a paper you might enjoy, which deals with extensible union types in the context of interpreters, using type classes to automate the embeddings: Monad Transformers and Modular Interpreters, Sheng Liang, Paul Hudak, and Mark P. Jones, In Conference Record of POPL'95: 22nd ACM SIGPLAN-SIGACT Symposium on Principles of Programming Languages, San Francisco, CA, January 1995. http://www.cse.ogi.edu/~mpj/pubs/modinterp.html Hth, Claus -- Haskell Communities and Activities Report (November 2002 edition) All contributions are due in by the end of October! http://www.haskell.org/communities/