RE: Data constructors and pattern matching

It seems to me that it might be very useful for a module to export the pattern matching operators for a datatype without exporting the constructors. Suppose we have

data A = X {a::Int, b::Float}

And we want to maintain the invariant that b is a floating-point representation of a. So normally we would have to use a completely abstract datatype, and provide constructors and destructors. But in this case, it would actually make sense to provide the pattern-matching operator X, but not the data constructor X. So I think it would be good to separate these in the module export syntax.

What's wrong with module M (A, a, b) where { ... } ? Simon