The existence of the Expression Problem is orthogonal to whether or not types promote modularity or not. In fact it has a type-safe solution. I recommend read

The existence of the Expression Problem is orthogonal to whether or not types promote modularity or not. In fact it has a type-safe solution. I recommend reading through the original post Wadler wrote to get a better sense of what it is saying: http://homepages.inf.ed.ac.uk/wadler/papers/expression/expression.txt http://homepages.inf.ed.ac.uk/wadler/papers/expression/expre... (There is also a link at the bottom of the Wikipedia article you linked to.)