RE: [Haskell-cafe] OO Design in Haskell Example (Draft)

Steve Downey wrote:

So, I've been working on a Composite example. I've used existential types to have a generic proxy to the base type, rather than a simple algebraic type, since adding new leaves to the algebraic type means modifying the whole type, a violation of the Open-Closed principle (open for extension, closed for modification)

Rather than using existential types, a simple record of functions can be often be useful. ie: data Component = Component { draw :: String add :: Component -> Component } It might be worth comparing this approach with the (more complex) one you have described. Tim