Gregg Reynolds wrote:
Imperative programmers also used it to describe programming patterns. Implementations of things like Observer/VIsitor etc. are ad-hoc, informal constructions; the equivalent in a functional language is a mathematical structure (feel free to fix my terminology). I don't think one needs design patterns for Haskell; it has mathematics and more-or-less formal semantics. Who needs "Visitor" when you have "For All"?
In broad strokes I agree with the thesis, but this example is precisely one where functional languages run into the same issues as OO languages. The Visitor pattern is obviated in functional languages if and only if you construct your recursive types as a fixed-point on a functor representing the open recursive form of your type. Given this you can invoke category-extras or similar libraries[1] to tie everything together for you. Otherwise you end up writing boilerplate functions which are groundings of cata and other recursion schemes. In the Visitor pattern the visitor itself is your F-algebra, and each subtype of \mu F defines its piece of cata (in order to reflect the algebra back on itself and to tell it where to recurse). The algebra has to be written no matter what, since it's the one that does the real work. With category-extras you can define a single implementation of cata which works for all F; without it you write boilerplate groundings of cata for your specific F, exactly as in the Visitor pattern. With a sufficiently expressive reflection system in OO you can do the same CT trick and use introspection to determine the pieces of cata. The functional CT version is generally cleaner, more efficient, and safer, but it still requires taking the steps toward CT as a templating language since functional programming by itself doesn't obviate the Visitor problem. [1] Similar to OO reflection, you could instead use Template Haskell to crack open fixed-recursion types into their open-recursion variants and generate an instance of some Cata class, but once more it's the same trick all over again. -- Live well, ~wren