On Thu, Jun 4, 2009 at 9:13 AM, wren ng thornton <w...@freegeek.org> wrote:
> The Visitor pattern isn't a functor, it's a collection of things. The type > being visited is the functor[1], the set of methods on that type for > accepting a visitor is a catamorphism[2], and the visitor itself is an > algebra for the functor[3]. > > > [1] Or rather, the coproduct of all related classes that can chain a > visitor forms a type, and that type is a functor. > > > [2] For the recursive Visitor pattern I use most often, that is. For the > non-recursive version it's usually fmap. This is the part where the pattern > gets a bit shaky because there are actually many different patterns all > called "Visitor". The main points of interest are whether it's recursive or > not, and whether it applies the visitor to itself, to its children, or both. > > non-recursive + itself == ($) > > non-recursive + children == fmap (under open-recursion interpretation of > the type, aka all nodes are elements) > > recursive + children == fmap (under closed-recursion interpretation, aka > only fringe nodes are elements) > > recursive + both == cata (usually, though it depends how you aggregate) > > recursive + itself == This is actually a variant of the Iterator pattern Could you be so kind to give an example for each? Cheers, Johan
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