You work for me, Computer.

By Brandon Bloom

Clojure's Multimethod Dispatch as a Library

In keeping with the theme, this post motivates dispatch-map

Clojure’s multimethods embody a powerful polymorphic dispatch mechanism. With multimethods, you can define a function which is polymorphic with respect to a given hierarchy and arbitrary dispatch value. The hierarchy is actually a directed acyclic graph, but you can adjust method priority to address the diamond problem.

Unfortunately, Clojure’s dispatch mechanism is baked into the defmulti and defmethod forms. If you want multimethod-style dispatch, you need a Var to hold your multimethod. Similar to the circumstances which prompted the creation of backtick, I discovered that there was an opportunity to decomplect a part of the core library to extract a useful subcomponent. In this case, the datastructure which describes dispatch rules was complected with the identity which points to those dispatch rules. As a result, it’s impossible to treat a multimethod as an immutible value.

So I created dispatch-map. A dispatch map is just like a regular Clojure map, but it comes coupled with a dispatch function and hierarchy. Looking up keys in a dispatch map has the same dispatch behavior and caching functionality of multimethods. However, unlike a multimethod, a dispatch-map is a true value. The standard map operators assoc, dissoc, etc, return a new dispatch-map, leaving the original unmodified. This enables you to manipulate dispatch maps like any other Clojure data structure, without the need for a named, mutable Var. Clojure’s multimethods and all related functions could trivially be reimplemented in terms of a dispatch-map and an atom (edit: and now are!).

One use for a dispatch-map as a value is to store them within some other data structure, without breaking the value guarantees of the enclosing data structure. In my case, I’ve got a dispatch-map of data types to their GUI representations. In a hierarchical definition of a GUI, each node can have a :templates key which is a (dispatch-map class). If you want to render a (Person.) inside a list box, you can rely on automatic dispatch by type to find the appropriate template: Simply walk up the GUI hierarchy and look in the :templates map. Thanks to the powerful dispatch functionality, this will also work if you have an (Employee.) too!


tgoossens said…

Doesn’t one restrict himself then. What to do when you have a new type that you need to get working with the existing code? You cannot add any new methods. Its a neat idea and I’m sure you can use it for a lot of stuff. But I think – if my previous statement is correct – that the advantage of having it as a value doesn’t compensate for losing the ability to extend (which is basically what you want to do to solve the expression problem right?)

Brandon Bloom said…

You cannot add any new methods.

If that were true, you couldn’t add any new key value pairs to an immutable map…

the advantage of having it as a value doesn't compensate for losing the ability to extend

You’re not losing your ability to extend, you’re simply shifting it’s responsibility elsewhere.

Just put your dispatch-map in an atom, and you’ve basically got a multimethod. More accurately, a multimethod is a dispatch-map plus a Var. The goal of dispatch-map is to decomplect dispatch from state.

Consider, for example, if I had two separate multimethods (call them f and g) and wanted to add a method to both atomically. With a multimethod, you would have to (locking xx (defmethod f …) (defmethod g …)) when you could instead (swap! yy #(–> (assoc-in [:x a] …) (assoc-in [:y a] …)))

tgoossens said…

Ok cool. I think that now I get what you were trying to achieve. Thanks for the explanation!

Paul Stadig said…

Conceptually, an atom and a dispatch-map could replace multimethods. It might be possible with multimethods, since they are so slow anyway. However, there could be performance issues.

I did some similar experiments trying to extract type based dispatch from protocol functions. I started with an atom and a dispatch table, and it turned out to be pretty slow.

Brandon Bloom said…

“Slow” is a relative term. Surely multimethods are slower than protocol dispatch. And protocol dispatch is surely slower than direct dispatch.

However, each yields an additional level of indirection for an additional abstraction need. Direct dispatch offers no abstraction. Protocol dispatch offers abstraction by type. Multimethods offer arbitrary dispatch with respect to a directed acyclic graph. In theory, you could also create a predicate-dispatch-map, which would allow dispatch with respect to an arbitrary decision tree.

With each level of abstraction, you gain a little more flexibility and dynamism at the cost of a little more performance. However, if you can statically determine a dispatch value, you can eliminate those performance costs at compile time. That’s likely not a worthwhile exercise, though, since dynamism is often the whole point of that level of abstraction.

In the case of both multimethods and my dispatch-maps, dispatch values are memoized. Subsequent calls given the same dispatch key do not require traversing the hierarchy to find a “best” match. The result is that the amortized cost of a lookup is the cost of the dispatch-fn plus the cost of one tree lookup. That’s quite reasonable in my mind. December 14, 2012 at 1:26 PM