Multiple dispatch in Hy

While tinkering with some AI routines, adderall and Hy in general, I came to conclusion that I needed something akin to multimethods in common lisp or clojure. Luckily I wasn’t the only one who had been wondering about them and quick googling found various articles detailing ways to implement them.

Adam Bard wrote a nice post with fully working code example. That I borrowed and more or less directly translated into Hy (just changed some names in the end and hacked in doc-string):

(defn multi-decorator [dispatch-fn]
  (setv inner (fn [&rest args &kwargs kwargs]
                (setv dispatch-key (apply dispatch-fn args kwargs))
                (if (in dispatch-key inner.--multi--)
                  (apply (get inner.--multi-- dispatch-key) args kwargs)
                  (apply inner.--multi-default-- args kwargs))))
  (setv inner.--multi-- {})
  (setv inner.--doc-- dispatch-fn.--doc--)
  (setv inner.--multi-default-- (fn [&rest args &kwargs kwargs] nil))
  inner)

(defn method-decorator [dispatch-fn &optional [dispatch-key nil]]
  (fn [func]
    (if (is dispatch-key nil)
      (setv dispatch-fn.--multi-default-- func)
      (assoc dispatch-fn.--multi-- dispatch-key func))
    dispatch-fn))

That’s really not much code at all and still it does exactly what I was after (dispatching by certain key in a dictionary). One could maybe add some extra guard clauses to raise an exception if multiple methods with same dispatch-key were registered, but that wouldn’t add much more code.

But, writing functions with decorators gets bothersome eventually, so I created three small macros to automate that part for me:

(defmacro defmulti [name params &rest body]
  `(with-decorator multi-decorator
    (defn ~name ~params ~@body)))

(defmacro defmethod [name multi-key params &rest body]
  `(with-decorator (method-decorator ~name ~multi-key)
    (defn ~name ~params ~@body)))

(defmacro default-method [name params &rest body]
  `(with-decorator (method-decorator ~name)
    (defn ~name ~params ~@body)))

With these Bard’s example can be written in Hy as:

=> (defmulti area [shape]
...  (:type shape))

=> (defmethod area "square" [square]
...  (* (:width square)
...     (:height square)))

=> (defmethod area "circle" [circle]
...  (* (** (:radius circle) 2)
...     3.14))

=> (default-method area [shape]
...  (raise (Exception "Can't calculate area of this shape")))

=> (area {:type "circle" :radius 0.5})
0.785

=> (area {:type "square" :width 1 :height 1})
1

=> (area {:type "rhombus"})
Traceback (most recent call last):
  File "<input>", line 1, in <module>
  File "<input>", line 6, in inner
  File "<input>", line 2, in area
Exception: Can't calculate area of this shape

Quite succinct, without being obtuse. And this system allows extensions (adding new implementations) for new cases, without the need for old ones being modified.

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One thought on “Multiple dispatch in Hy

  1. Pingback: Generating scrolls – redux | Engineer's Journey

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