fredag 17 februari 2012

Scary Macros and Nice Gates

I was finally forced into the realms of macros. What I wanted was this:

(defn logic-tree [gate1 gate2 gate3]
(fn [a b c d] (gate1 (gate2 a b) (gate3 c d))))

My idea was to be able to write a function that generated a function which took four args and combined them in a logic gate, like this:

(gate1 (gate2 A B) (gate3 C D))



This is not possible to solve with a function, since a function cannot easily distinguish between the functional arguments and the arguments to feed in later on. This meant I had to go for macros, which have always scared me.

Learning clojure has a good tutorial.

(defmacro logic4 [f]
(list 'fn ['a 'b]
(list f 'a 'b)))

(macroexpand '(logic4 and))
;;=> (fn* ([a b] (and a b)))

((logic4 and) true true) => true

which would easily be expanded to the final form.

Macros does seem less scary after this.

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