Karthikeyan A K

Karthikeyan A K

Hello, I am Karthikeyan A K, a software architect, and these writings are my experiments with Clojure. Contact me @ +91 8428050777 for Clojure consulation in weekends.

I kind of understand Clojure macros

2 minute read

Looks like I have understood about macros in Clojure (a bit). So whats a macro? Lets say you want to add 2 and 3 and you will write it like this:

(+ 2 3)

Where + is a function and 2 and 3 are arguments. But let’s say that I am more comfortable writing it as (2 + 3) rather than (+ 2 3), how I can do that in Clojure?

When you consider any Clojure code like (+ 2 3) its basically a list, you can verify it in REPL as shown

user=> (type '(+ 2 3))
clojure.lang.PersistentList

The single quote here '(+ 2 3) tells to Clojure that not to execute the list. The basic thing is, any program in Clojure is a list. So even (2 + 3) is a list. Now all we need to do is convert (2 + 3) to (+ 2 3) and let it execute.

So let’s imagine we capture (2 + 3) in a variable called a-list. We take the first element:

(first a-list) ; This gets the 2

Then after the first we put the last element:

(first a-list) ; This gets the 2
(last a-list)  ; This gets the 3

We need to grab the plus sign and put it in front, and we see the plus is in the middle of a-list which is (2 + 3), that is the + is the second element:

(second a-list)
(first a-list) ; This gets the 2
(last a-list)  ; This gets the 3

Now we pack it into a list:

(list ; convert (2 + 3) to (+ 2  3)
  (second a-list)
  (first a-list)
  (last a-list))

And we say the above thing is a macro named calculate which take a single argument called a-list as input:

(defmacro calculate [a-list]
  (list ; convert (2 + 3) to (+ 2  3)
    (second a-list)
    (first a-list)
    (last a-list)))

Now one can try this code:

;; macro.clj

(defmacro calculate [a-list]
  (list ; convert (2 + 3) to (+ 2  3)
    (second a-list)
    (first a-list)
    (last a-list)))

(println
  (calculate (2 + 3)))

This will give 5 as output.

You can also use macroexpand keyword to expand a macro, for example the code below:

(println
  (macroexpand
    '(calculate (2 + 3))))

Will print out:

(+ 2 3)

Thats what macro calculate is supposed to do after you give (2 + 3) as input.

The entire macro.clj code is listed below:

;; macro.clj

(defmacro calculate [a-list]
  (list ; convert (2 + 3) to (+ 2  3)
    (second a-list)
    (first a-list)
    (last a-list)))

(println
  (calculate (2 + 3)))

(println
  (macroexpand
    '(calculate (2 + 3))))

Run it and you should get output like this:

5
(+ 2 3)

References

Updated:

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