Build small self contained units of code (possibly pure) and combine them to make them do more complex stuff.

It fit nicely into what I was doing when playing with dates and generating histograms.

I had to get to a certain date in the past to start building my histogram. Libraries like moment.js make it pretty trivial:

moment().subtract('2', 'months');
moment().subtract(moment.duration(2, 'months'))

This will give you a date 2 months ago from now.

Although this works fine and I am reasonably happy with it, I find it somewhat limiting. Using Clojure (and clj-time specifically), you could do this something like:

(-> 2 months ago)

which is equivalent to saying

(ago (months 2))

Other than being easier on eyes (matter of opinion I’ve been told), we have Clojure’s composability at work here. ago and months are two functions, and not literals which have a set meaning. You can write your own functions to further extend this.

Lets say I want to clamp the time to midnight (which basically means setting hours, minutes and seconds to 0). You could write a function that just does that:

(defn at-midnight [t]
    (year t)
    (month t)
    (day t))))

Basically, just pick the year, month and day part out of the supplied time value and construct a new date-midnight instance out of it. I can then use this function as a part of my compositional chain:

(-> 2 months ago at-midnight)

which now gives you time at midnight, 2 months ago.


  1. clj-time - A date and time library for Clojure, wrapping the Joda Time library.
  2. cljs-time - A clj-time inspired date library for clojurescript.