Jay Fields is a software developer at DRW Trading. He has a passion for discovering and maturing innovative solutions. His most recent work has been in the Domain Specific Language space where he's delivered applications that empowered subject matter experts to write the business rules of the applications. He is also very interested in maturing software design through software testing. Jay is a DZone MVB and is not an employee of DZone and has posted 115 posts at DZone. You can read more from them at their website. View Full User Profile

Clojure: Check For nil In a List

08.24.2011
| 4089 views |
  • submit to reddit

The every? function in Clojure is very helpful for determining if every element of a list passes a predicate. From the docs:

Usage: (every? pred coll)

Returns true if (pred x) is logical true for every x in coll, else false.
The usage of every? is very straightforward, but a quick REPL session is always nice to verify our assumptions.
Clojure 1.2.0

user=> (every? nil? [nil nil nil])

true

user=> (every? nil? [nil 1])      

false
As expected, every? works well when you know exactly what predicate you need to use.

Yesterday, I was working on some code that included checking for nil - similar to the example below.
user=> (def front 1)

#'user/front

user=> (def back 2)

#'user/back

user=> (when (and front back) [front back])        

[1 2]
This code works perfectly if you have the individual elements "front" and "back", but as the code evolved I ended up representing "front" and "back" simply as a list of elements. Changing to a list required a way to verify that each entry in the list was not nil.

I was 99% sure that "and" was a macro; therefore, combining it with apply wasn't an option. A quick REPL reference verified my suspicion.
user=> (def legs [front back])               

#'user/legs

user=> (when (apply and legs) legs)      

java.lang.Exception: Can't take value of a macro: #'clojure.core/and (NO_SOURCE_FILE:8)
Several other options came to mind, an anonymous function that checked for (not (nil? %)), map the values to (not (nil? %)) and use every? with true?; however, because of Clojure's truthiness the identity function is really all you need. The following REPL session shows how identity works perfectly as our predicate for this example.
(when (every? identity legs) legs)

[1 2]
For a few more looks at behavior, here's a few examples that include nil and false.
user=> (every? identity [1 2 3 4])

true

user=> (every? identity [1 2 nil 4])

false

user=> (every? identity [1 false 4])

false
As you can see, using identity will cause every? to fail if any element is falsey (nil or false). In my case the elements are integers or nil, so this works perfectly; however, it's worth noting so you don't see unexpected results if booleans ever end up in your list.

 

From http://blog.jayfields.com/2011/08/clojure-check-for-nil-in-list.html

Published at DZone with permission of Jay Fields, author and DZone MVB.

(Note: Opinions expressed in this article and its replies are the opinions of their respective authors and not those of DZone, Inc.)