Clojure: Check For nil In a List
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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)The usage of every? is very straightforward, but a quick REPL session is always nice to verify our assumptions.
Returns true if (pred x) is logical true for every x in coll, else false.
Clojure 1.2.0 user=> (every? nil? [nil nil nil]) true user=> (every? nil? [nil 1]) falseAs 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]) falseAs 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.
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