Using Identity in Clojure

Callback Blocked

Sometimes, I want to be able to pass in a single value to a function that requires a callback function. Just earlier today, for example, I was demonstrating the Open-Closed Principle using defmulti and defmethod (more on those here). I wanted to dispatch each defmethod based on the keyword that was passed into the multimethod. defmulti, though, doesn't accept a single value like keywords, strings, or integers; it requires a callback function.

Enter: the identity function.

Hot Potato

I like to think of identity as the "hot potato" function, because it simply returns whatever argument it receives. It's the same as writing (fn [x] x). So when used as a callback function to functions like map, filter, or defmulti, it simply takes in the value that you give it and spits it right back out to map, filter, or defmulti.

For example:

(defmulti coffee-bot identity)
(defmethod coffee-bot :V60 [_] "Here’s your pour over!")
(defmethod coffee-bot :Nespresso [_] "Here’s your Nespresso!")

(coffee-bot :V60) => "Here's your pour over!"

As far as we're concerned when calling the function, it takes our keyword just the same as if defmulti didn't require a callback.

To tie that in with yesterday's post, this works great if you want to sort a collection into sub-collections of the same item:

user=> (partition-by identity (sort "abcdaabccc"))
((\a \a \a) (\b \b) (\c \c \c \c) (\d))