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Six Unusual Reasons why Clojure is a Delight

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Clojure is a delightful language, and here are six uncommonly discussed reasons why.

1 - Dead Simple Unit Test Mocking

Clojure is the easiest language to unit test I have ever seen. “Mocking” a function in a test only requires a simple replacement of the function definition. No extraneous interfaces, no dependency injection, no mocking framework. The built-in function with-redefs will replace any function in any library or namespace with a new definition.

(defn next-id [connection]
  (+ 1 (get-current-id connection)))

(testing "next-id"
  ;; bind get-current-id to a lambda that always returns 4 
  (with-redefs [get-current-id (fn [_] 4)]
    (is (= 5 (next-id nil)))))

We “mock” the get-current-id function to always return 4 inside the scope of with-redefs. Couldn’t be more simple! The binding only is in scope for code inside and called by the s-expression of the with-redefs, so no need to unbind it after the test.

2 - Amazing Editing

Many arguments have been made over those contentious parentheses. While the most powerful use of s-expressions is to easily allow macros, for the day to day, s-expressions have a very important use: amazing editing!

With ParEdit (available in most editors), it is trivial to select, move, replace, grow, or shrink any s-expression, string, map, or list. This animated guide shows excellent examples of ParEdit that are too complex to explain here.

Languages that don’t have a surrounding delimiter for expressions leave you jumping around with the mouse and arrow keys a whole lot more. Because it is so much easier to write a parser to select (add 1 2) than it is for add(1, 2), the tooling can be so much better.

No local editing tool I have seen comes close to Vim with ParEdit for effective editing.

3 - Live Attached Repl

Developing in Clojure against a running version of the program is a huge bonus for development speed. While possible to get similar behavior with an attached debugger in other languages, the fluidity of an always-on live attached repl is incredible. At any point, it is possible to run and rerun any given expression to see the results. More than once, I have seen an exception caused by calling a certain function. I trace that function to see the exact inputs that cause the exception, and am able to quickly run every line of the offending function to see the source.

If a debugger sheds light on a single line at a time when running an application, a live attached repl sheds light on the entire application.

4 - No-fuss Polymorphism

One of the best claims about “traditional” Java OO is polymorphism. The ability to make an interface with concrete classes gives the powerful ability to replace behavior dynamically. The trouble is, Java’s polymorphic dynamic dispatch is single dispatch - the decision of which method to call is limited to a single thing: the type of the callee. If this was the only type of dynamic dispatch you ever knew of, it might be hard for you to consciously realize it was ever a limitation, especially if you’ve never seen examples of multiple dispatch.

While most of the time, in any language with first class functions, it is possible to achieve a similar effect by passing functions, it is also possible to get a similar value with something called multimethods.

(defmulti speak :animal)
(defmethod speak :dog [this] (str "woof says " (:name this)))
(defmethod speak :cat [this] (str "mow says " (:name this)))

(speak {:animal :dog :id 1 :name "Spike"})
;; => "woof says Spike"
(speak {:animal :cat :id 2 :name "Mr Cat"})
;; => "mow says Mr Cat"

In this example, we use the :animal keyword to be the “route” function, and the two methods fill in two of the possible concrete types. We are not limited to just a keyword, we can dispatch on anything on the passed map, for example, the oddness of the id:

(defmulti odds? (comp odd? :id))
(defmethod odds? true [d] "odd id")
(defmethod odds? false [c] "even id")

(odds? {:animal :dog :id 1 :name "Spike"})
;; => "odd id"
(odds? {:animal :cat :id 2 :name "Mr Cat"})
;; => "even id"

While both examples are a bit silly, they should demonstrate the power of simple polymorphism. But you might think, what about inheritance? Multimethods allow that too!

5 - Simple Multiple Inheritance

We don’t build inheritance on a single type, but on a hierarchy of keywords. Those can be dispatched on just like any other keyword. First, an example hierarchy of keywords using the built-in functions derive and isa?. These :: keywords are namespaced, which prevents collisions.

(derive ::cat ::mammal)
(derive ::dog ::mammal)
(derive ::dog ::hairy)
(derive ::poodle ::dog)

(isa? ::poodle ::dog)
;; => true
(isa? ::poodle ::mammal)
;; => true
(isa? ::poodle ::hairy)
;; => true
(isa? ::poodle ::cat)
;; => false
(isa? ::mammal ::hairy)
;; => false

A ::dog is-a ::mammal and is-a ::hairy, the classical diamond problem (without the common ancestor, which is possible, but unneeded for the example).

(defmulti speak :animal)
(defmethod speak ::poodle [d] "chirps")
(defmethod speak ::mammal [c] "breathes")

(speak {:animal ::poodle :id 1 :name "Spike"})
;; => "chirps"
(speak {:animal ::dog :id 2 :name "Mr Dog"})
;; => "breathes"

(defmulti shave :animal)
(defmethod shave ::poodle [d] "shivers")
(defmethod shave ::hairy [c] "stuggles")
(defmethod shave ::mammal [c] "maybe cant be shaved!")
(prefer-method shave ::hairy ::mammal)

(shave {:animal ::poodle :id 1 :name "Spike"})
;; => "shivers"
(shave {:animal ::dog :id 2 :name "Rufs"})
;; => "stuggles"

We can see the ::dog keyword doesn’t have an explicit speak or shave implementation, which is fine, because it will then use the “preferred” parent implementation, which returns “breathes” for speak or “struggles” for shave. Since we can have a keyword be the child of multiple parents, we get a multiple inherited behavior, where the preferred match is the one returned.

This is possible because the default equality check of multimethod is the isa? function. Because of this, uses of multimethod hierarchies can have inherited behavior for complex structures.

6 - Mostly Monadic

Languages like Haskell and F# have tools like the maybe monad that add safety to operations. For example, using the maybe monad can completely prevent null reference exceptions by making you ensure you “unpack” the value every time.

How does Clojure address this? In a typical Clojure way, which gives 80% of the value for 20% of the effort, Clojure has a great relationship with empty lists and nil. Rather than wrapping every value that is nullable in a type, Clojure’s default functions all mostly deal with nil and empty without throwing exceptions. For example:

(get {:id 5} :id)
;; => 5
(get nil :id)
;; => nil

(first [3 2 1])
;; => 3
(first nil)
;; => nil
(count nil)
;; => 0

This allows functions to be chained without fear that along the way a nil will get returned.

Since most of the core functions are “smart” about nil, you gain much of the value and safety of monads without most of the hassle. Ultimately, a more rich type system would allow for custom types which can be domain specific, but in day-to-day working, primitive safety is still a huge win.


These are a few simple features that keep me coming back to Clojure, even from languages like F# and Haskell. While Clojure is a bit more wordy than the ML family, and not as type safe, the simplicity of these features keep me coming back for more!


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