The Clojure Style Guide

Role models are important.
-- Officer Alex J. Murphy / RoboCop

This Clojure style guide recommends best practices so that real-world Clojure programmers can write code that can be maintained by other real-world Clojure programmers. A style guide that reflects real-world usage gets used, and a style guide that holds to an ideal that has been rejected by the people it is supposed to help risks not getting used at all – no matter how good it is.

The guide is separated into several sections of related rules. I've tried to add the rationale behind the rules (if it's omitted I've assumed that is pretty obvious).

I didn't come up with all the rules out of nowhere - they are mostly based on my extensive career as a professional software engineer, feedback and suggestions from members of the Clojure community, and various highly regarded Clojure programming resources, such as "Clojure Programming" and "The Joy of Clojure".

The guide is still a work in progress - some sections are missing, others are incomplete, some rules are lacking examples, some rules don't have examples that illustrate them clearly enough. In due time these issues will be addressed - just keep them in mind for now.

You can generate a PDF or an HTML copy of this guide using Transmuter.

Source Code Layout & Organization

Nearly everybody is convinced that every style but their own is ugly and unreadable. Leave out the "but their own" and they're probably right...
-- Jerry Coffin (on indentation)

Use two spaces per indentation level. No hard tabs.

;; good
(when something
  (something-else))

;; bad - four spaces
(when something
    (something-else))

Align vertically function arguments.

;; good
(filter even?
        (range 1 10))

;; bad
(filter even?
  (range 1 10))

Align let bindings and map keywords.

;; good
(let [thing1 "some stuff"
      thing2 "other stuff"]
  {:thing1 thing1
   :thing2 thing2})

;; bad
(let [thing1 "some stuff"
  thing2 "other stuff"]
  {:thing1 thing1
  :thing2 thing2})

Optionally omit the new line between the function name and argument vector for defn when there is no docstring.

;; good
(defn foo
  [x]
  (bar x))

;; good
(defn foo [x]
  (bar x))

;; bad
(defn foo
  [x] (bar x))

Optionally omit the new line between the argument vector and a short function body.

;; good
(defn foo [x]
  (bar x))

;; good for a small function body
(defn foo [x] (bar x))

;; good for multi-arity functions
(defn foo
  ([x] (bar x))
  ([x y]
    (if (predicate? x)
      (bar x)
      (baz x))))

;; bad
(defn foo
  [x] (if (predicate? x)
        (bar x)
        (baz x)))

Indent each line of multi-line docstrings.

;; good
(defn foo
  "Hello there. This is
  a multi-line docstring."
  []
  (bar))

;; bad
(defn foo
  "Hello there. This is
a multi-line docstring."
  []
  (bar))

Use Unix-style line endings. (*BSD/Solaris/Linux/OSX users are covered by default, Windows users have to be extra careful.)
- If you're using Git you might want to add the following configuration setting to protect your project from Windows line endings creeping in:
```
bash$ git config --global core.autocrlf true
```
If any text precedes an opening bracket((, { and [) or follows a closing bracket(), } and ]), separate that text from that bracket with a space. Conversely, leave no space after an opening bracket and before following text, or after preceding text and before a closing bracket.
```
;; good
(foo (bar baz) quux)

;; bad
(foo(bar baz)quux)
(foo ( bar baz ) quux)
```
Don't use commas between the elements of sequential collection literals.
```
;; good
[1 2 3]
(1 2 3)

;; bad
[1, 2, 3]
(1, 2, 3)
```

Consider enhancing the readability of map literals via judicious use of commas and line breaks.

;; good
{:name "Bruce Wayne" :alter-ego "Batman"}

;; good and arguably a bit more readable
{:name "Bruce Wayne"
 :alter-ego "Batman"}

;; good and arguably more compact
{:name "Bruce Wayne", :alter-ego "Batman"}

Place all trailing parentheses on a single line instead of distinct lines.

;; good; single line
(when something
  (something-else))

;; bad; distinct lines
(when something
  (something-else)
)

Use empty lines between top-level forms.

;; good
(def x ...)

(defn foo ...)

;; bad
(def x ...)
(defn foo ...)

An exception to the rule is the grouping of related defs together.

;; good
(def min-rows 10)
(def max-rows 20)
(def min-cols 15)
(def max-cols 30)

Do not place blank lines in the middle of a function or macro definition. An exception can be made to indicate grouping of pairwise constructs as found in e.g. let and cond.
Where feasible, avoid making lines longer than 80 characters.
Avoid trailing whitespace.
Use one file per namespace.

Start every namespace with a comprehensive ns form, comprised of imports, requires, refers and uses.

(ns examples.ns
  (:refer-clojure :exclude [next replace remove])
  (:require [clojure.string :as s :refer [blank?]]
            [clojure.set :as set]
            [clojure.java.shell :as sh])
  (:use [clojure xml zip])
  (:import java.util.Date
           java.text.SimpleDateFormat
           [java.util.concurrent Executors
                                 LinkedBlockingQueue]))

Prefer using :require :refer :all over :use in ns macro.

;; good
(ns examples.ns
  (:require [clojure.zip :refer :all]))

;; bad
(ns examples.ns
  (:use clojure.zip))

Avoid single-segment namespaces.

;; good
(ns example.ns)

;; bad
(ns example)

Avoid the use of overly long namespaces(i.e. with more than 5 segments).
Avoid functions longer than 10 LOC (lines of code). Ideally, most functions will be shorter than 5 LOC.
Avoid parameter lists with more than three or four positional parameters.

Syntax

Avoid the use of namespace-manipulating functions like require and refer. They are entirely unnecessary outside of a REPL environment.
Use declare to enable forward references.
Prefer higher-order functions like map to loop/recur.

Prefer function pre and post conditions to checks inside a function's body.

;; good
(defn foo [x]
  {:pre [(pos? x)]}
  (bar x))

;; bad
(defn foo [x]
  (if (pos? x)
    (bar x)
    (throw (IllegalArgumentException "x must be a positive number!")))

Don't define vars inside functions.

;; very bad
(defn foo []
  (def x 5)
  ...)

Don't shadow clojure.core names with local bindings.

;; bad - you're forced to used clojure.core/map fully qualified inside
(defn foo [map]
  ...)

Use seq as a terminating condition to test whether a sequence is empty (this technique is sometimes called nil punning).

;; good
(defn print-seq [s]
  (when (seq s)
    (prn (first s))
    (recur (rest s))))

;; bad
(defn print-seq [s]
  (when-not (empty? s)
    (prn (first s))
    (recur (rest s))))

Use when instead of (if ... (do ...).

;; good
(when pred
  (foo)
  (bar))

;; bad
(if pred
  (do
    (foo)
    (bar)))

Use if-let instead of let + if.

;; good
(if-let [result (foo x)]
  (something-with result)
  (something-else))

;; bad
(let [result (foo x)]
  (if result
    (something-with result)
    (something-else)))

Use when-let instead of let + when.

;; good
(when-let [result (foo x)]
  (do-something-with result)
  (do-something-more-with result))

;; bad
(let [result (foo x)]
  (when result
    (do-something-with result)
    (do-something-more-with result)))

Use if-not instead of (if (not ...) ...).

;; good
(if-not (pred)
  (foo))

;; bad
(if (not pred)
  (foo))

Use when-not instead of (when (not ...) ...).

;; good
(when-not pred
  (foo)
  (bar))

;; bad
(when (not pred)
  (foo)
  (bar))

Use when-not instead of (if-not ... (do ...).

;; good
(when-not pred
  (foo)
  (bar))

;; bad
(if-not pred
  (do
    (foo)
    (bar)))

Use not= instead of (not (= ...)).

;; good
(not= foo bar)

;; bad
(not (= foo bar))

When doing comparisons keep in mind that Clojure's functions <, >, etc accept variable number of arguments.
```
;; good
(< 5 x 10)

;; bad
(and (> x 5) (< x 10))
```
Prefer % over %1 in function literals with only one parameter.
```
;; good
#(Math/round %)

;; bad
#(Math/round %1)
```
Prefer %1 over % in function literals with more than one parameter.
```
;; good
#(Math/pow %1 %2)

;; bad
#(Math/pow % %2)
```

Don't wrap functions in anonymous functions when you don't need to.

;; good
(filter even? (range 1 10))

;; bad
(filter #(even? %) (range 1 10))

Don't use function literals if the function body will consist of more than one form.

;; good
(fn [x]
  (println x)
  (* x 2))

;; bad (you need an explicit do form)
#(do (println %)
     (* % 2))

Favor the use of complement versus the use of an anonymous function.
```
;; good
(filter (complement some-pred?) coll)

;; bad
(filter #(not (some-pred? %)) coll)
```
This rule should obviously be ignored if the complementing predicate exists in the form of a separate function (e.g. even? and odd?).

Leverage comp when it would yield simpler code.

;; Assuming `(:require [clojure.string :as str])`...

;; good
(map #(str/capitalize (str/trim %)) ["top " " test "])

;; better
(map (comp str/capitalize str/trim) ["top " " test "])

Leverage partial when it would yield simpler code.

;; good
(map #(+ 5 %) (range 1 10))

;; (arguably) better
(map (partial + 5) (range 1 10))

Prefer the use of the threading macros -> (thread-first) and ->> (thread-last) to heavy form nesting.

;; good
(-> [1 2 3]
    reverse
    (conj 4)
    prn)

;; not as good
(prn (conj (reverse [1 2 3])
           4))

;; good
(->> (range 1 10)
     (filter even?)
     (map (partial * 2)))

;; not as good
(map (partial * 2)
     (filter even? (range 1 10)))

Prefer .. to -> when chaining method calls in Java interop.

;; good
(-> (System/getProperties) (.get "os.name"))

;; better
(.. System getProperties (get "os.name"))

Use :else as the catch-all test expression in cond.

;; good
(cond
  (< n 0) "negative"
  (> n 0) "positive"
  :else "zero"))

;; bad
(cond
  (< n 0) "negative"
  (> n 0) "positive"
  true "zero"))

Prefer condp instead of cond when the predicate & expression don't change.

;; good
(cond
  (= x 10) :ten
  (= x 20) :twenty
  (= x 30) :forty
  :else :dunno)

;; much better
(condp = x
  10 :ten
  20 :twenty
  30 :forty
  :dunno)

Prefer case instead of cond or condp when test expressions are compile-time constants.

;; good
(cond
  (= x 10) :ten
  (= x 20) :twenty
  (= x 30) :forty
  :else :dunno)

;; better
(condp = x
  10 :ten
  20 :twenty
  30 :forty
  :dunno)

;; best
(case x
  10 :ten
  20 :twenty
  30 :forty
  :dunno)

Use short forms in cond and related. If not possible give visual hints for the pairwise grouping with comments or empty lines.

;; good
(cond
  (test1) (action1)
  (test2) (action2)
  :else   (default-action))

;; ok-ish
(cond
;; test case 1
(test1)
(long-function-name-which-requires-a-new-line
   (complicated-sub-form
      (-> 'which-spans
          multiple-lines)))

(test2)
(another-very-long-function-name
   (yet-another-sub-form
      (-> 'which-spans
          multiple-lines)))

:else
(the-fall-through-default-case
  (which-also-spans 'multiple
                    'lines)))

Use a set as a predicate when appropriate.

;; good
(remove #{0} [0 1 2 3 4 5])

;; bad
(remove #(= % 0) [0 1 2 3 4 5])

;; good
(count (filter #{\a \e \i \o \u} "mary had a little lamb"))

;; bad
(count (filter #(or (= % \a)
                    (= % \e)
                    (= % \i)
                    (= % \o)
                    (= % \u))
               "mary had a little lamb"))

Use (inc x) & (dec x) instead of (+ x 1) and (- x 1).
Use (pos? x), (neg? x) & (zero? x) instead of (> x 0), (< x 0) & (= x 0).

Use the sugared Java interop forms.

;;; object creation
;; good
(java.util.ArrayList. 100)

;; bad
(new java.util.ArrayList 100)

;;; static method invocation
;; good
(Math/pow 2 10)

;; bad
(. Math pow 2 10)

;;; instance method invocation
;; good
(.substring "hello" 1 3)

;; bad
(. "hello" substring 1 3)

;;; static field access
;; good
Integer/MAX_VALUE

;; bad
(. Integer MAX_VALUE)

;;; instance field access
;; good
(.someField some-object)

;; bad
(. some-object someField)

Use the compact metadata notation for metadata that contains only slots whose keys are keywords and whose value is boolean true.
```
;; good
(def ^:private a 5)

;; bad
(def ^{:private true} a 5)
```

Denote private parts of your code.

;; good
(defn- private-fun [] ...)

(def ^:private private-var ...)

;; bad
(defn private-fun [] ...) ; not private at all

(defn ^:private private-fun [] ...) ; overly verbose

(def private-var ...) ; not private at all

Be careful regarding what exactly do you attach metadata to.

;; we attach the metadata to the var referenced by `a`
(def ^:private a {})
(meta a) ;=> nil
(meta #'a) ;=> {:private true}

;; we attach the metadata to the empty hash-map value
(def a ^:private {})
(meta a) ;=> {:private true}
(meta #'a) ;=> nil

Naming

The only real difficulties in programming are cache invalidation and naming things.
-- Phil Karlton

When naming namespaces favor the following two schemas:
- project.module
- organization.project.module
Use lisp-case in composite namespace segments(e.g. bruce.project-euler)

Use lisp-case for function and variable names.

;; good
(def some-var ...)
(defn some-fun ...)

;; bad
(def someVar ...)
(defn somefun ...)
(def some_fun ...)

Use CamelCase for protocols, records, structs, and types. (Keep acronyms like HTTP, RFC, XML uppercase.)

The names of predicate methods (methods that return a boolean value) should end in a question mark. (i.e. even?).

;; good
(defn palindrome? ...)

;; bad
(defn palindrome-p ...) ; Common Lisp style
(defn is-palindrome ...) ; Java style

The names of functions/macros that are not safe in STM transactions should end with an exclamation mark. (i.e. reset!)

Use -> instead of to in the names of conversion functions.

;; good
(defn f->c ...)

;; not so good
(defn f-to-c ...)

Use *earmuffs* for things intended for rebinding (ie. are dynamic).

;; good
(def ^:dynamic *a* 10)

;; bad
(def ^:dynamic a 10)

Don't use a special notation for constants; everything is assumed a constant unless specified otherwise.

Use _ for destructuring targets and formal arguments names whose value will be ignored by the code at hand.

;; good
(let [[a b _ c] [1 2 3 4]]
  (println a b c))

(dotimes [_ 3]
  (println "Hello!"))

;; bad
(let [[a b c d] [1 2 3 4]]
  (println a b d))

(dotimes [i 3]
  (println "Hello!"))

Follow clojure.core's example for idiomatic names like pred and coll.
- in functions:
  - f, g, h - function input
  - n - integer input usually a size
  - index - integer index
  - x, y - numbers
  - s - string input
  - coll - a collection
  - pred - a predicate closure
  - & more - variadic input
- in macros:
  - expr - an expression
  - body - a macro body
  - binding - a macro binding vector

Collections

It is better to have 100 functions operate on one data structure than to have 10 functions operate on 10 data structures.
-- Alan J. Perlis

Avoid the use of lists for generic data storage (unless a list is exactly what you need).

Prefer the use of keywords for hash keys.

;; good
{:name "Bruce" :age 30}

;; bad
{"name" "Bruce" "age" 30}

Prefer the use of the literal collection syntax where applicable. However, when defining sets, only use literal syntax when the values are compile-time constants.

;; good
[1 2 3]
#{1 2 3}
(hash-set (func1) (func2)) ; values determined at runtime

;; bad
(vector 1 2 3)
(hash-set 1 2 3)
#{(func1) (func2)} ; will throw runtime exception if (func1) = (func2)

Avoid accessing collection members by index whenever possible.

Prefer the use of keywords as functions for retrieving values from maps, where applicable.

(def m {:name "Bruce" :age 30})

;; good
(:name m)

;; more verbose than necessary
(get m :name)

;; bad - susceptible to NullPointerException
(m :name)

Leverage the fact that most collections are functions of their elements.

;; good
(filter #{\a \e \o \i \u} "this is a test")

;; bad - too ugly to share

Leverage the fact that keywords can be used as functions of a collection.
```
((juxt :a :b) {:a "ala" :b "bala"})
```
Avoid the use of transient collections, except for performance-critical portions of the code.
Avoid the use of Java collections.
Avoid the use of Java arrays, except for interop scenarios and performance-critical code dealing heavily with primitive types.

Mutation

Refs

Consider wrapping all I/O calls with the io! macro to avoid nasty surprises if you accidentally end up calling such code in a transaction.

Avoid the use of ref-set whenever possible.

(def r (ref 0))

;; good
(dosync (alter r + 5))

;; bad
(dosync (ref-set r 5))

Try to keep the size of transactions (the amount of work encapsulated in them) as small as possible.
Avoid having both short- and long-running transactions interacting with the same Ref.

Agents

Use send only for actions that are CPU bound and don't block on I/O or other threads.
Use send-off for actions that might block, sleep, or otherwise tie up the thread.

Atoms

Avoid atom updates inside STM transactions.

Try to use swap! rather than reset!, where possible.

(def a (atom 0))

;; good
(swap! a + 5)

;; not as good
(reset! a 5)

Strings

Prefer string manipulation functions from clojure.string over Java interop or rolling your own.
```
;; good
(clojure.string/upper-case "bruce")

;; bad
(.toUpperCase "bruce")
```

Exceptions

Reuse existing exception types. Idiomatic Clojure code, when it does throw an exception, throws an exception of a standard type (e.g. java.lang.IllegalArgumentException, java.lang.UnsupportedOperationException, java.lang.IllegalStateException, java.io.IOException).
Favor with-open over finally.

Macros

Don't write a macro if a function will do.
Create an example of a macro usage first and the macro afterwards.
Break complicated macros into smaller functions whenever possible.
A macro should usually just provide syntactic sugar and the core of the macro should be a plain function. Doing so will improve composability.
Prefer syntax-quoted forms over building lists manually.

Comments

Good code is its own best documentation. As you're about to add a comment, ask yourself, "How can I improve the code so that this comment isn't needed?" Improve the code and then document it to make it even clearer.
-- Steve McConnell

Endeavor to make your code as self-documenting as possible.
Write heading comments with at least four semicolons.
Write top-level comments with three semicolons.
Write comments on a particular fragment of code before that fragment and aligned with it, using two semicolons.
Write margin comments with one semicolon.

Always have at least one space between the semicolon and the text that follows it.

;;;; Frob Grovel

;;; This section of code has some important implications:
;;;   1. Foo.
;;;   2. Bar.
;;;   3. Baz.

(defn fnord [zarquon]
  ;; If zob, then veeblefitz.
  (quux zot
        mumble             ; Zibblefrotz.
        frotz))

Comments longer than a word begin with a capital letter and use punctuation. Separate sentences with one space.

Avoid superfluous comments.

;; bad
(inc counter) ; increments counter by one

Keep existing comments up-to-date. An outdated comment is worse than no comment at all.
Prefer the use of the #_ reader macro over a regular comment when you need to comment out a particular form.
```
;; good
(+ foo #_(bar x) delta)

;; bad
(+ foo
   ;; (bar x)
   delta)
```

Good code is like a good joke - it needs no explanation.
-- Russ Olsen

Avoid writing comments to explain bad code. Refactor the code to make it self-explanatory. ("Do, or do not. There is no try." --Yoda)

Comment Annotations

Annotations should usually be written on the line immediately above the relevant code.
The annotation keyword is followed by a colon and a space, then a note describing the problem.
If multiple lines are required to describe the problem, subsequent lines should be indented as much as the first one.

Tag the annotation with your initials and a date so its relevance can be easily verified.

(defn some-fun
  []
  ;; FIXME: This has crashed occasionally since v1.2.3. It may
  ;;        be related to the BarBazUtil upgrade. (xz 13-1-31)
  (baz))

In cases where the problem is so obvious that any documentation would be redundant, annotations may be left at the end of the offending line with no note. This usage should be the exception and not the rule.
```
(defn bar
  []
  (sleep 100)) ; OPTIMIZE
```
Use TODO to note missing features or functionality that should be added at a later date.
Use FIXME to note broken code that needs to be fixed.
Use OPTIMIZE to note slow or inefficient code that may cause performance problems.
Use HACK to note "code smells" where questionable coding practices were used and should be refactored away.
Use REVIEW to note anything that should be looked at to confirm it is working as intended. For example: REVIEW: Are we sure this is how the client does X currently?
Use other custom annotation keywords if it feels appropriate, but be sure to document them in your project's README or similar.

Existential

Code in a functional way, avoiding mutation when that makes sense.
Be consistent. In an ideal world, be consistent with these guidelines.
Use common sense.

Tooling

There are some tools created by the Clojure community that might aid you in your endeavor to write idiomatic Clojure code.

Slamhound is a tool that will automatically generate proper ns declarations from your existing code.
kibit is a static code analyzer for Clojure which uses core.logic to search for patterns of code for which there might exist a more idiomatic function or macro.

Contributing

Nothing written in this guide is set in stone. It's my desire to work together with everyone interested in Clojure coding style, so that we could ultimately create a resource that will be beneficial to the entire Clojure community.

Feel free to open tickets or send pull requests with improvements. Thanks in advance for your help!

License

This work is licensed under a Creative Commons Attribution 3.0 Unported License

Spread the Word

A community-driven style guide is of little use to a community that doesn't know about its existence. Tweet about the guide, share it with your friends and colleagues. Every comment, suggestion or opinion we get makes the guide just a little bit better. And we want to have the best possible guide, don't we?

Cheers,
Bozhidar

erichmond/clojure-style-guide

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