Role models are important.
|
Tip
|
You can find a beautiful version of this guide with much improved navigation at https://guide.clojure.style. |
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 it’s 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".
|
Note
|
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. Help us address those shortcomings and make this guide better together! |
Please note, that the Clojure developing community maintains a list of coding standards for libraries, too.
You can generate a PDF copy of this guide using AsciiDoctor PDF, and an HTML copy with AsciiDoctor using the following commands:
# Generates README.pdf
asciidoctor-pdf -a allow-uri-read README.adoc
# Generates README.html
asciidoctor|
Tip
|
Install the gem install rouge |
Translations of the guide are available in the following languages:
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…
Use spaces for indentation. No hard tabs.
Use 2 spaces to indent the bodies of
forms that have body parameters. This covers all def forms, special
forms and macros that introduce local bindings (e.g. loop, let,
when-let) and many macros like when, cond, as->, cond->, case,
with-*, etc.
;; good
(when something
(something-else))
(with-out-str
(println "Hello, ")
(println "world!"))
;; bad - four spaces
(when something
(something-else))
;; bad - one space
(with-out-str
(println "Hello, ")
(println "world!"))Vertically align function (macro) arguments spanning multiple lines.
;; good
(filter even?
(range 1 10))
;; bad
(filter even?
(range 1 10))Use a single space indentation for function (macro) arguments when there are no arguments on the same line as the function name.
;; good
(filter
even?
(range 1 10))
(or
ala
bala
portokala)
;; bad - two-space indent
(filter
even?
(range 1 10))
(or
ala
bala
portokala)Vertically align let (and let-like) bindings.
;; good
(let [thing1 "some stuff"
thing2 "other stuff"]
(foo thing1 thing2))
;; bad
(let [thing1 "some stuff"
thing2 "other stuff"]
(foo thing1 thing2})Align vertically map keys.
;; good
{:thing1 thing1
:thing2 thing2}
;; bad
{:thing1 thing1
:thing2 thing2}
;; bad
{:thing1 thing1
:thing2 thing2}Use Unix-style line endings.[1]
|
Tip
|
If you’re using Git you might want to add the following configuration setting to protect your project from Windows line endings creeping in: $ 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)Syntactic sugar causes semicolon cancer.
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 a single empty line between top-level forms.
;; good
(def x ...)
(defn foo ...)
;; bad
(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.
Avoid single-segment namespaces.
;; good
(ns example.ns)
;; bad
(ns example)Avoid the use of overly long namespaces (i.e., more than 5 segments).
Start every namespace with a comprehensive ns form, comprised of
refers, requires, and imports, conventionally in that order.
(ns examples.ns
(:refer-clojure :exclude [next replace remove])
(:require [clojure.string :as s :refer [blank?]])
(:import java.util.Date))When there are multiple dependencies, you may want to start them on a new line, then insert line breaks after each one for easier sorting, readability if it’s a long line, and to reduce diffing when only there is only a change related to one dependency.
;; better
(ns examples.ns
(:require
[clojure.string :as s :refer [blank?]]
[clojure.set :as set]
[clojure.java.shell :as sh])
(:import
java.util.Date
java.text.SimpleDateFormat
[java.util.concurrent Executors
LinkedBlockingQueue]))
;; good
(ns examples.ns
(:require [clojure.string :as s :refer [blank?]]
[clojure.set :as set]
[clojure.java.shell :as sh])
(:import java.util.Date
java.text.SimpleDateFormat
[java.util.concurrent Executors
LinkedBlockingQueue]))
;; bad
(ns examples.ns
(:require [clojure.string :as s :refer [blank?]] [clojure.set :as set] [clojure.java.shell :as sh])
(:import java.util.Date java.text.SimpleDateFormat [java.util.concurrent Executors LinkedBlockingQueue]))In the ns form prefer :require :as over :require :refer over :require
:refer :all. Prefer :require over :use; the latter form should be
considered deprecated for new code.
;; good
(ns examples.ns
(:require [clojure.zip :as zip]))
;; good
(ns examples.ns
(:require [clojure.zip :refer [lefts rights]]))
;; acceptable as warranted
(ns examples.ns
(:require [clojure.zip :refer :all]))
;; bad
(ns examples.ns
(:use clojure.zip))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))Place the dispatch-val of a multimethod on the same line as the
function name.
;; good
(defmethod foo :bar [x] (baz x))
(defmethod foo :bar
[x]
(baz x))
;; bad
(defmethod foo
:bar
[x]
(baz x))
(defmethod foo
:bar [x]
(baz 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 arity form of a function definition vertically aligned with its parameters.
;; good
(defn foo
"I have two arities."
([x]
(foo x 1))
([x y]
(+ x y)))
;; bad - extra indentation
(defn foo
"I have two arities."
([x]
(foo x 1))
([x y]
(+ x y)))Sort the arities of a function from fewest to most arguments. The common case of multi-arity functions is that some K arguments fully specifies the function’s behavior, and that arities N < K partially apply the K arity, and arities N > K provide a fold of the K arity over varargs.
;; good - it's easy to scan for the nth arity
(defn foo
"I have two arities."
([x]
(foo x 1))
([x y]
(+ x y)))
;; okay - the other arities are applications of the two-arity
(defn foo
"I have two arities."
([x y]
(+ x y))
([x]
(foo x 1))
([x y z & more]
(reduce foo (foo x (foo y z)) more)))
;; bad - unordered for no apparent reason
(defn foo
([x] 1)
([x y z] (foo x (foo y z)))
([x y] (+ x y))
([w x y z & more] (reduce foo (foo w (foo x (foo y z))) more)))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.
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!")))Avoid the use of namespace-manipulating functions like require and
refer. They are entirely unnecessary outside of a REPL
environment.
Avoid forward references. They are occasionally necessary, but such occasions are rare in practice.
Use declare to enable forward references when forward references are
necessary.
Prefer higher-order functions like map to loop/recur.
Don’t shadow clojure.core names with local bindings.
;; bad - you're forced to use clojure.core/map fully qualified inside
(defn foo [map]
...)Use alter-var-root instead of def to change the value of a var.
;; good
(def thing 1) ; value of thing is now 1
; do some stuff with thing
(alter-var-root #'thing (constantly nil)) ; value of thing is now nil
;; bad
(def thing 1)
; do some stuff with thing
(def thing nil)
; value of thing is now nilUse 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))))Prefer vec over into when you need to convert a sequence into a vector.
;; good
(vec some-seq)
;; bad
(into [] some-seq)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 printf instead of (print (format …)).
;; good
(printf "Hello, %s!\n" name)
;; ok
(println (format "Hello, %s!" name))When doing comparisons, keep in mind that Clojure’s functions <,
>, etc. accept a 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 doing so yields 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 doing so yields 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)))Use :else as the catch-all test expression in cond.
;; good
(cond
(neg? n) "negative"
(pos? n) "positive"
:else "zero")
;; bad
(cond
(neg? n) "negative"
(pos? n) "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) :thirty
:else :dunno)
;; much better
(condp = x
10 :ten
20 :twenty
30 :thirty
: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)))
;; test case 2
(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 #{1} [0 1 2 3 4 5])
;; bad
(remove #(= % 1) [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 list* instead of a series of nested cons invocations.
;; good
(list* 1 2 3 [4 5])
;; bad
(cons 1 (cons 2 (cons 3 [4 5])))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 allTo access a private var (e.g. for testing), use the @#'some.ns/var form.
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) ;=> nilThe only real difficulties in programming are cache invalidation and naming things.
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
(e.g., even?).
;; good
(defn palindrome? ...)
;; bad
(defn palindrome-p ...) ; Common Lisp style
(defn is-palindrome ...) ; Java styleThe names of functions/macros that are not safe in STM transactions
should end with an exclamation mark (e.g. 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 argument 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,i- integer index -
x,y- numbers -
xs- sequence -
m- map -
s- string input -
re- regular expression -
coll- a collection -
pred- a predicate closure -
& more- variadic input -
xf- xform, a transducer
-
-
in macros:
-
expr- an expression -
body- a macro body -
binding- a macro binding vector
-
It is better to have 100 functions operate on one data structure than to have 10 functions operate on 10 data structures.
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 shareLeverage 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.
Don’t use the interop syntax to
construct type and record instances. deftype and defrecord
automatically create constructor functions. Use those instead of
the interop syntax, as they make it clear that you’re dealing with a
deftype or a defrecord. See this
article
for more details.
(defrecord Foo [a b])
(deftype Bar [a b])
;; good
(->Foo 1 2)
(map->Foo {:b 4 :a 3})
(->Bar 1 2)
;; bad
(Foo. 1 2)
(Bar. 1 2)Note that deftype doesn’t define the map->Type
constructor. It’s available only for records.
Add custom type/record constructors when needed (e.g. to validate properties on record creation). See this article for more details.
(defrecord Customer [id name phone email])
(defn make-customer
"Creates a new customer record."
[{:keys [name phone email]}]
{:pre [(string? name)
(valid-phone? phone)
(valid-email? email)]}
(->Customer (next-id) name phone email))Feel free to adopt whatever naming convention or structure you’d like for such custom constructors.
Don’t override the auto-generated type/record constructor functions. People expect them to have a certain behaviour and you changing this behaviour violates the principle of the least surprise . See this article for more details.
(defrecord Foo [num])
;; good
(defn make-foo
[num]
{:pre [(pos? num)]}
(->Foo num))
;; bad
(defn ->Foo
[num]
{:pre [(pos? num)]}
(Foo. num))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.
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.
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)Prefer string manipulation functions from clojure.string over Java interop or rolling your own.
;; good
(clojure.string/upper-case "bruce")
;; bad
(.toUpperCase "bruce")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.
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.
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.
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 oneKeep 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.
Avoid writing comments to explain bad code. Refactor the code to make it self-explanatory. ("Do, or do not. There is no try." --Yoda)
Annotations should usually be written on the line immediately above the relevant code.
;; good
(defn some-fun
[]
;; FIXME: Replace baz with the newer bar.
(baz))
;; bad
;; FIXME: Replace baz with the newer bar.
(defn some-fun
[]
(baz))The annotation keyword is followed by a colon and a space, then a note describing the problem.
;; good
(defn some-fun
[]
;; FIXME: Replace baz with the newer bar.
(baz))
;; bad - no colon after annotation
(defn some-fun
[]
;; FIXME Replace baz with the newer bar.
(baz))
;; bad - no space after colon
(defn some-fun
[]
;; FIXME:Replace baz with the newer bar.
(baz))If multiple lines are required to describe the problem, subsequent lines should be indented as much as the first one.
;; good
(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))
;; bad
(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))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)) ; OPTIMIZEUse 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.
Docstrings are the primary way to document Clojure code. Many definition forms
(e.g. def, defn, defmacro, ns)
support docstrings and usually it’s a good idea to make good use of them, regardless
of whether the var in question is something public or private.
If a definition form doesn’t support docstrings directly you can still supply them via
the :doc metadata attribute.
This section outlines some of the common conventions and best practices for documenting Clojure code.
If a form supports docstrings directly prefer them over using :doc metadata:
;; good
(defn foo
"This function doesn't do much."
[]
...)
(ns foo.bar.core
"That's an awesome library.")
;; bad
(defn foo
^{:doc "This function doesn't do much."}
[]
...)
(ns ^{:doc "That's an awesome library.")
foo.bar.core)Let the first line in the doc string be a complete, capitalized sentence which concisely describes the var in question. This makes it easy for tooling (Clojure editors and IDEs) to display a short a summary of the docstring at various places.
;; good
(defn frobnitz
"This function does a frobnitz.
It will do gnorwatz to achieve this, but only under certain
circumstances."
[]
...)
;; bad
(defn frobnitz
"This function does a frobnitz. It will do gnorwatz to
achieve this, but only under certain circumstances."
[]
...)Document all positional arguments, and wrap them them with backticks (`) so that editors and IDEs can identify them and potentially provide extra functionality for them.
;; good
(defn watsitz
"Watsitz takes a `frob` and converts it to a znoot.
When the `frob` is negative, the znoot becomes angry."
[frob]
...)
;; bad
(defn watsitz
"Watsitz takes a frob and converts it to a znoot.
When the frob is negative, the znoot becomes angry."
[frob]
...)Wrap any var references in the docstring with ` so that tooling can identify them.
;; good
(defn wombat
"Acts much like `clojure.core/identity` except when it doesn't.
Takes `x` as an argument and returns that. If it feels like it."
[x]
...)
;; bad
(defn wombat
"Acts much like clojure.core/identity except when it doesn't.
Takes `x` as an argument and returns that. If it feels like it."
[x]
...)Docstrings should be comprised from proper English sentences - this means every sentences should start with an capitalized word and should end with the proper punctuation. Sentences should be separated with a single space.
;; good
(def foo
"All sentences should end with a period (or maybe an exclamation mark).
And the period should be followed by a space, unless it's the last sentence.")
;; bad
(def foo
"all sentences should end with a period (or maybe an exclamation mark).
And the period should be followed by a space, unless it's the last sentence")Indent multi-line docstrings by two spaces.
;; good
(ns my.ns
"It is actually possible to document a ns.
It's a nice place to describe the purpose of the namespace and maybe even
the overall conventions used. Note how _not_ indenting the doc string makes
it easier for tooling to display it correctly.")
;; bad
(ns my.ns
"It is actually possible to document a ns.
It's a nice place to describe the purpose of the namespace and maybe even
the overall conventions used. Note how _not_ indenting the doc string makes
it easier for tooling to display it correctly.")Neither start nor end your doc strings with any whitespace.
;; good
(def foo
"I'm so awesome."
42)
;; bad
(def silly
" It's just silly to start a doc string with spaces.
Just as silly as it is to end it with a bunch of them. "
42)When adding a docstring — especially to a function using the above form — take care to correctly place the docstring after the function name, not after the argument vector. The latter is not invalid syntax and won’t cause an error, but includes the string as a form in the function body without attaching it to the var as documentation.
;; good
(defn foo
"docstring"
[x]
(bar x))
;; bad
(defn foo [x]
"docstring"
(bar x))Code in a functional way, using mutation only when it makes sense.
Be consistent. In an ideal world, be consistent with these guidelines.
Use common sense.
Store your tests in a separate directory, typically test/yourproject/ (as
opposed to src/yourproject/). Your build tool is responsible for making
them available in the contexts where they are necessary; most templates
will do this for you automatically.
Name your ns yourproject.something-test, a file which usually lives in
test/yourproject/something_test.clj (or .cljc, cljs).
When using clojure.test, define your tests
with deftest and name them something-test.
;; good
(deftest something-test ...)
;; bad
(deftest something-tests ...)
(deftest test-something ...)
(deftest something ...)If you are publishing libraries to be used by others, make sure to
follow the Central Repository
guidelines
for choosing your groupId and artifactId. This helps to prevent
name conflicts and facilitates the widest possible use. A good
example is Component.
Avoid unnecessary dependencies. For example, a three-line utility function copied into a project is usually better than a dependency that drags in hundreds of vars you do not plan to use.
There are some lint 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
nsdeclarations 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.
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!
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?