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. we’ve tried to add the rationale behind the rules (if it’s omitted, we’ve assumed that it’s pretty obvious).
We didn’t come up with all the rules out of nowhere; they are mostly based on the experience of the style guide’s editors, feedback and suggestions from numerous members of the Clojure community, and various highly regarded Clojure programming resources, such as "Clojure Programming" and "The Joy of Clojure".
Nothing written here is set in stone. This style guide evolves over time as additional conventions are identified and past conventions are rendered obsolete by changes in Clojure itself.
|
Note
|
Clojure’s developers also maintain a list of coding guidelines for libraries.[1] They were one of the sources of inspiration for the document, you’re currently reading. |
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 |
Programs must be written for people to read, and only incidentally for machines to execute.
Structure and Interpretation of Computer Programs
It’s common knowledge that code is read much more often than it is written. The guidelines provided here are intended to improve the readability of code and make it consistent across the wide spectrum of Clojure code. They are also meant to reflect real-world usage of Clojure instead of a random ideal. When we had to choose between a very established practice and a subjectively better alternative we’ve opted to recommend the established practice.[2]
There are some areas in which there is no clear consensus in the Clojure community regarding a particular style (like semantic indentation vs fixed indentation, semantic comments vs uniform comments, etc). In such scenarios all popular styles are acknowledged and it’s up to you to pick one and apply it consistently.
Fortunately Clojure is a Lisp, and Lisps are fundamentally simple. Even though this guide was created a few years after Clojure (the first version was published in early 2013), you could see that most Clojure code in the wild was fairly uniform. We attribute this to both the simplicity we already mentioned and to the fact that since day 1 Clojurists adopted many of the style conventions of other established Lisp dialects (e.g. Common Lisp and Scheme). This made the work on this guide fairly easy and straight-forward, especially compared to the massive exercise in frustration that was the Community Ruby Style Guide.[3]
Clojure is famously optimized for simplicity and clarity. We’d like to believe that this guide is going to help you optimize for maximum simplicity and clarity.
A foolish consistency is the hobgoblin of little minds, adored by little statesmen and philosophers and divines.
A style guide is about consistency.[4] Consistency with this style guide is important. Consistency within a project is more important. Consistency within one class or method is the most important.
However, know when to be inconsistent — sometimes style guide recommendations just aren’t applicable. When in doubt, use your best judgment. Look at other examples and decide what looks best. And don’t hesitate to ask!
In particular: do not break backwards compatibility just to comply with this guide!
Some other good reasons to ignore a particular guideline:
-
When applying the guideline would make the code less readable, even for someone who is used to reading code that follows this style guide.
-
To be consistent with surrounding code that also breaks it (maybe for historic reasons) — although this is also an opportunity to clean up someone else’s mess (in true XP style).
-
Because the code in question predates the introduction of the guideline and there is no other reason to be modifying that code.
-
When the code needs to remain compatible with older versions of Clojure that don’t support the feature recommended by the style guide.
Translations of the guide are available in the following languages:
|
Note
|
These translations are not maintained by our editor team, so their quality and level of completeness may vary. The translated versions of the guide often lag behind the upstream English version. |
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…
Where feasible, avoid making lines longer than 80 characters.
A lot of people these days feel that a maximum line length of 80 characters is just a remnant of the past and makes little sense today. After all - modern displays can easily fit 200+ characters on a single line. Still, there are some important benefits to be gained from sticking to shorter lines of code.
First, and foremost - numerous studies have shown that humans read much faster vertically and very long lines of text impede the reading process. As noted earlier, one of the guiding principles of this style guide is to optimize the code we write for human consumption.
Additionally, limiting the required editor window width makes it possible to have several files open side-by-side, and works well when using code review tools that present the two versions in adjacent columns.
The default wrapping in most tools disrupts the visual structure of the code, making it more difficult to understand. The limits are chosen to avoid wrapping in editors with the window width set to 80, even if the tool places a marker glyph in the final column when wrapping lines. Some web based tools may not offer dynamic line wrapping at all.
Some teams strongly prefer a longer line length. For code maintained exclusively or primarily by a team that can reach agreement on this issue, it is okay to increase the line length limit up to 100 characters, or all the way up to 120 characters. Please, restrain the urge to go beyond 120 characters.
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 - argument aligned with function name (one space indent)
(filter even?
(range 1 10))
;; bad - two space indent
(filter even?
(range 1 10))The reasoning behind this guideline is pretty simple - the arguments are easier to process by the human brain if they stand out and stick together.
|
Note
|
Generally, you should stick to the formatting outlined in the previous guideline, unless you’re limited by the available horizontal space. |
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)This may appear like some weird special rule to people without Lisp background, but the reasoning behind it is quite simple. Function calls are nothing but regular list literals and normally those are aligned in the same way as other collection type literals when spanning multiple lines:
;; list literal
(1
2
3)
;; vector literal
[1
2
3]
;; set literal
#{1
2
3}Admittedly, list literals are not very common in Clojure, that’s why it’s understandable that for many people lists are nothing but an invocation syntax.
As a side benefit, function arguments are still aligned in this scenario as well. They just happen to accidentally be aligned with the function name as well.
The guidelines to indent differently macros with body forms from all other macro and function calls are collectively known as "semantic indentation". Simply put, this means that the code is indented differently, so that the indentation would give the reader of the code some hints about its meaning.
The downside of this approach is that requires Clojure code formatters to be
smarter. They either need to process macro arglists and rely on the fact
that people named their parameters consistently, or process some additional
indentation metadata.
Some people in the Clojure community have argued that’s not worth it and that everything should simply be indented in the same fashion. Here are a few examples:
;;; Fixed Indentation
;;
;; macros
(when something
(something-else))
(with-out-str
(println "Hello, ")
(println "world!"))
;; function call spanning two lines
(filter even?
(range 1 10))
;; function call spanning three lines
(filter
even?
(range 1 10))This suggestion has certainly gotten some ground in the community, but it also goes against much of the Lisp tradition and one of the primary goals of this style guide - namely to optimize code for human consumption.
There’s one exception to the fixed indentation rule - data lists (those that are not a function invocation):
;;; Fixed Indentation
;;
;; list literals
;; we still do
(1
2
3
4
5
6)
;; and
(1 2 3
4 5 6)
;; instead of
(1 2 3
4 5 6)
;; or
(1
2
3
4
5
6)This makes sure that lists are consistent with how other collection types are normally indented.
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.[5]
|
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 |
End each file with a newline.
|
Tip
|
This should be done by through editor configuration, not manually. |
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, in case those don’t
fit on the same line.
;; good
(defn fibo-iter
([n] (fibo-iter 0 1 n))
([curr nxt n]
(cond
(zero? n) curr
:else (recur nxt (+' curr nxt) (dec n)))))
;; okay - the line break delimits a cond pair
(defn fibo-iter
([n] (fibo-iter 0 1 n))
([curr nxt n]
(cond
(zero? n)
curr
:else
(recur nxt (+' curr nxt) (dec n)))))
;; bad
(defn fibo-iter
([n] (fibo-iter 0 1 n))
([curr nxt n]
(cond
(zero? n) curr
:else (recur nxt (+' curr nxt) (dec n)))))Occasionally, it might seem like a good idea to add a blank line here and there in a longer function definition, but if you get to this point you should also consider whether this long function isn’t doing too much and could potentially be broken down.
Avoid trailing whitespace.
Use one file per namespace and one namespace per file.
;; good
(ns foo.bar)
;; bad
(ns foo.bar)
(ns baz.qux)
;; bad
(in-ns quux.quuz)
(in-ns quuz.corge)
;; bad
(ns foo.bar) or (in-ns foo.bar) in multiple filesAvoid single-segment namespaces.
;; good
(ns example.ns)
;; bad
(ns example)Namespaces exist to disambiguate names. Using a single segment namespace puts you in direct conflict with everyone else using single segment namespaces, thus making it more likely you will conflict with another code base.
In practice this means that libraries should never use single-segment namespace to avoid namespace conflicts with other libraries. Within your own private app of course, you can do whatever you like.
|
Tip
|
It’s common practice to use the convention domain.library-name
or library-name.core for libraries with a single namespace in them.
Read on for more coverage of the namespace naming topic.
|
There are other reasons why might want to avoid single-segment namespaces, so you should think long and hard before making any use of them.
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 give each one its own line. This facilitates sorting, readability, and cleaner diffs for dependency changes.
;; 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))In the ns form, sort your requirements and imports. This facilitates readability and avoids duplication, especially when the list of required / imported namespaces is very long.
;; good
(ns examples.ns
(:require
[baz.core :as baz]
[clojure.java.shell :as sh]
[clojure.set :as set]
[clojure.string :as s :refer [blank?]]
[foo.bar :as foo]))
;; bad
(ns examples.ns
(:require
[clojure.string :as s :refer [blank?]]
[clojure.set :as set]
[baz.core :as baz]
[foo.bar :as foo]
[clojure.java.shell :as sh]))Many core Clojure namespaces have idiomatic aliases that you’re
encouraged to use within your projects - e.g. the most common way to
require clojure.string is: [clojure.string :as str].
|
Note
|
This may appear to mask
clojure.core.str, but it doesn’t. It’s expected that
clojure.core/str and clojure.string/* to be used in a namespace as
str and str/whatever without conflict.
|
;; good
(ns ... (:require [clojure.string :as str] ...)
(str/join ...)
;; not as good - just be idiomatic and use as `str/`
(ns ... (:require [clojure.string :as string] ...)
(string/join ...)As noted in the next section, it’s generally considered idiomatic to use
an alias that is the last segment of the namespace, if that makes it unique,
or else the two segments, typically dropping redundant parts like clj or core.
Amongst Clojure’s core and Contrib namespaces, the following namespaces have idiomatic aliases following that pattern:
Namespace |
Idiomatic Alias |
clojure.datafy |
datafy |
clojure.edn |
edn |
clojure.java.io |
io |
clojure.math |
math |
clojure.set |
set |
clojure.walk |
walk |
clojure.zip |
zip |
clojure.core.async |
async |
clojure.data.csv |
csv |
clojure.data.xml |
xml |
clojure.tools.cli |
cli |
Then there are some core and Contrib namespaces that have shorter idiomatic aliases:
Namespace |
Idiomatic Alias |
clojure.java.shell |
sh |
clojure.pprint |
pp |
clojure.spec.alpha |
s |
clojure.string |
str |
clojure.core.matrix |
mat |
clojure.tools.logging |
log |
clojure.core.protocols |
p |
clojure.core.reducers |
r |
And amongst commonly used community libraries, there are also many that have widely-used, idiomatic aliases for several namespaces:
Namespace |
Idiomatic Alias |
cheshire.core |
json |
clj-yaml.core |
yaml |
clj-http.client |
http |
hugsql.core |
sql |
java-time |
time |
next.jdbc |
jdbc |
Above we covered a handful of popular namespaces and their idiomatic aliases. You might have noticed that those are a bit inconsistent:
-
clojure.stringbecomesstr -
clojure.pprintbecomespp -
clojure.walkbecomeswalk -
clojure.spec.alphabecomess
It’s clear that the one thing they have in common is that they aim to be concise, but still carry some meaning (aliasing clojure.walk to w would
be concise, but won’t carry much meaning).
But what to do about all the other namespaces out there that don’t have idiomatic aliases? Well, you better be consistent in your approach to deriving aliases for them, otherwise the people working on a shared Clojure codebase are going to experience a great deal of confusion. Here are a few rules that you should follow.[6]
-
Make the alias the same as the namespace name with the leading parts removed.
(ns com.example.application
(:require
[clojure.java.io :as io]
[clojure.reflect :as reflect]))-
Keep enough trailing parts to make each alias unique.
[clojure.data.xml :as data.xml]
[clojure.xml :as xml]|
Tip
|
Yes, namespace aliases can have dots in them. Make good use of them. |
-
Eliminate redundant words such as "core" and "clj" in aliases.
[clj-time.core :as time]
[clj-time.format :as time.format]Across a project, it’s good to be consistent with namespace aliases; e.g., don’t require clojure.string as str in one namespace but string in another.
If you follow the previous two guidelines you’re basically covered, but if you opt for custom namespace aliasing scheme it’s still important to apply it
consistently within your projects.
The only real difficulties in programming are cache invalidation and naming things.
When naming namespaces favor the following two schemas:
-
project.module -
organization.project.module
When you’re following the project.module naming scheme and your project
has a single (implementation) namespace it’s common to name it project.core.
Avoid the project.core name in all other cases, as more informative names
are always a better idea.
Use lisp-case in composite namespace segments (e.g. bruce.project-euler).
|
Note
|
Many non-Lisp programming communities refer to lisp-case as
kebab-case, but we all know that Lisp existed way before kebab
was invented.
|
Use lisp-case for function and variable names.
;; good
(def some-var ...)
(defn some-fun ...)
;; bad
(def someVar ...)
(defn somefun ...)
(def some_fun ...)Boolean variables should end in a question mark.
;; good
(def allow-foo? true)
(def allow-bar? false)
;; bad
(def allow-foo true)
(def allow-bar false)Use CapitalCase for protocols, records, structs, and types. (Keep
acronyms like HTTP, RFC, XML uppercase.)
|
Note
|
CapitalCase is also known as UpperCamelCase, `CapitalWords
and PascalCase.
|
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.
;; good
(def max-size 10)
;; bad
(def MAX-SIZE 10) ; Java style
(def +max-size+ 10) ; Common Lisp style, global constant
(def *max-size* 10) ; Common Lisp style, global variable|
Note
|
Famously *clojure-version* defies this convention, but you should
treat this naming choice as a historical oddity and not as an example to
follow.
|
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!"))However, when it can help the understanding of your code, it can be useful to explicitly name unused arguments or maps you’re destructuring from. In this case, prepend the name with an underscore to explicitly signal that the variable is supposed to be unused.
;; good
(defn myfun1 [context _]
(assoc context :foo "bar"))
(defn myfun2 [context {:keys [id]}]
(assoc context :user-id id))
;; better
(defn myfun1 [context _user]
(assoc context :foo "bar"))
(defn myfun2 [context {:keys [id] :as _user}]
(assoc context :user-id id))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 -
sym- symbol -
coll- a collection -
pred- a predicate closure -
& more- variadic input -
xf- xform, a transducer -
el- element -
sel- selector -
evt- event -
addr- address -
val- value -
ns- namespace[7]
-
-
in macros:
-
expr- an expression -
body- a macro body -
binding- a macro binding vector
-
-
in methods (when specified in
defprotocol,deftype,defrecord,reify, etc):-
this- for the first argument, indicating a reference to the object - or alternatively, a consistent name which describes the object
-
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 - clojure.core/map must be fully qualified inside the function
(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 the boolean function if you need to convert something to an actual boolean value (true or false).
;; good
(boolean (foo bar))
;; bad
(if (foo bar) true false)|
Note
|
Don’t forget that the only values in Clojure that are "falsey" are false and nil. Everything else
will evaluate to true when passed to the boolean function.
|
You’ll rarely need an actual boolean value in Clojure, but it’s useful to know how to obtain one when you do.
Use when instead of if with just the truthy branch, as in (if condition (something…)) or (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)))Prefer printf over (print (format …)).
;; good
(printf "Hello, %s!\n" name)
;; ok
(println (format "Hello, %s!" name))When doing comparisons, leverage the fact 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))Prefer anonymous functions over complement, comp and partial, as this results
in simpler code most of the time.[8]
;; good
(filter #(not (some-pred? %)) coll)
;; okish
(filter (complement some-pred?) coll);; Assuming `(:require [clojure.string :as str])`...
;; good
(map #(str/capitalize (str/trim %)) ["top " " test "])
;; okish
(map (comp str/capitalize str/trim) ["top " " test "])comp is quite useful when composing transducer chains, though.
;; good
(def xf
(comp
(filter odd?)
(map inc)
(take 5)));; good
(map #(+ 5 %) (range 1 10))
;; okish
(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)))Parentheses are not required when using the threading macros for functions having no argument specified, so use them only when necessary.
;; good
(-> x fizz :foo first frob)
;; bad; parens add clutter and are not needed
(-> x (fizz) (:foo) (first) (frob))
;; good, parens are necessary with an arg
(-> x
(fizz a b)
:foo
first
(frob x y))The arguments to the threading macros -> (thread-first) and ->>
(thread-last) should line up.
;; good
(->> (range)
(filter even?)
(take 5))
;; bad
(->> (range)
(filter even?)
(take 5))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 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) ;=> nilIt 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 changing this behaviour violates the principle of 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 math functions from clojure.math over (Java) interop or rolling your own.
;; good
(clojure.math/pow 2 5)
;; okish
(Math/pow 2 5)The JDK package java.lang.Math provides access to many useful math
functions. Prior to version 1.11, Clojure relied on using these via interop, but this had
issues with discoverability, primitive performance, higher order application,
and portability. The new clojure.math namespace provides wrapper functions for
the methods available in java.lang.Math for long and double overloads with fast
primitive invocation.
Prefer string manipulation functions from clojure.string over Java interop or rolling your own.
;; good
(clojure.string/upper-case "bruce")
;; bad
(.toUpperCase "bruce")|
Note
|
Several new functions were added to clojure.string in Clojure
1.8 (index-of, last-index-of, starts-with?, ends-with? and
includes?). You should avoid using those if you need to support
older Clojure releases.
|
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.
In this section we’ll go over some common metadata for namespaces and vars that Clojure development tools can leverage.
The most common way to document when a public API was added to a
library is via the :added metadata.
(def ^{:added "0.5"} foo
42)
(ns foo.bar
"A very useful ns."
{:added "0.8"})
(defn ^{:added "0.5"} foo
(bar))|
Tip
|
If you’re into SemVer, it’s a good idea to omit the patch version.
This means you should use 0.5 instead of 0.5.0. This applies
for all metadata data that’s version related.
|
The most common way to document when a public API was changed in a
library is via the :changed metadata. This metadata makes sense only for
vars and you should be using it sparingly, as changing the behavior of
a public API is generally a bad idea.
Still, if you decide to do it, it’s best to make that clear to the API users.
(def ^{:added "0.5"
:changed "0.6"} foo
43)The most common way to mark deprecated public APIs is via the :deprecated
metadata. Normally you’d use as the value the version in which something
was deprecated in case of versioned software (e.g. a library) or simply
true in the case of unversioned software (e.g. some web application).
;;; good
;;
;; in case we have a version
(def ^{:deprecated "0.5"} foo
"Use `bar` instead."
42)
(ns foo.bar
"A deprecated ns."
{:deprecated "0.8"})
(defn ^{:deprecated "0.5"} foo
(bar))
;; otherwise
(defn ^:deprecated foo
(bar))
;;; bad
;;
;; using the docstring to signal deprecation
(def foo
"DEPRECATED: Use `bar` instead."
42)
(ns foo.bar
"DEPRECATED: A deprecated ns.")Often you’d combine :deprecated with :superseded-by, as there would be
some newer API that supersedes whatever got deprecated.
Typically for vars you’ll use a non-qualified name if the replacement lives in the same namespace, and a fully-qualified name otherwise.
;; in case we have a version
(def ^{:deprecated "0.5"
:superseded-by "bar"} foo
"Use `bar` instead."
42)
(ns foo.bar
"A deprecated ns."
{:deprecated "0.8"
:superseded-by "foo.baz"})
(defn ^{:deprecated "0.5"
:superseded-by "bar"} foo
(bar))
;; otherwise
(defn ^{:deprecated true
:superseded-by "bar"} foo
(bar))|
Tip
|
You can also consider adding :supersedes metadata to the newer APIs, basically the inverse of :superseded-by.
|
From time to time you might want to point out some related vars/namespaces that the users of your library might be interested in.
The most common way to do so would be via the :see-also metadata, which takes a vector of related items.
When talking about vars - items in the same namespace don’t need to fully qualified.
;; refers to vars in the same ns
(def ^{:see-also ["bar" "baz"]} foo
"A very useful var."
42)
;; refers to vars in some other ns
(defn ^{:see-also ["top.bar" "top.baz"]} foo
(bar))|
Note
|
Many Clojure programming tools will also try to extract references to other vars from the docstring, but it’s both
simpler and more explicit to use the :see-also metadata instead.
|
Documentation tools like Codox like cljdoc recognize :no-doc metadata.
When a var or a namespace has :no-doc metadata, it indicates to these tools that it should be excluded from generated API docs.
To exclude an entire namespace from API docs:
(ns ^:no-doc my-library.impl
"Internal implementation details")
...To exclude vars within a documented namespace:
(ns my-library.api)
;; private functions do not get documented
(defn- clearly-private []
...)
;; nor do public functions with :no-doc metadata
(defn ^:no-doc shared-helper []
...)
;; this function will be documented
(defn api-fn1
"I am useful to the public"
[]
...)Unlike other Lisp dialects, Clojure doesn’t have a standard metadata format to specify the indentation of macros. CIDER proposed a tool-agnostic indentation specification based on metadata in 2015.[9] Here’s a simple example:
;; refers to vars in the same ns
(defmacro with-in-str
"[DOCSTRING]"
{:style/indent 1}
[s & body]
...cut for brevity...)This instructs the indentation engine that this is a macro with one ordinary parameter and a body after it.
;; without metadata (indented as a regular function)
(dop-iin-str some-string
foo
bar
baz)
;; with metadata (indented as macro with one special param and a body)
(with-in-str some-string
foo
bar
baz)Unfortunately, as of 2020 there’s still no widespread adoption of :style/indent and many editors and IDEs would just
hardcode the indentation rules for common macros.
|
Note
|
This approach to indentation ("semantic indentation") is a contested topic in the Clojure community, due to the need for the additional metadata and tooling support. Despite the long tradition of that approach in the Lisp community in general, some people argue to just stop treating functions and macros differently and simply indent everything with a fixed indentation. This article is one popular presentation of that alternative approach. |
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-explanatory as possible. If you fail to achieve this follow the rest of the guidelines in this section.
Write heading comments with at least four semicolons. Those typically serve to outline/separate major section of code, or to describe important ideas. Often you’d have a section comment followed by a bunch of top-level comments.
;;;; Section Comment/Heading
;;; Foo...
;;; Bar...
;;; Baz...Write top-level comments with three semicolons.
;;; I'm a top-level comment.
;;; I live outside any definition.
(defn foo [])|
Note
|
While the classic Lisp tradition dictates the use of ;;; for
top-level comments, you’ll find plenty of Clojure code in the wild
that’s using ;; or even ;.
|
Write comments on a particular fragment of code before that fragment and aligned with it, using two semicolons.
(defn foo [x]
;; I'm a line/code fragment comment.
x)|
Note
|
While the classic Lisp tradition dictates the use of ;; for
line comments, you’ll find plenty of Clojure code in the wild
that’s using only ;.
|
Write margin comments with one semicolon.
(defn foo [x]
x ; I'm a line/code fragment comment.
)Avoid using those in situations that would result in hanging closing parentheses.
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.
;; This is a good comment.
;; this is a bad commentObviously punctuation is not the most important thing about a comment, but a bit of extra effort results in better experience for the readers of our comments.
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 docstring 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."
[]
...)Important tools such as cljdoc support Markdown in docstrings so leverage it for nicely formatted documentation.
;; good
(defn qzuf-number
"Computes the [Qzuf number](https://wikipedia.org/qzuf) of the `coll`.
Supported options in `opts`:
| key | description |
| --------------|-------------|
| `:finite-uni?`| Assume finite universe; default: `false`
| `:complex?` | If OK to return a [complex number](https://en.wikipedia.org/wiki/Complex_number); default: `false`
| `:timeout` | Throw an exception if the computation doesn't finish within `:timeout` milliseconds; default: `nil`
Example:
```clojure
(when (neg? (qzuf-number [1 2 3] {:finite-uni? true}))
(throw (RuntimeException. \"Error in the Universe!\")))
```"
[coll opts]
...)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. Wrap them with [[..]] if you want to link to 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.
See also [[kangaroo]]."
[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.
See also kangaroo."
[x]
...)Docstrings should be composed of well-formed English sentences. Every sentence should start with a capitalized word, be grammatically coherent, and end with appropriate punctuation. Sentences should be separated with a single space.
;; good
(def foo
"All sentences should end with a period (or maybe an exclamation mark).
The sentence should be followed by a space, unless it concludes the docstring.")
;; bad
(def foo
"all sentences should end with a period (or maybe an exclamation mark).
The sentence should be followed by a space, unless it concludes the docstring.")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 docstring 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 docstring makes
it easier for tooling to display it correctly.")Neither start nor end your docstrings with any whitespace.
;; good
(def foo
"I'm so awesome."
42)
;; bad
(def silly
" It's just silly to start a docstring 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))|
Note
|
Place docstrings for (defprotocol MyProtocol
"MyProtocol docstring"
(foo [this x y z]
"foo docstring")
(bar [this]
"bar docstring")) |
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 - its
coordinates are com.stuartsierra/component.
Another approach that’s popular in the wild is to use a project (or organization)
name as the groupId instead of domain name. Examples of such naming would be:
-
cider/cider-nrepl -
nrepl/nrepl -
nrepl/drawbridge -
clj-commons/fs
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.
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.
One problem with style guides is that it’s often hard to remember all the guidelines and to apply them consistently. We’re only humans, after all. Fortunately, there are a bunch of tools that can do most of the work for us.
|
Tip
|
It’s a great idea run such tools as part of your continuous integration (CI). This ensure that all the code in one project is consistent with the style you’re aiming for. |
There are some lint tools created by the Clojure community that might aid you in your endeavor to write idiomatic Clojure 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.
-
clj-kondo is a linter that detects a wide number of discouraged patterns and suggests improvements, based on this style guide.
While most Clojure editors and IDEs can format the code, according to the layout guidelines outlined here, it’s always handy to have some command-line code formatting tools. There are a couple of options for Clojure that do a great job when it comes to formatting the code as suggested in this guide:
|
Note
|
When it comes to editors - Emacs’s clojure-mode by default will format the code exactly as outlined in the guide.
Other editors might require some configuration tweaking to produce the same results.
|
This guide was started in 2013 by Bozhidar Batsov, following the success of a similar project he had created in the Ruby community.
Bozhidar was very passionate about both Clojure and good programming style and he wanted to bridge the gap between what was covered by the Clojure library coding guidelines and what the style guides for languages like Java, Python and Ruby would typically cover. Bozhidar still serves as the guide’s primary editor, but there’s an entire editor team supporting the project.
Since the inception of the guide we’ve received a lot of feedback from members of the exceptional Clojure community around the world. Thanks for all the suggestions and the support! Together we can make a resource beneficial to each and every Clojure developer out there.
Many people, books, presentations, articles and other style guides influenced the community Clojure style guide. Here are some of them:
The Clojure style guide is stewarded by an editor team of experienced Clojurists that aims to reduce all the input we get (e.g. feedback and suggestions) to a better reference for everyone.
The guide is still a work in progress - some guidelines are lacking examples, some guidelines don’t have examples that illustrate them clearly enough. Improving such guidelines is a great (and simple way) to help the Clojure community!
In due time these issues will (hopefully) be addressed - just keep them in mind for now.
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!
You can also support the style guide (and all my Clojure projects like CIDER, nREPL, orchard, etc) with financial contributions via one of the following platforms:
It’s easy, just follow the contribution guidelines below:
-
Fork bbatsov/clojure-style-guide on GitHub
-
Make your feature addition or bug fix in a feature branch.
-
Include a good description of your changes
-
Push your feature branch to GitHub
-
Send a Pull Request
This guide is written in AsciiDoc and is published as HTML using AsciiDoctor. The HTML version of the guide is hosted on GitHub Pages.
Originally the guide was written in Markdown, but was converted to AsciiDoc in 2019.
This work is licensed under a
Creative Commons Attribution 3.0 Unported License
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?
ns macro, but it’s extremely unlikely you’ll ever need it in the body of a function.