Linters

This page contains an overview of all available linters and their corresponding configuration. For general configurations options, go here.

Table of Contents

Linters
Underscore in namespace

Aliased namespace symbol

Keyword: :aliased-namespace-symbol.

Description: warn when the namespace of a qualified symbol has a defined alias.

Default level: :off.

Example trigger:

(ns foo
  (:require [clojure.string :as str]))

(clojure.string/join ", " (range 10))

Example message: An alias is defined for clojure.string: str.

Config: to suppress the above warning:

{:linters {:aliased-namespace-symbol {:exclude [clojure.string]}}}

Aliased namespace var usage

Keyword: :aliased-namespace-var-usage.

Description: warn when a var from a namespace that was used with :as-alias is used.

Default level: :warning.

Example trigger:

(ns foo
  (:require [clojure.data.xml :as-alias xml]))

(xml/parse-str "<foo/>")

Example message: Namespace only aliased but wasn't loaded: clojure.data.xml

Case

Case duplicate test

Keyword: :case-duplicate-test.

Description: identify duplicate case test constants.

Default level: :error.

Example trigger: (case x :a 1 :b 2 :a 3)

Example message: Duplicate case test constant: :a.

Case quoted test

Keyword: :case-quoted-test

Description: Warn on quoted test constants in case, a common mistake when users don't yet understand that test constants in case are not evaluated.

Default level: :warning

Example trigger:

(case 'x
  'x 1)

Example message: Case test is compile time constant and should not be quoted.

Case symbol test constant

Keyword: :case-symbol-test

Description: Warn on symbol test constants in case. Sometimes this is intentional, but often users expect the symbol to be evaluated. To avoid this confusion, enable this opt-in linter. Another reason to enable it might this extra corner case in CLJS-2209. To opt out after enabling this linter, you can prepend the case expression with #_{:clj-kondo/ignore [:case-symbol-test]}.

Default level: :off

Example trigger:

(let [x 1]
  (case x
    x 1))

Example message: Case test symbol is compile time constant and is never evaluated.

Clj-kondo config

Keyword: :clj-kondo-config

Description: warn on common errors in .clj-kondo/config files

Default level: :warning

Example trigger:

.clj-kondo/config.edn:

{:linters {:foo 1}}

Example message: Unexpected linter name: :foo.

Cond-else

Keyword: :cond-else.

Description: warn on cond with a different constant for the else branch than :else.

Default level: :warning.

Example trigger: (cond (odd? (rand-int 10)) :foo :default :bar).

Example message: use :else as the catch-all test expression in cond.

Condition always true

Keyword: :condition-always-true.

Description: warn on a condition that evaluates to an always truthy constant, like when passing a function instead of calling it. This linter intentionally doesn't check for literally true values of vars since this is often a dev/production setting.

Default level: :off (will be :warning in a future release).

Example trigger: (if odd? :odd :even).

Example message: Condition always true.

Conflicting-alias

Keyword: :conflicting-alias.

Description: warn on conflicting alias.

Default level: :error.

Example trigger:

(require '[clojure.string :as s]
         '[clojure.spec.alpha :as s])

Example message: Conflicting alias for clojure.spec.alpha.

Consistent-alias

Keyword: :consistent-alias

Description: Sometimes it's desirable to have a consistent alias for certain namespaces in a project. E.g. in the below code it could be desirable if every alias for old.api was old-api:

Default level: :warning.

Example trigger:

(ns foo (:require [new.api :as api]))
(ns bar (:require [old.api :as old-api]))
(ns baz (:require [old.api :as api]))

Config:

The consistent alias linter needs pre-configured aliases for namespaces that should have a consistent alias. This configuration:

{:linters {:consistent-alias {:aliases {old.api old-api}}}}

will produce this warning:

Inconsistent alias. Expected old-api instead of api.

Datalog syntax

Keyword: :datalog-syntax.

Description: warn on invalid datalog syntax. This linter is implemented using io.lambdaforge/datalog-parser. Also see this blog post.

Default level: :error.

Example trigger:

(ns user (:require [datahike.api :refer [q]]))

(q '[:find ?a :where [?b :foo _]] 42)

Example message: Query for unknown vars: [?a].

Deprecated var

Keyword: :deprecated-var.

Description: warn on usage of var that is deprecated.

Default level: :warning.

Example trigger: (def ^:deprecated x) x

Example warning: #'user/x is deprecated.

Config:

Say you have the following function:

(ns app.foo)
(defn foo {:deprecated "1.9.0"} [])

and you still want to be able to call it without getting a warning, for example in function in the same namespace which is also deprecated:

(defn bar {:deprecated "1.9.0"} []
  (foo))

or in test code:

(ns app.foo-test
  (:require
   [app.foo :refer [foo]]
   [clojure.test :refer [deftest is]]))

(deftest foo-test [] (is (nil? (foo))))

To achieve this, use this config:

{:linters
 {:deprecated-var
  {:exclude
   {app.foo/foo
    {:defs [app.foo/bar]
     :namespaces [app.foo-test]}}}}}

A regex is also permitted, e.g. to exclude all test namespaces:

{:linters {:deprecated-var {:exclude {app.foo/foo {:namespaces [".*-test$"]}}}}}

Deprecated namespace

Keyword: :deprecated-namespace.

Description: warn on usage of namespace that is deprecated.

Default level: :warning.

Example trigger:

(ns foo {:deprecated true})
(def x 1)

(ns bar (:require [foo]))

Example warning: Namespace foo is deprecated..

Config:

To exclude warnings about specific namespaces, use:

{:linters {:deprecated-namespace {:exclude [the-deprecated.namespace]}}}

Deps.edn

Keyword: :deps.edn

Description: warn on common errors in deps.edn and bb.edn files.

Default level: :warning

Example trigger:

deps.edn:

{:deps {foo/bar "2020.10.11"}}

Example message:

Expected map, found: java.lang.String

Bb.edn

Bb.edn dependency on undefined task

Keyword: :bb.edn-undefined-task

Description: warn on taks undefined task dependencies in bb.edn files.

Default level: :error

Example trigger:

bb.edn:

{:tasks {run {:depends [compile]}}}

Example message:

Depending on undefined task: compile

Bb.edn cyclic task dependency

Keyword: :bb.edn-cyclic-task-dependency

Description: warn on cyclic dependencies bb.edn files.

Default level: :error

Example trigger:

bb.edn:

{:tasks {a {:depends [b]
            :task (println "a")}
         b {:depends [a]}}}

Example message:

Cyclic task dependency: a -> b -> a

Bb.edn Unexpected key

Keyword: :bb.edn-unexpected-key

Description: warn on unexpected keys in bb.edn

Default level: :warning

Example trigger:

bb.edn:

{:requires [[babashka.fs :as fs]]}

Example message:

Global :requires belong in the :tasks map.

Bb.edn task docstring missing

Keyword: :bb.edn-task-missing-docstring

Description: warn on missing docstring for map tasks.

Default level: :off

Example trigger:

bb.edn:

{:tasks {a {:task (call-fn}]}

Example message:

Docstring missing for task: a

Discouraged var

Keyword: :discouraged-var

Description: warn on the usage of a var that is discouraged to be used.

Default level: :warning

Config:

{:linters {:discouraged-var {clojure.core/read-string {:message "Use edn/read-string instead of read-string"}}}}

The matching namespace symbol may be given a group name using a regex pattern. The warning can be made undone on the namespace level (e.g. via :config-in-ns or ns metadata) by providing :level on the var level:

{:linters {:discouraged-var {clojure.core/read-string {:level :off}}}}

Example trigger:

With the configuration above:

(read-string "(+ 1 2 3)")

Example message:

Use edn/read-string instead of read-string

Discouraged namespace

Keyword: :discouraged-namespace

Description: warn on the require or usage of a namespace that is discouraged to be used.

Default level: :warning

Config:

{:linters {:discouraged-namespace {clojure.java.jdbc {:message "Use next.jdbc instead of clojure.java.jdbc"}}}}

The matching namespace symbol may be given a group name using a regex pattern.

{:ns-groups [{:pattern "clojure\\.java\\.jdbc.*"
              :name jdbc-legacy}]
 :linters {:discouraged-namespace {jdbc-legacy {:message "Use next.jdbc instead of clojure.java.jdbc"}}}}

Add :discouraged-namespace linter into :config-in-ns to specify that specific namespaces are discouraged to be used in some namespace of ns-group.

{:config-in-ns {app.jdbc {:linters {:discouraged-namespace {clojure.java.jdbc {:message "Use next.jdbc instead of clojure.java.jdbc"}}}}}}

Example trigger:

With the configuration above:

(require '[clojure.java.jdbc :as j])

Discouraged tag

Keyword: :discouraged-tag

Description: warn on the usage of a tagged literal that is discouraged to be used.

Default level: :warning

Config:

{:linters {:discouraged-tag {inst {:message "Prefer #java-time/instant" }}}}

Example trigger:

Given the above configuration:

{:date #inst "2020"}

Example message:

Prefer #java-time/instant

Docstring blank

Keyword: :docstring-blank.

Description: warn on blank docstring.

Default level: :warning.

Example trigger: (defn foo "" [a b] 1)

Example message: Docstring should not be blank..

Docstring no summary

Keyword: :docstring-no-summary.

Description: warn when first line of docstring is not a complete sentence. This linter is based on the community style guide.

Explanation by Bozhidar Batsov:

The idea is simple - each docstring should start with a one-line sentence. This minimizes the work tools have to do to extract some meaningful summary of what a var does (and as a bonus - it plays great with the Emacs minibuffer, that happens to have a height of 1 line).

Default level: :off.

Example trigger: (defn foo "not a sentence" [a b] 1)

Example message: First line of the docstring should be a capitalized sentence ending with punctuation.

Docstring leading trailing whitespace

Keyword: :docstring-leading-trailing-whitespace.

Description: warn when docstring has leading or trailing whitespace

Default level: :off.

Example trigger: (defn foo "Has trailing whitespace.\n" [a b] 1)

Example message: Docstring should not have leading or trailing whitespace.

Duplicate map key

Keyword: :duplicate-map-key.

Description: warn on duplicate key in map.

Default level: :error.

Example trigger: {:a 1 :a 2}

Example message: duplicate key :a.

Duplicate require

Keyword: :duplicate-require.

Description: warns on namespace that has been required more than once within a namespace.

Example trigger:

(ns foo
  (:require [clojure.string :as str]
            [clojure.string :as str]))

Example message: duplicate require of clojure.string

Duplicate set key

Keyword: :duplicate-set-key.

Description: similar to :duplicate-map-key but for sets.

Example trigger: #{:a :a}

Example message: duplicate set element :a.

Duplicate field name

Keyword: :duplicate-field-name.

Description: identify duplicate fields in deftype/defrecord fields definition.

Default level: :error.

Example trigger: (deftype T [x y z y])

Example message: Duplicate field name: y.

Dynamic vars

Dynamic var not earmuffed

Keyword: :dynamic-var-not-earmuffed

Description: warn when dynamic var doesn't have an earmuffed name.

Default level: :off.

Example trigger: (def ^:dynamic foo)

Example message: "Var is declared dynamic but name is not earmuffed: foo"

Earmuffed var not dynamic

Keyword: :earmuffed-var-not-dynamic

Description: warn when var with earmuffed name isn't declared dynamic.

Default level: :warning.

Example trigger: (def *foo*)

Example message: "Var has earmuffed name but is not declared dynamic: *foo*"

Quoted case test constant

Keyword: :quoted-case-test-constant.

Description: warn when encountering quoted test case constants.

Default level: :warning.

Example trigger: (case x 'a 1 :b 2)

Example message: Case test is compile time constant and should not be quoted.

Equals expected position

Keyword: :equals-expected-position

Description: warn on usage of = with the expected value, a constant, that is not in the expected (first by default) position

Default level: :off

Example trigger: (= (+ 1 2 3) 6)

Example message: Write expected value first

Config:

The default configuration for this linter is:

{:linters {:equals-expected-position {:level :off
                                      :position :first
                                      :only-in-test-assertion false}}}

Possible values for :position are :first and :last The :only-in-test-assertion boolean activates the linter only in a test assertion context, e.g. (clojure.test/is (= (+ 1 2 3) 1))

Equals false

Keyword: :equals-false

Description: warn on usage of (= false x) or (= x false) rather than (false? x)

Default level: :off

Example trigger: (fn [x] (= false x))

Example message: Prefer (false? x) over (= false x).

Equals true

Keyword: :equals-true

Description: warn on usage of (= true x) or (= x true) rather than (true? x)

Default level: :off

Example trigger: (fn [x] (= true x))

Example message: Prefer (true? x) over (= true x).

File

Keyword: :file.

Description: warn on error while reading file.

Default level: :error.

Example trigger: clj-kondo --lint foo.clje.

Example message: file does not exist.

Format

Keyword: :format.

Description: warn on unexpected amount of arguments in format.

Default level: :error.

Example trigger: (format "%s" 1 2).

Example message: Format string expects 1 arguments instead of 2..

Def + fn instead of defn

Keyword: :def-fn.

Description: tells about closures defined with the combination of def and fn with optional let in-between. In almost all cases defn can be used instead which has the benefit of adding :arglists metadata to vars.

The practice of using defn instead of def + fn has the following benefits:

:argslists* metadata on the var
more readable stacktraces

Default level: :off.

Example triggers:

(def f (fn [] nil)) which can be written as: (defn f [] nil)).
(def f (let [y 1] (fn [x] (+ x y)))) which can be written as (let [y 1] (defn f [x] (+ x y))).

Example messages:

Use defn instead of def + fn

Config:

{:linters {:def-fn {:level :warning}}}

More info:

See issue.

Destructured or binding of same map

Keyword: :destructured-or-binding-of-same-map

Description: an :or default value refers to a destructured binding of the same map. This may result in ambigious behavior since the order of bindings is undefined. See this issue for more details and discussion.

Default level: :warning

Example triggers:

(fn [x {:keys [a b] :or {b a}}]). Note that this works in Clojure, but relies on incidental behavior of how destructuring is implemented. If you reverse the order of a and b in :keys you will get an error from the Clojure compiler: (fn [x {:keys [b a] :or {b a}}]) It is better to not rely on this behavior at all since adding bindings or changing the order of bindings will break it.

Example messages:

Destructured :or refers to binding of same map: a

Config:

{:linters {:destructured-or-binding-of-same-map {:level :warning}}}

Inline def

Keyword: :inline-def.

Description: warn on non-toplevel usage of def (and defn, etc.).

Default level: :warning.

Example trigger: (defn foo [] (def x 1)).

Example message: inline def.

Invalid arity

Keyword: :invalid-arity.

Description: warn when a function (or macro) is called with an invalid amount of arguments.

Default level: :error.

Example trigger: (inc).

Example message: clojure.core/inc is called with 0 args but expects 1.

Config:

Some macros rewrite their arguments and therefore can cause false positive arity errors. Imagine the following silly macro:

(ns silly-macros)

(defmacro with-map [m [fn & args]]
  `(~fn ~m ~@args))

which you can call like:

(silly-macros/with-map {:a 1 :d 2} (select-keys [:a :b :c])) ;;=> {:a 1}

Normally a call to this macro will give an invalid arity error for (select-keys [:a :b :c]), but not when you use the following configuration:

{:linters {:invalid-arity {:skip-args [silly-macros/with-map]}}}

Conflicting arity

Keyword: :conflicting-fn-arity.

Description: warn when an overloaded function defines multiple argument vectors with the same arity.

Default level: :error.

Example trigger: (fn ([x] x) ([y]) x).

Example message: More than one function overload with arity 2..

Reduce without initial value

Keyword: :reduce-without-init.

Description: warn when reduce is called without an explicit initial value. Read this article why this can be problematic.

Default level: :off.

Example trigger: (reduce max []).

Example message: Reduce called without explicit initial value.

Config: to suppress the above warning:

{:linters {:reduce-without-init {:exclude [clojure.core/max cljs.core/max]}}}

Loop without recur

Keyword: :loop-without-recur.

Description: warn when loop does not contain recur.

Default level: :warning.

Example trigger: (loop []).

Example message: Loop without recur.

Line length

Keyword: :line-length.

Description: warn when lines are longer than a configured length.

Default level: :warning.

Default line length: :max-line-length is nil by default, which disables line length linting.

Config:

The line length linter needs to know how long you are prepared to allow your lines to be. This configuration:

{:linters {:line-length {:max-line-length 120}}}

will produce this warning:

Line is longer than 120 characters.

Config:

To exclude lines with URLs use: :exclude-urls true

{:linters {:line-length {:max-line-length 120
                         :exclude-urls true}}}

To exclude lines that matches a pattern via re-find, use: :exclude-pattern ";; :ll/ok":

{:linters {:line-length {:max-line-length 120
                         :exclude-pattern ";; :ll/ok"}}}

Keyword in binding vector

Keyword: :keyword-binding

Description: warn when a keyword is used in a :keys binding vector

Default level: :off.

Example trigger: (let [{:keys [:a]} {:a 1}] a).

Example message: Keyword binding should be a symbol: :a

Main without gen-class

Keyword: :main-without-gen-class.

Description: warn when -main function is present without corresponding :gen-class.

Default level: :off.

Example trigger: (ns foo) (defn -main [& _args]).

Example message: Main function without gen-class.

Minus one

Keyword: :minus-one

Description: warn on usages of - that can be replaced with dec.

Default level: :off

Example trigger:

(def x 1)
(- x 1)

Example message: Prefer (dec x) over (- x 1)

Also see :plus-one.

Misplaced docstring

Keyword: :misplaced-docstring.

Description: warn when docstring appears after argument vector instead of before.

Default level: :warning.

Example trigger: (defn foo [] "cool fn" 1).

Example message: Misplaced docstring.

Missing body in when

Keyword: :missing-body-in-when.

Description: warn when when is called only with a condition.

Default level: :warning.

Example trigger: (when true).

Example message: Missing body in when.

Missing clause in try

Keyword: :missing-clause-in-try.

Description: warn when try expression misses catch or finally clause.

Default level: :warning.

Example trigger: (try 1).

Example message: Missing catch or finally in try.

Missing docstring

Keyword: :missing-docstring.

Description: warn when public var misses docstring.

Default level: :off.

Example trigger: (defn foo [] 1).

Example message: Missing docstring.

Missing else branch

Keyword: :missing-else-branch.

Description: warns about missing else branch in if expression.

Default level: :warning.

Example trigger: (if :foo :bar).

Example message: Missing else branch..

Missing map value

Keyword: :missing-map-value.

Description: warn on key with uneven amount of elements, i.e. one of the keys misses a value.

Default level: :error.

Example trigger: {:a 1 :b}

Example message: missing value for key :b.

Missing test assertion

Keyword: :missing-test-assertion.

Description: warn on deftest expression without test assertion.

Default level: :warning.

Example trigger:

(require '[clojure.test :as test])
(test/deftest foo (pos? 1))

Example message: missing test assertion.

Namespace name mismatch

Keyword: :namespace-name-mismatch.

Description: warn when the namespace in the ns form does not correspond with the file name of the file.

Default level: :error.

Example trigger: a file named foo.clj containing a namespace (ns bar).

Example message: Namespace name does not match file name: bar

Example trigger: a folder/file containing dashes instead of underscores, example: example-namespace/foo.clj containing a namespace (ns example-namespace.foo).

Example message: Namespace name does not match file name: example-namespace.foo

Non-arg vec return type hint

Keyword: :non-arg-vec-return-type-hint.

Description: warn when a return type in defn is not placed on the argument vector (CLJ only).

Default level: :warning.

Example trigger: (defn ^String foo [] "cool fn").

Example message: Prefer placing return type hint on arg vector: String

Read this issue for more background information on this linter.

Not empty?

Keyword: :not-empty?

Description: warn on (not (empty? ...)) idiom. According to the docstring of empty? seq is prefered.

Default level: :warning.

Example trigger: (not (empty? []))

Example message: use the idiom (seq x) rather than (not (empty? x)).

Plus one

Keyword: :plus-one

Description: warn on usages of + that can be replaced with inc.

Default level: :off

Example trigger:

(def x 1)
(+ x 1)

Example message: Prefer (inc x) over (+ 1 x)

Also see :minus-one.

Private call

Keyword: :private-call.

Description: warn when private var is used. The name of this linter should be renamed to "private usage" since it will warn on usage of private vars and not only inside calls.

Default level: :error.

Example trigger:

(ns foo) (defn- f [])

(ns bar (:require [foo]))
(foo/f)

Example message: #'foo/f is private.

To suppress the above message, refer to foo/f using the var #'foo/f or write:

#_{:clj-kondo/ignore [:private-call]}
(foo/f)

Protocol method varargs

Keyword: :protocol-method-varargs.

Description: warn on definition of varargs protocol method.

Default level: :error.

Example trigger: (defprotocol Foo (foo [x & xs]))

Example message: Protocol methods do not support varargs.

Redefined var

Keyword: :redefined-var.

Description: warn on redefined var.

Default level: :warning.

Example trigger: (def x 1) (def x 2)

Example message: redefined var #'user/x.

When redefining a var from another namespace that was referred, e.g. inc, the message is: inc already refers to #'clojure.core/inc

Var same name except case

Keyword: :var-same-name-except-case.

Description: warn on vars that share the same name with different case (only in Clojure mode) as these could cause clashing class file names on case insensitive filesystems.

Default level: :warning.

Example trigger: (defmacro One [] 1) (defn one [] 1)

Example message: warning: Var name one differs only in case from: One.

Redundant do

Keyword: :redundant-do.

Description: warn on usage of do that is redundant. The warning usually arises because of an explicit or implicit do as the direct parent s-expression.

Default level: :warning.

Example trigger: (defn foo [] (do 1)).

Example message: redundant do.

Redundant call

Keyword: :redundant-call

Description: warn on redundant calls. The warning arises when a single argument is passed to a function or macro that that returns its arguments.

Default level: :off.

clojure.core and cljs.core functions and macros that trigger this lint:

->, ->>
cond->, cond->>
some->, some->>
comp, partial
merge

Config:

{:linters {:redundant-call {:exclude #{clojure.core/->}
                            :include #{clojure.core/conj!}}}}

Use :exclude to suppress warnings for the built-in list. Use :include to warn on additional vars.

Example trigger: (-> 1).

Example message: Single arg use of -> always returns the arg itself.

Redundant fn wrapper

Keyword: :redundant-fn-wrapper

Description: warn on redundant function wrapper.

Default level: :off.

Example trigger: #(inc %).

Example message: Redundant fn wrapper.

Redundant ignore

Keyword: :redundant-ignore

Description: warn on redundant ignore, i.e. when an ignored expression doesn't trigger any lint warning

Default level: :info.

Example trigger: #_:clj-kondo/ignore (+ 1 2 3).

Example message: Redundant ignore.

Redundant nested call

Keyword: :redundant-nested-call

Description: warn on redundant nested call of functions and macros

Default level: :info.

Example trigger: (+ 1 2 (+ 1 2 3)).

Example message: Redundant nested call: +.

Redundant let

Keyword: :redundant-let.

Description: warn on usage of let that is redundant. The warning usually arises because directly nested lets.

Default level: :warning.

Example trigger: (let [x 1] (let [y 2] (+ x y))).

Example message: Redundant let expression.

Redundant str call

Keyword: :redundant-str-call

Description: warn on redundant str calls. The warning arises when a single argument is passed to a str that is already a string, which makes the str unnecessary.

Default level: :info.

Example triggers: (str "foo"), (str (str 1)).

Example message: Single argument to str already is a string.

Refer

Keyword: :refer

Description: warns when :refer is used. This can be used when one wants to enforce usage of aliases.

Default level: :off.

Example trigger: (ns foo (:require [clojure.set :refer [union]])).

Example warning: require with :refer.

Config: to suppress the above warning:

{:linters {:refer {:exclude [clojure.set]}}}

Refer all

Keyword: :refer-all

Description: warns when :refer :all is used.

Default level: :warning.

Example trigger: (ns foo (:require [clojure.set :refer :all])).

Example message: use alias or :refer.

Config: to suppress the above warning:

{:linters {:refer-all {:exclude [clojure.set]}}}

Schema misplaced return

Keyword: :schema-misplaced-return

Description: warn on a misplaced return Schema

Default level: :warning

Example trigger: (require '[schema.core :as s]) (s/defn foo [] :- s/Str)

Example message: Return schema should go before vector.

Self-requiring namespace

Keyword: :self-requiring-namespace

Description: warn on a namespace that requires itself

Default level: :off

Example trigger: (ns foo (:require [foo]))

Example message: Namespace is requiring itself: foo

Single key in

Keyword: :single-key-in.

Description: warn on associative path function with a single value path.

Default level: :off.

Example trigger: (get-in {:a 1} [:a]).

Example message: get-in with single key.

Single logical operand

Keyword: :single-logical-operand.

Description: warn on single operand logical operators with always the same value.

Default level: :warning.

Example trigger: (and 1).

Example message: Single arg use of and always returns the arg itself.

Single operand comparison

Keyword: :single-operand-comparison.

Description: warn on comparison with only one argument.

Default level: :warning.

Example trigger: (< 1).

Example message: Single operand use of clojure.core/< is always true.

Shadowed fn param

Keyword: :shadowed-fn-param

Description: warn on fn param that has same name as previously defined one (in the same fn expression)

Default level: :warning.

Example trigger: (fn [x x]).

Example message: Shadowed fn param: x

Shadowed var

Keyword: :shadowed-var.

Description: warn on var that is shadowed by local.

Default level: :off.

Example trigger: (def x 1) (let [x 2] x).

Example message: Shadowed var: user/x.

Config:

{:linters {:shadowed-var {:level :warning
                          :exclude [ns]
                          :suggest {name nom}}}}

(fn [name] name)
     ^--- Shadowed var: clojure.core/name. Suggestion: nom

Use :exclude to suppress warnings for specific binding names. Use :include to warn only for specific names.

To avoid shadowing core vars you can also use :refer-clojure + :exclude in the ns form.

Static field call

Keyword: :java-static-field-call.

Description: warn when invoking a static field on a Java object.

Default level: :warning.

Example trigger: (clojure.lang.PersistentQueue/EMPTY)

Example message: Static fields should be referenced without parens unless they are intended as function calls

Syntax

Keyword: :syntax.

Description: warn on invalid syntax.

Default level: :warning.

Example trigger: [)

Example messages:

Mismatched bracket: found an opening [ and a closing ) on line 1
Mismatched bracket: found an opening [ on line 1 and a closing )

Type mismatch

Keyword: :type-mismatch.

Description: warn on type mismatches, e.g. passing a keyword where a number is expected.

Default level: :error.

Example trigger: (inc :foo)

Example message: Expected: number, received: keyword.

Config:

You can add or override type annotations. See types.md.

Unbound destructuring default

Keyword: :unbound-destructuring-default.

Description: warn on binding in :or which does not occur in destructuring.

Default level: :warning.

Example trigger: (let [{:keys [i] :or {i 2 j 3}} {}] i)

Example message: j is not bound in this destructuring form.

Uninitialized var

Keyword: :uninitialized-var

Description: warn on var without initial value

Default level: :warning

Example trigger: (def x)

Example message: Uninitialized var

Unused alias

Keyword: :unused-alias.

Description: warn on unused alias introduced in ns form.

Default level: :off.

Example trigger: (ns foo (:require [foo :as-alias bar]))

Example message: Unused alias: bar.

Unused binding

Keyword: :unused-binding.

Description: warn on unused binding.

Default level: :warning.

Example trigger: (let [x 1] (prn :foo))

Example message: unused binding x.

Config:

To exclude unused bindings from being reported, start their names with underscores: _x or add regex patterns to :exclude-patterns [].

To exclude warnings about key-destructured function arguments, use:

{:linters {:unused-binding {:exclude-destructured-keys-in-fn-args true}}}

This will disable warnings for the following example:

(defn f [{:keys [a b c]} d])

To disable warnings about :as bindings (which can be useful for documentation), use:

{:linters {:unused-binding {:exclude-destructured-as true}}}

This will disable the warning in:

(defn f [{:keys [a b c] :as g}] a b c)

(defn g [[a :as b]] a)

To exclude warnings about defmulti dispatch function arguments, use:

{:linters {:unused-binding {:exclude-defmulti-args true}}}

This will disable the warning in:

(defmulti f (fn [a b] a))

To exclude bindings named "this" use:

{:linters {:unused-binding {:exclude-patterns ["^this"]}}}

Patterns are matched via re-find.

Unused value

Keyword: :unused-value

Description: warn on unused value: constants, unrealized lazy values, pure functions and transient ops (assoc!, conj! etc).

Default level: :warning.

Example triggers:

(do 1 2)
(do (map inc [1 2 3]) 2)
(do (assoc {} :foo :bar) 2)
(do (assoc! (transient {}) :foo :bar) 2)

Example message: Unused value: 1.

Used underscored bindings

Keyword: :used-underscored-binding.

Description: warn when a underscored (ie marked as unused) binding is used.

Default level: :off.

Example trigger: (let [_x 0] _x).

Example message: Using binding marked as unused: _x

These warnings can be enabled by setting the level to :warning or :error in your config.

{:linters {:used-underscored-binding {:level :warning}}}

To suppress the above warning:

{:linters {:used-underscored-binding {:level :warning
                                      :exclude [_x]}}}

A regex is also supported:

{:linters {:used-underscored-binding {:level :warning
                                      :exclude ["^_x.*$"]}}}

This will exclude all bindings starting with _x.

Unknown :require option

Keyword: :unknown-require-option

Description: warn on unknown :require option pairs.

Default level: :warning.

Example trigger: (ns foo (:require [bar :s b])).

Example message: Unknown :require option: :s.

Config: use :exclude [:s] to suppress the above warning.

Unreachable code

Keyword: :unreachable-code.

Description: warn on unreachable code.

Default level: :warning.

Example trigger: (cond :else 1 (odd? 1) 2).

Example message: unreachable code.

Unused import

Keyword: :unused-import.

Description: warn on unused import.

Default level: :warning.

Example trigger: (ns foo (:import [java.util UUID])).

Example message: Unused import UUID.

Unresolved namespace

Keyword: :unresolved-namespace.

Default level: :error.

Example trigger: foo.bar/baz.

Example message: Unresolved namespace foo.bar. Are you missing a require?

Config: use :exclude [foo.bar] to suppress the above warning.

You can report duplicate warnings using:

{:linters {:unresolved-namespace {:report-duplicates true}}}

Unresolved symbol

Keyword: :unresolved-symbol.

Default level: :error.

Example trigger: x.

Example message: Unresolved symbol: x.

Config:

In the following code, match? is a test assert expression brought in by matcher-combinators.test. We don't want it to be reported as an unresolved symbol.

(ns foo
  (:require [clojure.test :refer [deftest is]]
            [matcher-combinators.test]))

(deftest my-test
  (is (match? [1 odd?] [1 3])))

The necessary config:

{:linters
  {:unresolved-symbol
    {:exclude [(clojure.test/is [match?])]}}}

If you want to exclude unresolved symbols from being reported:

for all symbols under calls to clojure.test/is, omit the vector of symbols: :exclude [(clojure.test/is)]
for symbol match? globally for your project, specify only the vector of symbols: :exclude [match?]

Sometimes vars are introduced by executing macros, e.g. when using HugSQL's def-db-fns. You can suppress warnings about these vars by using declare. Example:

(ns hugsql-example
  (:require [hugsql.core :as hugsql]))

(declare select-things)

;; this will define a var #'select-things:
(hugsql/def-db-fns "select_things.sql")

(defn get-my-things [conn params]
  (select-things conn params))

If the amount of symbols introduced by HugSQL becomes too unwieldy, consider introducing a separate namespace in which HugSQL generates the vars: foo.db.hugsql. You can then refer to this namespace from foo.db with (require '[foo.db.hugsql :as sql]) (sql/insert! ...) and clj-kondo will not complain about this.

Furthermore, the :lint-as option can help treating certain macros like built-in ones. This is in clj-kondo's own config:

:lint-as {me.raynes.conch/programs clojure.core/declare
          me.raynes.conch/let-programs clojure.core/let}

and helps preventing false positive unresolved symbols in this code:

(ns foo (:require [me.raynes.conch :refer [programs let-programs]]))

(programs rm mkdir echo mv)
(let-programs [clj-kondo "./clj-kondo"]
  ,,,)

You can report duplicate warnings using:

{:linters {:unresolved-symbol {:report-duplicates true}}}

:exclude-patterns

Since v2023.04.14 you can use :exclude-patterns to suppress symbols by regex patterns (as strings, processed via re-find):

(ns scratch)

(defmacro match
  {:clj-kondo/config
   '{:linters {:unresolved-symbol {:exclude-patterns ["^\\?"]}}}}
  [& _xs])

(match {:foo ?foo} {:foo :bar}
       [?foo x])

In the above example, only x is reported as an unresolved symbol, while symbols starting with a question mark are not reported.

Unresolved var

Keyword: :unresolved-var.

Description: warns on unresolved var from other namespace.

Default level: :warning.

Example trigger: (require '[clojure.set :as set]) (set/onion).

Example message: Unresolved var: set/onion.

Config:

Given this example:

(ns foo)
(defmacro gen-vars [& names]) (gen-vars x y z)

(ns bar (:require foo))
foo/x
(foo/y)

you can exclude warnings for all unresolved vars from namespace foo using:

{:linters {:unresolved-var {:exclude [foo]}}}

or exclude a selection of unresolved vars using qualified symbols:

{:linters {:unresolved-var {:exclude [foo/x]}}}

You can report duplicate warnings using:

{:linters {:unresolved-var {:report-duplicates true}}}

Unsorted imports

Keyword: :unsorted-imports.

Description: warns on non-alphabetically sorted imports in ns and require forms.

Default level: :off.

Example trigger: (ns foo (:import [foo A] [bar B])).

Example message: Unsorted import: [bar B].

Unsorted required namespaces

Keyword: :unsorted-required-namespaces.

Description: warns on non-alphabetically sorted libspecs in ns and require forms.

Default level: :off.

Example trigger: (ns foo (:require b a)).

Example message: Unsorted namespace: a.

Config:

The default sort will consider lower-cased namespace name. To enable keeping the namespace name as is:

{:linters {:unsorted-required-namespaces {:sort :case-sensitive}}}

Possible values for :sort are :case-insensitive (default) and :case-sensitive.

Unused namespace

Keyword: :unused-namespace.

Description: warns on required but unused namespace.

Default level: :warning.

Example trigger: (ns foo (:require [bar :as b])).

Example message: namespace bar is required but never used.

Config:

Given this example:

(ns foo (:require [foo.specs :as specs]))

you will get a warning about foo.specs being unused.

To suppress this, you can either leave out the alias specs if it isn't used anywhere in the namespace or use this config:

{:linters {:unused-namespace {:exclude [foo.specs]}}}

A regex is also supported:

{:linters {:unused-namespace {:exclude [".*\\.specs$"]}}}

This will exclude all namespaces ending with .specs.

Namespaces without :as or :refer are assumed to be loaded for side effects, e.g. for clojure.spec or defining a protocol or multi-method, so the following will not trigger a warning:

(ns foo (:require [foo.specs]))

If you'd like to have namespaces without :as or :refer trigger warnings, you can enable this by setting the :simple-libspec option

{:linters {:unused-namespace {:simple-libspec true}}}

Unused private var

Keyword: :unused-private-var.

Description: warns on unused private vars.

Default level: :warning.

Example trigger: (ns foo) (defn- f [])

Example message: Unused private var foo/f

Config:

To suppress the above warning:

{:linters {:unused-private-var {:exclude [foo/f]}}}

When defining a private var with defonce just for side effects, you can start the name with an underscore:

(defonce ^:private _dude (launch-missiles))

Unused referred var

Keyword: :unused-referred-var.

Description: warns about unused referred vars.

Default level: :warning.

Example trigger: (ns foo (:require [clojure.set :refer [union]])).

Example message: #'clojure.set/union is referred but never used.

Config:

Imagine you want to have taoensso.timbre/debug available in all of your namespaces. Even when you don't use it, you don't want to get a warning about it. That can be done as follows:

{:linters {:unused-referred-var {:exclude {taoensso.timbre [debug]}}}}

Use

Keyword: :use.

Description: warns about :use or use.

Default level: :warning.

Example trigger: (ns foo (:use [clojure.set])).

Example message: use :require with alias or :refer.

Config:

This linter is closely tied to Refer All. Namespaces configured to suppress the :refer-all warning will also suppress the :use warning.

Warn on reflection

Keyword: :warn-on-reflection

Description: warns about not setting *warn-on-reflection* to true in Clojure namespaces. Defaults to only warning when doing interop.

Default level: :off

Example trigger: (.length "hello")

Example message: Var *warn-on-reflection* is not set in this namespace.

Config:

:warn-on-reflection {:level :off
                     :warn-only-on-interop true}

The value of :warn-only-on-interop can be set to false to always warn in Clojure namespaces.

Underscore in namespace

Keyword: :underscore-in-namespace

Description: warns about the usage of the _ character in the declaration of namespaces (as opposed to -).

Default level: :warning

Example trigger: (ns special_files)

Example message: Avoid underscore in namespace name: special_files

Files

linters.md

Latest commit

History

linters.md

File metadata and controls

Linters

Aliased namespace symbol

Aliased namespace var usage

Case

Case duplicate test

Case quoted test

Case symbol test constant

Clj-kondo config

Cond-else

Condition always true

Conflicting-alias

Consistent-alias

Datalog syntax

Deprecated var

Deprecated namespace

Deps.edn

Bb.edn

Bb.edn dependency on undefined task

Bb.edn cyclic task dependency

Bb.edn Unexpected key

Bb.edn task docstring missing

Discouraged var

Discouraged namespace

Discouraged tag

Docstring blank

Docstring no summary

Docstring leading trailing whitespace

Duplicate map key

Duplicate require

Duplicate set key

Duplicate field name

Dynamic vars

Dynamic var not earmuffed

Earmuffed var not dynamic

Quoted case test constant

Equals expected position

Equals false

Equals true

File

Format

Def + fn instead of defn

Destructured or binding of same map

Inline def

Invalid arity

Conflicting arity

Reduce without initial value

Loop without recur

Line length

Keyword in binding vector

Main without gen-class

Minus one

Misplaced docstring

Missing body in when

Missing clause in try

Missing docstring

Missing else branch

Missing map value

Missing test assertion

Namespace name mismatch

Non-arg vec return type hint

Not empty?

Plus one

Private call

Protocol method varargs

Redefined var

Var same name except case

Redundant do

Redundant call

Redundant fn wrapper

Redundant ignore

Redundant nested call

Redundant let

Redundant str call

Refer