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clj-kondo

A minimal and opinionated linter for Clojure code that sparks joy.

Rationale

You don't mind the occasional inline def for debugging, but you would like to get rid of them before making your code public. Also, unnecessary do and let nestings don't really add any value to your life. Let clj-kondo help you tidy your code.

Features

clj-kondo detects:

inline def expressions
redundant do and let wrappings
arity errors:
- within the same namespace and across namespaces
- of static Java method calls
- of local let and letfn binding calls
- of recursive calls (including recur)
private function calls
required but unused namespaces
unused function arguments and let bindings
redefined vars
duplicate map keys and set elements
missing map keys
invalid number of forms in binding vectors
missing assertions in clojure.test/deftest

before your REPL knows it.

It suggests style guide recommendations:

use :else as the catch-all test expression in cond (see style guide)

This linter is:

compatible with .clj, .cljs and .cljc files
build tool and editor agnostic
a static code analyzer
compiled to native code using GraalVM

Status

Under active development, but already useful. None of the code is meant to be exposed as a public API, except the command line interface.

For new features I'd like to focus on things that joker doesn't support yet, so I recommend enabling that one as well.

Installation

Running on the JVM

Running with Docker

Usage

Command line

Lint from stdin:

$ echo '(def x (def x 1))' | clj-kondo --lint -
<stdin>:1:8: warning: inline def

Lint a file:

$ echo '(def x (def x 1))' > /tmp/foo.clj
$ clj-kondo --lint /tmp/foo.clj
/tmp/foo.clj:1:8: warning: inline def

Lint a directory:

$ clj-kondo --lint src
src/clj_kondo/test.cljs:7:1: warning: redundant do
src/clj_kondo/calls.clj:291:3: error: Wrong number of args (1) passed to clj-kondo.calls/analyze-calls

Lint a project classpath:

$ clj-kondo --lint "$(lein classpath)"

Project setup

To detect lint errors across namespaces in your project, a cache is needed. To create one, make a .clj-kondo directory in the root of your project. A cache will be created inside of it when you run clj-kondo with the --cache option. Before linting inside your editor, it is recommended to lint the entire classpath to teach clj-kondo about all the libraries you are using, including Clojure and/or ClojureScript itself:

$ clj-kondo --lint "<classpath>" --cache

Build tool specific ways to get a classpath:

lein classpath
boot with-cp -w -f
clojure -Spath

So for lein the entire command would be:

$ clj-kondo --lint "$(lein classpath)" --cache

Now you are ready to lint single files using editor integration. A simulation of what happens when you edit a file in your editor:

$ echo '(select-keys)' | clj-kondo --lang cljs --cache --lint -
<stdin>:1:1: error: Wrong number of args (0) passed to cljs.core/select-keys

Since clj-kondo now knows about your version of ClojureScript via the cache, it detects that the number of arguments you passed to select-keys is invalid. Each time you edit a file, the cache is incrementally updated, so clj-kondo is informed about new functions you just wrote.

0: no errors or warnings were found
2: more than one warning was found
3: more than one error was found

All other error codes indicate an unexpected error.