hooks.md

Hooks

Hooks are a way to enhance linting via user provided code.

API

Hooks are interpreted using the Small Clojure Interpreter.

Hooks receive Clojure code as rewrite-clj nodes, not only for performance reasons, but also because rewrite-clj nodes carry the line and row numbers for every Clojure element. Note that when we refer to a "rewrite-clj node", we are referring to clj-kondo's version of rewrite-clj node. Clj-kondo's version of rewrite-clj is catered to its use case, includes some bug fixes, but most notably: strips away all whitespace.

A hook can leverage the clj-kondo.hooks-api namespace for transformation and analysis of rewrite-clj nodes.

API functions for producing nodes:

• list-node: produce a new list node from a seqable nodes.
• vector-node: produce a new vector node from a seqable of nodes.

• keyword-node: produce a new keyword. Use (api/keyword-node :foo) for a normal keyword and (api/keyword-node :foo true) to produce a node for ::foo.
• string-node: produce a new node for a single string or multiple strings (passed as seq)
• token-node: produce a new token node. Used for all remaining tokens (mainly used for symbols or nil).

Each producing function has a predicate counterpart for checking a type of node:

• list-node?: returns true if given node is a list node
• etc.

Other API functions:

• sexpr: turns a node into a Clojure s-expression. Useful for analyzing concrete values.
• reg-finding!: registers a finding. Expects a map with:
  • :message: the lint message
  • :row and :col: the location of the finding. These values can be derived from the metadata of a node.
  • :type: the type of lint warning. A level must be set for this type in the clj-kondo config under :linters. If the level is not set, the lint warning is ignored.

The namespaces clojure.core, clojure.set and clojure.string are also available. Use println or prn for debugging and time to measure performance.

Hooks must be configured in clj-kondo's config.edn under :hooks, e.g.:

{:hooks {:analyze-call {foo.weird-macro hooks.foo/weird-macro}}}

analyze-call

The analyze-call hook offers a way to lint macros that are unrecognized by clj-kondo and cannot be supported by :lint-as.

It receives Clojure macro (or function) call code as input in the form of a rewrite-clj node, and can:

• Transform the code to teach clj-kondo about its effect.
• Inspect call arguments and emit findings about them.

Transformation

As an example, let's take this macro:

(ns mylib)
(defmacro with-bound [binding-vector & body] ,,,)

Users can call this macro like so:

(require '[my-lib])
(my-lib/with-bound [a 1 {:with-bound/setting true}] (inc a))

Clj-kondo does not recognize this syntax and will report the symbol a as unresolved. If the macro didn't expect an option map in the third position of the binding vector, we could have used :lint-as {my-lib.with-bound clojure.core/let}, but unfortunately that doesn't work for this macro. We will now write a hook that transforms the call into:

(let [a 1] {:with-bound/setting true} (inc a))

It is not important that the code is rewritten exactly according to the macroexpansion. What counts is that the transformation rewrites into code that clj-kondo can understand.

This is the code for the hook:

(ns hooks.with-bound
  (:require [clj-kondo.hooks-api :as api]))

(defn with-bound [{:keys [:node]}]
  (let [[binding-vec & body] (rest (:children node))
        [sym val opts] (:children binding-vec)]
    (when-not (and sym val)
      (throw (ex-info "No sym and val provided" {})))
    (let [new-node (api/list-node
                    (list*
                     (api/token-node 'let)
                     (api/vector-node [sym val])
                     opts
                     body))]
      {:node new-node})))

This code will be placed in a file hooks/with_bound.clj in your .clj-kondo directory.

To register the hook, use this configuration:

{:hooks {:analyze-call {my-lib/with-bound hooks.with-bound/with-bound}}}

The symbol hooks.with-bound/with-bound corresponds to the file .clj-kondo/hooks/with_bound.clj and the with-bound function defined in it. Note that the file has to declare a namespace corresponding to its directory structure and file name, just like in normal Clojure.

An analyze-call hook function receives a :node in its argument map. This is a rewrite-clj node representing the hooked Clojure macro (or function) call code clj-kondo has found in the source code it is linting. The hook uses the clj-kondo.hooks-api namespace to validate then rewrite this node into a new rewrite-clj node:

1. The with-bound hook function checks if the call has at least a sym and val node. If not, it will throw an exception, which will result in a clj-kondo warning.

2. As a last step, the hook function constructs a new node using api/list-node, api/token-node and api/vector-node. This new node is returned in a map under the :node key.

Now clj-kondo fully understands the my-lib/with-bound macro and you will no longer get false positives when using it. Moreover, it will report unused bindings and will give warnings customized to this macro.

Custom lint warnings

Analyze-call hooks can also be used to create custom lint warnings, without transforming the original rewrite-clj node.

This is an example for re-frame's dispatch function which checks if the dispatched event used a qualified keyword.

(ns hooks.re-frame
  (:require [clj-kondo.hooks-api :as api]))

(defn dispatch [{:keys [:node]}]
  (let [sexpr (api/sexpr node)
        event (second sexpr)
        kw (first event)]
    (when (and (vector? event)
               (keyword? kw)
               (not (qualified-keyword? kw)))
      (let [{:keys [:row :col]} (some-> node :children second :children first meta)]
        (api/reg-finding! {:message "keyword should be fully qualified!"
                           :type :re-frame/keyword
                           :row row
                           :col col})))))

The hook uses the api/sexpr function to convert the rewrite-clj node into a Clojure s-expression, which is easier to analyze. In case of an unqualified keyword we register a finding with api/reg-finding! which has a :message, and :type. The :type should also occur in the clj-kondo configuration with a level set to :info, :warning or :error in order to appear in the output:

{:linters {:re-frame/keyword {:level :warning}}
 :hooks {:analyze-call {re-frame.core/dispatch hooks.re-frame/dispatch}}}

Additionally, the finding has :row and :col, derived from the node's metadata to show the finding at the appropriate location.

Clojure code as rewrite-clj nodes

If you develop a hook you will likely need some familiarity with rewrite-clj node structure. A couple of examples might help:

(my-macro 1 2 3) becomes:

• a list node with :children:
  • token node my-macro
  • token node 1
  • token node 2
  • token node 3

(my-lib/with-bound [a 1 {:with-bound/setting true}] (inc a)) becomes:

• a list node with :children
  • token node my-lib/with-bound
  • vector node with :children
    • token-node a
    • token-node 1
    • map node with :children
      • keyword node :with-bound/setting
      • token node true
  • list node
    • token node inc
    • token node a

Clj-kondo uses a different approach to metadata than the original rewrite-clj library. Metadata nodes are stored in the :meta key on nodes correponding to the values carrying the metadata:

^:foo ^:bar [] becomes:

• a vector node with :meta
  • a seq of nodes with:
    • keyword node :foo
    • keyword node :bar

Example Hooks

• rewrite-cljc-playground: import-vars-with-mod

More examples of hooks can be found in the config project.

Tips and tricks

Here are some tips and tricks for developing hooks.

Debugging

For debugging the output of a hook function, you can use println or prn. To get a sense of what a newly generated node looks like, you can use (prn (api/sexpr node)).

Performance

Less code to process will result in faster linting. If only one hook is used in certain files and another hook is used in other files, divide them up into multiple files and namespaces. If the hooks use common code, you can put that in a library namespace and use require to load it from each hook's namespace.

To test performance of a hook, you can write code which triggers the hook and repeat that expression n times (where n is a large number like 1000000). Then lint the file with clj-kondo --lint and measure timing. The time macro is also available within hooks code.

Clojurists Together

The initial work on hooks was sponsored by Clojurists Together as part of their Summer of Bugs program.