REPL adventures

Mostly about Clojure and ClojureScript

Clj-kondo Hooks

Clj-kondo is a Clojure linter that uses static analysis. This means it only looks at source code, but does not execute it. While the information available to produce good lint warnings is more limited with static analysis, static analyzing is generally more performant, and works independently from a runtime (JVM, nodeJS, browser, etc.). Static analysis does not suffer from causing unwanted side effects when executing code. It often yields good enough results. Where static analysis falls short, clj-kondo offers configuration options where the user can help clj-kondo understand more of their code.

One area where static analysis of Clojure code becomes hard is macros. Macros can introduce new syntactical constructs. Often macros are syntactically similar to existing Clojure core macros. This is where you can use clj-kondo’s :lint-as configuration. In places where this isn’t possible, for example because the macro had irregular binding patterns, one could use :unresolved-symbol + :exclude which would simply ignore unresolved symbol errors in an entire s-expression.

I’ve been asking myself the following question for a while now: can clj-kondo make more sense of custom macros with a little help from the user? Clj-kondo could invent some DSL to express a transformation, but DSLs often cover just 80% of what you want to achieve. To get 20% more power, you’d have to turn the DSL into something like Clojure itself. So why not just use Clojure directly?

Clj-kondo is distributed in a couple of different ways. A widely used distribution is the binary compiled with GraalVM. One limitation of a GraalVM-compiled binary is that one cannot introduce new classes at runtime. And this is what clojure.core/eval does, so that’s off the table. Since August 2019 I’ve been working on the Small Clojure Interpreter. It’s not a compiler, like Clojure, but it allows you to interpret Clojure expressions within a GraalVM binary. The interpreter is used in babashka but it has other uses as well and also works in JavaScript.

This interpreter can be used in clj-kondo to execute hooks that users can provide to transform custom macro calls into constructs that clj-kondo can understand. And this is what I’ve worked on.

Clj-kondo uses a vendored version of rewrite-clj to analyze source code. My first attempt at the hooks API was to transform the rewrite-clj nodes into Clojure s-expressions. Then the user’s hook function would transform these s-expressions in a similar fashion as the macro would, returning new s-expressions. Lastly clj-kondo would then translate these s-expressions back into rewrite-clj nodes and continue analysis. Ostensibly this worked great for several test cases, but ultimately it wasn’t good enough. The main problem is that numbers, strings and keywords cannot carry metadata. Metadata on sexprs was used to keep track of the original locations. When (some of) these locations are lost, clj-kondo cannot accurately position lint warnings anymore. And this is unacceptable in my opinion. You can read more about this problem on ClojureVerse here and in the issue on Github here.

After more experimentation I decided that the transformation should happen direcly on rewrite-clj nodes in order to preserve location information. This led to the current implementation of the :analyze-call hook, documented here. Additionally, some library specific example config + hook code is provided here, showing how to make clj-kondo understand Rum’s defc macro and slingshot’s try+ macro.

I consider this new feature a powerful feature but not an easy to use one. It does provide a higher degree of linting quality while still enjoying the benefits of static analysis. Luckily we only have to figure out the right code for each library once. I urge library authors and users to contribute their configurations to the clj-kondo repository so we can all benefit.

Clojurist Together has sponsored this work as part of their Summer of Bugs program. Thanks to the people who have made this possible: the Clojurists Together staff and of course the people who donate.

Hope you enjoy. Happy linting!

Michiel Borkent (a.k.a. @borkdude)