Linter: ! suffix consistency

[vemv]

Context

A defn that is not suffixed in ! but which body contains a defn/macro call which is suffixed with ! is "inconsistent":

(defn foo [] ;; inconsistent: should be named `foo!`
  (swap! bar inc))

This gives the impression that foo is pure, while it is not.

The overall result is that the whole ! convention becomes weaker and less useful.

Task

Create a linter which analyzes all top-level defns/etc, detecting these inconsistencies.

Considerations

• Renaming defns to satisfy the linter could constitute breaking changes; consumers should act carefully
• A naive codebase that complied with this linter could have !s everywhere
  • That'd be a good thing: it would be evident that the codebase could benefit from a more functional architecture.

Additional resources

• https://clojure.org/community/contrib_howto :

Use the bang! only for things not safe in an STM transaction.

• https://www.destroyallsoftware.com/screencasts/catalog/functional-core-imperative-shell

[vemv]

Tried here clj-kondo/clj-kondo#751 proved tricky