Warn on cond & condp without default case

[neotyk]

Is your feature request related to a problem? Please describe.
It's perfectly valid to have cond or condp without default case, though exception is thrown when no matching case is found.

Describe the solution you'd like
Warn on cond & condp used without default case.

Assuming this is really wanted I have some OSS time this Friday, just sayin'

[borkdude]

Cond doesn't throw an exception without a default case:

user=> (cond (odd? 1) :true)
:true
user=> (cond (odd? 2) :true)

For case we don't lint this as a warning since case without a default case is meant to throw
I did consider this once, but this was met with feedback from e.g. Alex Miller (@puredanger) that case (and I think the same goes for condp) is meant to cover a limited set of cases and should throw when a case is unhandled.

https://twitter.com/puredanger/status/1116696133315760128

So as it stands, I don't think any changes are needed here.

[neotyk]

Maybe I was too trigger happy about cond, but condp is throwing:

(condp = 0 1 :something)
;=> Execution error (IllegalArgumentException) at dev/eval60650 (form-init4047019337742748236.clj:1).
No matching clause: 0

[neotyk]

Yeah ok, throwing by design. I'd still like the warning for it.

[borkdude]

@neotyk Currently you can write a custom hook to check for condp without defaults:

$ clj-kondo --lint src
src/program.clj:3:1: warning: condp without default
linting took 14ms, errors: 0, warnings: 1

$ cat src/program.clj
(ns program)

(condp = 1
  2 :foo)

$ cat .clj-kondo/config.edn
{:hooks {:analyze-call {clojure.core/condp hooks/condp-hook}}
 :linters {:condp-without-default {:level :warning}}}
 
$ cat .clj-kondo/hooks.clj
(ns hooks
  (:require [clj-kondo.hooks-api :as api]))

(defn condp-hook [{:keys [:node]}]
  (let [children (rest (:children node))]
    (when (even? (count children))
      (api/reg-finding! (assoc (meta node)
                               :message "condp without default"
                               :type :condp-without-default)))))

[serioga]

For me the cond without default case is fine and handy.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.