Unexpected result for non-tail usage of recur

[jeroenvandijk]

version

Babashka 0.2.6 (or 5aa9031)

platform

Mac OSX

problem

loop / recur allows recur to be called from a non-tail position. In this particular case this gives unexpected result (doesn't recur) with an error message.

repro

(loop []
    (println "hi")
    (recur)
    2)
hi
2

expected behavior

Clojure throws an error:

Syntax error (UnsupportedOperationException) compiling recur at (form-init7929880134577873038.clj:3:5).
Can only recur from tail position

Maybe the clojure behaviour would be most consistent. I'm not sure what the benefits are for not doing this (apart from having to add this check). In this particular case (adding an extra item behind (recur)) caught me by surprise.

[borkdude]

Note for self:

The last expression in fn, loop, let and do forms are always in tail position; the then and else expression of an if form are also tail positions.

[borkdude]

A more efficient way to detect this is using a linter: clj-kondo/clj-kondo#1126
In this way the analyzer doesn't have to do extra work. If you aren't already using it, I recommend setting up editor integration with clj-kondo and then hopefully not too far in the future, you will get a warning about this.

[jeroenvandijk]

I agree that clj-kondo is a nice tool to catch this. Is it already possible to use clj-kondo as a (cljs) library so the linter could be used in other environments? Embedding Sci on a web page for evaluating user code would be an example use case.

[borkdude]

@jeroenvandijk Clj-kondo is currently CLJ only, but portions of it could maybe be ported to CLJS. This is a substantial amount of work though. The way I'm using it currently is as a server side service:

https://clj-kondo.michielborkent.nl/

The front-end just sends the code to the backend, the backend lints it and then returns the warnings.

[borkdude]

To be clear, clj-kondo can be used as a JVM library.

[Cnly]

I also hit this problem lately... While linters would be nice, should sci also do something about this? It should be more than a linter warning IMO.

[borkdude]

I will first address this in clj-kondo. I am also contemplating how I could address this in SCI without affecting performance too much.

[borkdude]

It's in clj-kondo now. I'll have a stab at it in SCI soon.

[borkdude]

In babashka master:

Type:     clojure.lang.ExceptionInfo
Message:  Can only recur from tail position
Location: /tmp/dude.clj:2:3
Phase:    analysis

----- Context ------------------------------------------------------------------
1: (defn foo []
2:   (recur)
     ^--- Can only recur from tail position
3:   1)

[borkdude]

Already released a while ago.