Clash Royale CLAN TAG#URR8PPP

Assertions in Clojure applications enabled by default when running uberjar built with leiningen

Should assertions be enabled by default when running a Clojure application as a jar?

We use leiningen to build an uberjar and then java -jar ... to run it and I've just found out that *assert* is true.

java -jar ...

*assert*

I couldn't find what default value should this dynamic variable hold but I thought it would be false (and only set to true in REPL development environment).

This lead to a nasty issue in combination with future where a precondition assertion error caused the future thread to die and we didn't know what happened (because we caught only Exception not Throwable).

future

Exception

Throwable

I went through the source code of clojure.lang.RT but I couldn't figure out where is the *assert* var set to true.

clojure.lang.RT

*assert*

I've also found the option :global-vars in the leinigen sample project (this is mentioned in How to disable Clojure assertions, including preconditions?).
That looks reasonable but I thought that for "production" build I would get *assert* false automatically.

:global-vars

*assert* false

EDIT: I missed that little T thing (boolean true) used to initialize *assert* variable in RT.java. However, still curious why that's the default and what is the recommended way to turn off assertions (if it's even recommended).

T

*assert*

RT.java

T

*assert*

RT.java

1 Answer
1

From the source, clojure.core/assert is a macro that evaluates *assert* at compile time:

clojure.core/assert

*assert*

(defmacro assert
"Evaluates expr and throws an exception if it does not evaluate to
logical true."
:added "1.0"
([x]
(when *assert*
`(when-not ~x
(throw (new AssertionError (str "Assert failed: " (pr-str '~x)))))))
([x message]
(when *assert*
`(when-not ~x
(throw (new AssertionError (str "Assert failed: " ~message "n" (pr-str '~x))))))))


This means that both alter-var-root and binding cannot change the behavior of assert, since they operate at runtime, after assert has already evaluated *assert*. Therefore, the :global-vars option in project.clj:

alter-var-root

binding

assert

assert

*assert*

:global-vars

project.clj

:global-vars *warn-on-reflection* false
*assert* false


is the only solution that will work, since it is evaluated before macro expansion. With this test code:

(dotest
(spyxx *assert*)
(assert false "Don't do that!")
(println "past the assertion"))


we get result:

----------------------------------
Clojure 1.9.0 Java 10.0.1
----------------------------------

lein test tst.demo.core
*assert* => <#java.lang.Boolean false>
past the assertion

Ran 2 tests containing 0 assertions.
0 failures, 0 errors.
lein test 32.28s user 0.56s system 349% cpu 9.387 total


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