Hello,

On the Clojure REPL I notice this:

user=> (+ 1 2)
3
user=> (+ 1 "er")
ClassCastException java.lang.String cannot be cast to
java.lang.Number  clojure.lang.Numbers.add (Numbers.java:126)

user=> (+ "we" "er")
ClassCastException java.lang.String cannot be cast to
java.lang.Number  clojure.lang.Numbers.add (Numbers.java:126)

user=> (1 "we" 1)
ClassCastException java.lang.Long cannot be cast to clojure.lang.IFn
user/eval952 (NO_SOURCE_FILE:1)


However, on the ClojureScript REPL I notice the following:


ClojureScript:cljs.user> (+ 1 2)
3
ClojureScript:cljs.user> (+ 1 "er")
"1er"
ClojureScript:cljs.user> (+ "we" "er")
"weer"
ClojureScript:cljs.user> (- "we" 1)
NaN


This difference in behavior can be explained by platform-specific
semantics, but I want to know whether this is by design.

Shantanu

No one has offered a way to prevent that without slowing things down.

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To expound:

(+ "we" "er") is a user error.

We rely on the runtime type system of the host to efficiently detect
type errors. Unfortunately, the JS runtime does not consider that an
error, thus it is not generating an exception.

That is much different from there being semantics for (+ "we" "er") -
there aren't. Nor is it guaranteed to continue to produce "weer".

Nowhere does Clojure promise that all user errors will become
exceptions, and it is a constant balancing act to add more checks
without compromising the performance of correct programs.

If and when we have a debug mode of runtime these kind of checks would
be possible there.

Rich
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Thanks so much for the insight, Rich.

Shantanu