Why are errors in nested futures suppressed?

[Colin Yates]

Hi all,

If the function executed in a future throws an error it is printed out in the repl immediately.  If that function is executed in a future which itself is executed in a future then it isn't.

For example, imagine somebody wrote the following code (please, suspend belief and just accept people do do this when learning Clojure :)):

[code]

;; some silly code

user> (swap! atom inc 0)

ClassCastException clojure.core$atom cannot be cast to clojure.lang.Atom  clojure.core/swap! (core.clj:2161)

;; silly code wrapped in a future

user> (future (swap! atom inc 0))

ClassCastException clojure.core$atom cannot be cast to clojure.lang.Atom  clojure.core/swap! (core.clj:2161)

;; silly code wrapped in a future wrapped in a future

user> (future (future (swap! atom inc 0)))

#<core$future_call$reify__6267@11e55d39: :pending>

user> 

[/code]

My understanding is that future executes its delegate in a separate thread, hence the "(future (swa...))" code prints out the exception almost immediately.  I don't understand why the nested future doesn't print out the error though as it should surely be executed almost immediately as well?

Of course, if you dereference the call then it prints out the stack trace.

As to why you would want a future in a future...that is a different kettle of fish :).

[atkaaz]

=> (future (swap! atom inc 0))

#<core$future_call$reify__6267@38db2b7e: :pending>

=> @(future (swap! atom inc 0))

ClassCastException clojure.core$atom cannot be cast to clojure.lang.Atom  clojure.core/swap! (core.clj:2161)

(both in ccw, but i notice that the first statement does throw in lein repl)

guessing the error is actually thrown in that thread, but nobody can see it unless they are trying to deref it in current thread or something else along those lines

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