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[CLJ-1053] Locals still cleared too aggressively on delay in specific cases Created: 30/Aug/12  Updated: 03/Sep/13  Resolved: 03/Sep/13

Status: Closed
Project: Clojure
Component/s: None
Affects Version/s: Release 1.4
Fix Version/s: Release 1.6

Type: Defect Priority: Minor
Reporter: Jean Niklas L'orange Assignee: Unassigned
Resolution: Completed Votes: 0
Labels: None


This seems to be an extension of #CLJ-232. Locals are still cleared too aggressively in some specific instances: If the delay throws an exception when realized, and you dereference a local within the delay, the local may or may not be cleared depending on its need in the tail positions (without any obvious pattern). Examples of functions which works as intended and doesn't work as intended follows.

(def d (let [y (delay 0)]
         (delay (if (zero? @y) (/ 0 0) (/ @y 1))))) ; works properly

(def d (let [y (delay 0)]
         (delay (if (zero? @y) (/ 0 0) (/ 1 1))))) ; does not work properly

(def d (let [y (delay 0)]
         (delay (if (zero? @y) (/ @y 0) (/ 1 1))))) ; does not work properly

(def d (let [y (delay 0)]
         (delay (if (zero? @y) (/ @y 0) (/ @y 1))))) ; does not work properly

(def d (let [y (delay 0)]
         (delay @y (/ 0 0)))) ; does not work properly

(def d (let [y (delay 0)]
         (delay @y (/ 0 0) @y))) ; works properly

By "works", d will throw "ArithmeticException Divide by zero" every single time it is dereferenced. By "does not work", d will throw "ArithmeticException Divide by zero" on first dereferencing, but from second and onwards it will throw "NullPointerException [trace missing]".

Comment by Jean Niklas L'orange [ 09/Apr/13 11:55 AM ]

With Clojure 1.5.1, the trace is given and returns NullPointerException clojure.core/deref-future (core.clj:2NullPointerException clojure.core/deref-future (core.clj:2108)108), which refers to a .get call. The stacktrace reveals the following:

	clojure.core/deref-future (core.clj:2108)
	clojure.core/deref (core.clj:2129)
	user/fn--1/fn--4 (NO_SOURCE_FILE:2)
	clojure.lang.Delay.deref (Delay.java:33)
	clojure.core/deref (core.clj:2128)
	user/eval13 (NO_SOURCE_FILE:-1)
	clojure.lang.Compiler.eval (Compiler.java:6619)
	clojure.lang.Compiler.eval (Compiler.java:6582)
	clojure.core/eval (core.clj:2852)
	clojure.main/repl/read-eval-print--6588/fn--6591 (main.clj:259)
	clojure.main/repl/read-eval-print--6588 (main.clj:259)
	clojure.main/repl/fn--6597 (main.clj:277)
Comment by Alex Miller [ 03/Sep/13 12:17 PM ]

This was fixed by CLJ-1175 which is now in 1.6.0-master. All of the given examples now work there.

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