Mathematica Stack Exchange is a question and answer site for users of Mathematica. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I ran across a scoping puzzle while experimenting with ScheduledTasks, and I'd be grateful for an explanation from the sages here. I found a workaround by intuitive horse sense, but was unable to explain to myself adequately what was wrong with my original attempt.

First, a straightforward, tail-recursive, synchronous Module

Module[{state = 0, doNextIteration},
 doNextIteration = Function[
   state = state + 1;
   If[state < 4,
    (Print[state]; doNextIteration[])]];
(* 1 2 3 *)

Next, my first attempt to do the same with asynchronous tasks, which I need to tail-chain as one-shot ScheduledTasks. If CreateScheduledTask has a second List argument, then it creates a one-shot task delayed by the time in seconds in the list, just what I need for my bigger application. In this simplified sample, the ScheduledTask tail-chains a new one-shot ScheduledTask by a tail-recursive call -- just like the successful synchronous code -- once every tenth of a second until the condition on state produces False (ignore cleanup of the task objects for simplicity, here).

Module[{state = 0, runNextTask},
 runNextTask = Function[
     state = state + 1;
     If[state < 4,
      (Print[state]; runNextTask[])],

OK, the problem is this only Prints once, not the desired three times. I had a sickening hunch that the problem has something to do with runNextTask being local to the Module and somehow not being able to refer to itself recursively -- even though there is only one Module -- in the bizzaro-land of asynchrony, so I "fixed" the code as follows:

Module[{state = 0, runNextTask = Unique[]},
 runNextTask[] :=
  If[state < 3,
      state = state + 1;

I made runNextTask refer to a Unique global symbol, and made my function into a rewrite rule attached to runNextTask[]. This works great, but I don't understand well why it does. I hate being as dumb as a horse, even if I can jump the fences.

Clues, advice, explanations: all appreciated.

share|improve this question
up vote 9 down vote accepted

You hit a rather subtle behavior, related to the garbage-collection and the Temporary attribute, and the semantics of Module regarding returning expressions. The thing is, to achieve your goal, you need the Module-generated variable (function)'s definition(s) to be exported outside Module (to be persistent). But, since you return not the symbol itself, but its r.h.s. (which is a chunk of code with delayed evaluation, either Function or ScheduledTaskObject in this case), the definitions of runNextTask are destroyed because it has a Temporary attribute, despite the fact that code containing this variable has been exported outside Module.

To illustrate my point, here are two work-arounds which both will lead to your desired effect:

  1. Instead of assigning runNextTask to Unique[], insert a line ClearAll[runNextTask] right after the Module declarations. This will remove the Temporary attribute and the definition of runNextTask will persist.

  2. Wrap your final call in Hold (so, return Hold[runNextTask[]]), and then use ReleaseHold outside Module.

This is your code for this option:

res = 
 Module[{state = 0, runNextTask},
   runNextTask[] :=
         state = state + 1;
         If[state < 4,
           (Print[state]; runNextTask[])


In this case, the definition also persists.

Note that this problem is not seen in examples where all evaluation happens at the time when Module is left: the following code will execute promptly

Module[{f, n = 0},
  f[] := (n++; If[n < 10, Print["*"]; f[]]);

even though all definitions for f are destroyed at the end - because the execution here is immediate (the same evaluation process, so that f still has all definitions during this evaluation), while delayed and asynchronous code induces a separate evauation process.

share|improve this answer
Thanks for this clue! I have a follow-on that seems to be related coming in a new question. – Reb.Cabin Apr 1 '12 at 22:10
@Reb.Cabin Glad I could help. I actually did not previously encounter this exact behavior, so I learned something too. Thanks for the accept. – Leonid Shifrin Apr 1 '12 at 22:17
good explanation, thanks for that. I just wanted to mention that, if the function is just called for the side effect like here, one could just do this to make the definition persist: Module[{state = 0, runNextTask}, runNextTask = Function[StartScheduledTask@CreateScheduledTask[state = state + 1; If[state < 4, (Print[state]; runNextTask[])], {0.10}]]; runNextTask[]; Hold@runNextTask] – Albert Retey Apr 2 '12 at 9:32
@Albert Indeed, a good option! I did not think about it, thanks. – Leonid Shifrin Apr 2 '12 at 11:47
Well, actually shortly after my comment I decided that I would actually recommend to rather use Unique than Module to create the symbol: Not only because no additional "tricks" are needed to make things work. It also seems easier to guess what the code actually is supposed to do for a reader. As far as I can see the OP is using the version I'd recommend anyway... – Albert Retey Apr 2 '12 at 14:12

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.