958905 - Task.Result API has unintuitive behavior

Status: UNCONFIRMED

(Toolkit :: General, defect)

Description

[Jeremy Maitin-Shepard]

User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:26.0) Gecko/20100101 Firefox/26.0 conkeror/1.0pre (Beta/Release)
Build ID: git

Steps to reproduce:

Consider the code below:

Components.utils.import("resource://gre/modules/Task.jsm");
Task.spawn(function() {
  try {
    // do some stuff that may throw
    throw new Task.Result(1);
  } catch(e) {
      dump("Got exception: " + e + "\n");
      throw e;
  }
}).then(function(x) { dump("Result: " + x + "\n"); });



Actual results:

Got exception: [object Object]
Result: 1

Obviously this behavior is inherent to how exceptions work, but it is inconvenient that the only API for returning a value from a Task differs significantly in semantics from the normal return statement.


Expected results:

Result: 1

A better API would be to use the syntax:

yield Task.Result(x);

Supporting this syntax would be nearly backwards-compatible with existing code, in that only the edge case of using yield my_function(), where my_function() is a non-generation function that happens to return an instance of Task.Result, would be broken by supporting the new syntax (and it seems this would constitute a misuse of Task.Result in that case).  The existing throw-based syntax could also still be supported at the same time, but deprecated.

Note: The Conkeror web browser (based on Mozilla) has a coroutine implementation that is very similar in function to Task.jsm.  I faced this same issue in writing it, having initially chosen a throw-based API for returning values, and then switching to a yield-based one after realizing throw would have the wrong behavior with respect to catch clauses.