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Re: [jslint] Circular Function Definitions

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  • Erik Eckhardt
    My apologies. My code was written too hastily. I thought you were using a Boolean to indicate whether to make the return call to f. The point I was trying to
    Message 1 of 17 , Jan 3, 2011
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      My apologies. My code was written too hastily. I thought you were using a
      Boolean to indicate whether to make the return call to f.

      The point I was trying to get across is that you don't need the circularity.

      There are many ways to solve this. Here's one: Any time you want something
      to happen after the callback, pass the call to the callback in an anonymous
      function:

      f = function() {
      ___// do stuff
      ___xhr(params, callback); // plain call to cb func
      ___xhr(params, function() {callback(); f();}); // circular call back to f
      }

      Or pass the after-callback function as a parameter to the callback function
      (again through use of an anonymous function).

      or: `callback.returnfn = f; // now check property in that function and call
      if needed

      Or use a library like jquery and define a custom event, then bind all the
      handlers to the event you want, and trigger it any time.

      Circularity isn't required.

      Erik

      On Fri, Dec 31, 2010 at 12:12 PM, Felix E. Klee <felix.klee@...> wrote:

      >
      >
      > On Fri, Dec 31, 2010 at 7:21 PM, Erik Eckhardt <erik@...<erik%40eckhardts.com>>
      > wrote:
      > > function callback(morefn) {
      > > // do something ...
      > > if (typeof(morefn) === 'function') {
      > > moref();
      > > }
      > > }
      > >
      > > f = function () {
      > > sendToServer('xyz', callback ? f : null);
      > > };
      >
      > Are you sure this makes sense? The second last line will always evaluate
      > to:
      >
      > sendToServer('xyz', f);
      >
      > And I don't see how this solves the issue I raised:
      >
      > var f;
      >
      > function callback(moreToDo) {
      > // ...
      > }
      >
      > f = function () {
      > // ...
      > };
      >
      > That's inconsistent, and thus confusing. Of course one could write:
      >
      > var callback, f;
      >
      > callback = function (moreToDo) {
      > // ...
      > };
      >
      > f = function () {
      > // ...
      > };
      >
      > Now things are consistent. But is that the way to go?
      >
      >


      [Non-text portions of this message have been removed]
    • Felix E. Klee
      ... No, that was correct. In the example code, I *was* using a boolean. Only in the real code, things are more complex: Based on what the XHR returns some
      Message 2 of 17 , Jan 4, 2011
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        On Tue, Jan 4, 2011 at 7:53 AM, Erik Eckhardt <erik@...> wrote:
        > I thought you were using a Boolean to indicate whether to make the
        > return call to f.

        No, that was correct. In the example code, I *was* using a boolean. Only
        in the real code, things are more complex:

        Based on what the XHR returns some calculations are performed, and - if
        needed - another XHR is made.

        > The point I was trying to get across is that you don't need the
        > circularity.

        What's bad about the circularity?

        > xhr(params, function() {callback(); f();}); // circular call back to f

        That would again be an infinite loop. The body of the anonymous function
        would need to be at least slightly more complex. I don't think this is
        increases readability.
      • Erik Eckhardt
        If your example isn t the full story, please post an example of the full story! One thing that s wrong with the circularity is the cross-dependence as JSLint
        Message 3 of 17 , Jan 4, 2011
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          If your example isn't the full story, please post an example of the full
          story!

          One thing that's wrong with the circularity is the cross-dependence as
          JSLint is showing you. It's not a huge deal, but an annoying one, perhaps.

          In any case I thought you might find it useful to use some of the techniques
          I mentioned, where if a function determines that an additional step is
          needed, it manages it within the function rather than making the callback
          function have a hard coded reference back. The scheme you're using now seems
          to make sense but as your application grows you may find that it becomes
          unworkable. For example, what if you eventually have two functions that may
          or may not need to be called after an xhr? Are you going to pass two
          booleans? Or God forbid, a code indicating which functions to run? If from
          the start you simply passed around single callback functions (which
          themselves could be a string of functions) this problem would be solved.

          function myxhr(url, params, callback) {
          ___//do stuff;
          doXhr(function() {myxhrreturn(xmlhttp, callback):});
          }

          function myxhrreturn(xmlhttp, callback) {
          ___//process xmlhttp object into a result
          ___callback(result);
          }

          function getsomething() {
          ___myxhr('http://example.com', 'a=1', function(result) {
          ______if (result.blah === 'gorp') {
          _________dosomething;
          ______} else {
          _________dosomethingelse();
          ______}
          ___}
          }

          Now your getsomething function controls everything and all your xmlhttp
          requests can be handled the same way without having to make a hardcoded
          call. If you want the logic in the myxhrreturn function, you can put it
          there, but it's not required. Here's another way to do it:

          function getsomething() {
          var cb;
          if (somecondition) {
          ______cb = function(result) {dosomething(result)};
          ___} else {
          ______cb = function(result) {dosomethingelse(result);};
          ___}
          ___myxhr('http://example.com', 'a=1', cb}
          }

          See how versatile this is? You only need a single callback, no extra Boolean
          parameter is needed to indicate whether to call a hard-coded function name.
          Any time you want more stuff to happen, you can load it into the passed
          callback function somehow.

          Erik

          On Tue, Jan 4, 2011 at 1:47 AM, Felix E. Klee <felix.klee@...> wrote:

          >
          >
          > On Tue, Jan 4, 2011 at 7:53 AM, Erik Eckhardt <erik@...<erik%40eckhardts.com>>
          > wrote:
          > > I thought you were using a Boolean to indicate whether to make the
          > > return call to f.
          >
          > No, that was correct. In the example code, I *was* using a boolean. Only
          > in the real code, things are more complex:
          >
          > Based on what the XHR returns some calculations are performed, and - if
          > needed - another XHR is made.
          >
          >
          > > The point I was trying to get across is that you don't need the
          > > circularity.
          >
          > What's bad about the circularity?
          >
          >
          > > xhr(params, function() {callback(); f();}); // circular call back to f
          >
          > That would again be an infinite loop. The body of the anonymous function
          > would need to be at least slightly more complex. I don't think this is
          > increases readability.
          >
          >


          [Non-text portions of this message have been removed]
        • Felix E. Klee
          On Tue, Jan 4, 2011 at 7:35 PM, Erik Eckhardt ... There may be a misunderstanding. Thus, for your pleasure, below a more real-life
          Message 4 of 17 , Jan 5, 2011
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            On Tue, Jan 4, 2011 at 7:35 PM, Erik Eckhardt <erik@...>
            wrote:
            > no extra Boolean parameter is needed to indicate whether to call a
            > hard-coded function name.

            There may be a misunderstanding.

            Thus, for your pleasure, below a more real-life example. Asides from
            being more verbose, the only difference to my original example is the
            addition of the parameter "liveCommentary". Together with the flag
            "matchIsStillRunning" (formerly: "moreToDo") it forms the data returned
            from the server.

            var sendToServer; // defined elsewhere

            function onCommentaryReceived(commentary, matchIsStillRunning) {
            // write live commentary: ...
            if (matchIsStillRunning) {
            requestCommentary();
            }
            }

            function requestCommentary() {
            sendToServer('Berlin:Munich', onCommentaryReceived);
            }

            Naturally, one could rewrite this:

            var sendToServer; // defined elsewhere

            function requestCommentary() {
            sendToServer('Berlin:Munich',
            function (commentary, matchIsStillRunning) {
            // write live commentary: ...
            if (matchIsStillRunning) {
            requestCommentary();
            }
            });
            }

            The second form avoids the circular function definition, but is it more
            readable?
          • Jordan
            I think the confusion here is that you shouldn t be using the circular function pattern you are using. In addition, you are using function declarations
            Message 5 of 17 , Jan 5, 2011
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              I think the confusion here is that you shouldn't be using the "circular function" pattern you are using. In addition, you are using function declarations rather than assigning a function statement to a variable.

              What you want to do, it seems, is immediately run requestCommentary after a matchIsStillRunning response comes back from the server.

              What I can think of off the top of my head is:

              var onCommentaryReceived = function (commentary, matchIsStillRunning) {
              // write live commentary: ...
              if (matchIsStillRunning) {
              sendToServer('Berlin:Munich', onCommentaryReceived);
              }
              },
              requestCommentary = function () {
              return onCommentaryReceived(null, true);
              };

              Alternatively, and with less refactoring:
              var onCommentaryReceived, requestCommentary;
              onCommentaryReceived = function (commentary, matchIsStillRunning) {
              // write live commentary: ...
              if (matchIsStillRunning) {
              requestCommentary();
              }
              };
              requestCommentary = function () {
              sendToServer('Berlin:Munich', onCommentaryReceived);
              };

              --- In jslint_com@yahoogroups.com, "Felix E. Klee" <felix.klee@...> wrote:
              >
              > On Tue, Jan 4, 2011 at 7:35 PM, Erik Eckhardt <erik@...>
              > wrote:
              > > no extra Boolean parameter is needed to indicate whether to call a
              > > hard-coded function name.
              >
              > There may be a misunderstanding.
              >
              > Thus, for your pleasure, below a more real-life example. Asides from
              > being more verbose, the only difference to my original example is the
              > addition of the parameter "liveCommentary". Together with the flag
              > "matchIsStillRunning" (formerly: "moreToDo") it forms the data returned
              > from the server.
              >
              > var sendToServer; // defined elsewhere
              >
              > function onCommentaryReceived(commentary, matchIsStillRunning) {
              > // write live commentary: ...
              > if (matchIsStillRunning) {
              > requestCommentary();
              > }
              > }
              >
              > function requestCommentary() {
              > sendToServer('Berlin:Munich', onCommentaryReceived);
              > }
              >
              > Naturally, one could rewrite this:
              >
              > var sendToServer; // defined elsewhere
              >
              > function requestCommentary() {
              > sendToServer('Berlin:Munich',
              > function (commentary, matchIsStillRunning) {
              > // write live commentary: ...
              > if (matchIsStillRunning) {
              > requestCommentary();
              > }
              > });
              > }
              >
              > The second form avoids the circular function definition, but is it more
              > readable?
              >
            • Felix E. Klee
              ... The question is: Why? Is that considered bad practice? If so, a reference please. ... See my original post. There I mentioned that I would like to enforce
              Message 6 of 17 , Jan 6, 2011
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                On Thu, Jan 6, 2011 at 6:27 AM, Jordan <ljharb@...> wrote:
                > I think the confusion here is that you shouldn't be using the
                > "circular function" pattern you are using.

                The question is: Why? Is that considered bad practice? If so, a
                reference please.

                > In addition, you are using function declarations rather than assigning
                > a function statement to a variable.

                See my original post. There I mentioned that I would like to enforce
                that convention for the whole project using JSLint. Only I wonder why
                there is no such option.

                > What I can think of off the top of my head is:
                >
                > [...]
                > requestCommentary = function () {
                > return onCommentaryReceived(null, true);
                > };

                That would work, but it's confusing to read. At least the function name
                "onCommentaryReceived" should be changed.

                > Alternatively, and with less refactoring:

                See my original post. It's there already.
              • Felix E. Klee
                On Fri, Dec 31, 2010 at 6:33 PM, Felix E. Klee ... Just figured that this is akin to: http://en.wikipedia.org/wiki/Mutual_recursion And I
                Message 7 of 17 , Jan 8, 2011
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                  On Fri, Dec 31, 2010 at 6:33 PM, Felix E. Klee <felix.klee@...>
                  wrote:
                  > If I write code such as the following

                  Just figured that this is akin to:

                  http://en.wikipedia.org/wiki/Mutual_recursion

                  And I checked: The latest version of JSLint, 2011-01-06, still
                  complains about an undefined function.
                • Douglas Crockford
                  ... ` var sendToServer; ` ` function f() { ` sendToServer( xyz , function (moreToDo) { ` // do something ... ` if (moreToDo) { `
                  Message 8 of 17 , Jan 8, 2011
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                    --- In jslint_com@yahoogroups.com, "Felix E. Klee" <felix.klee@...> wrote:
                    >
                    > If I write code such as the following, then JSLint complains that "f" is
                    > not defined.
                    >
                    > var sendToServer;
                    >
                    > function callback(moreToDo) {
                    > // do something ...
                    > if (moreToDo) {
                    > f();
                    > }
                    > }
                    >
                    > function f() {
                    > sendToServer('xyz', callback);
                    > }
                    >
                    > What's the most elegant solution to get rid of the error message?


                    ` var sendToServer;
                    `
                    ` function f() {
                    ` sendToServer('xyz', function (moreToDo) {
                    ` // do something ...
                    ` if (moreToDo) {
                    ` f();
                    ` }
                    ` });
                    ` }
                  • Felix E. Klee
                    On Sat, Jan 8, 2011 at 4:05 PM, Douglas Crockford ... Thanks for the suggestion! I guess I ll take this approach and put the lengthy do something into a
                    Message 9 of 17 , Jan 8, 2011
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                      On Sat, Jan 8, 2011 at 4:05 PM, Douglas Crockford
                      <douglas@...> wrote:
                      > ` var sendToServer;
                      > `
                      > ` function f() {
                      > ` sendToServer('xyz', function (moreToDo) {
                      > ` // do something ...
                      > ` if (moreToDo) {
                      > ` f();
                      > ` }
                      > ` });
                      > ` }

                      Thanks for the suggestion! I guess I'll take this approach and put the
                      lengthy "do something" into a separate function.

                      Just thinking about it: I assume that having JSLint to *not* report an
                      error on circular function definitions is non-trivial. That is, if one
                      still wants to be warned about cases were an undefined function would be
                      called, such as:

                      function f() {
                      g();
                      }

                      f();

                      function g() {
                      // do something
                      }
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