Problems with error conversion and propagation

[dionjwa]

I'm trying to catch a raw error, and convert it to a more useful error. Something like:

doSomethingReturningAPromise()
  .errorPipe(function(err) {
    //This error is not helpful. My own errors are better because they are haxe enums
    var myNicerError = convert(err);
    //Now I want to re-throw this, so it will be correctly propagated
    //to downstream error handlers
   //but how 

  //Neither of these work:
   var p = new DeferredPromise();
    // p.throwError('Some new thrown error');
    Node.setTimeout(function() {
      p.boundPromise.reject('errorPiped boundPromise reject');
    }, 50);

    return p.boundPromise;
  })
  .then(function(stuff) {
  })
   .then(function(stuff) {
  })
  .catchError(function(err) {
    //Final handler. If err is a fancy error I can:
    switch(err) {
      //Nice compile time checked error handling
    }
  })

The examples that do something similar involve catching the error, using haxe's nice catch type checking, and rethrowing. But that cannot work here because the return type of Promise.catchError() is an AsyncBase, and errorPipe doesn't handle errors throw in it, or from promises it returns.

So I'm confused if this is possible. It seems like a reasonable use case, but I'm a bit stumped.

[dionjwa]

Basically, I want a conditional catch/errorPipe: when I examine the error, I may want to rethrow, or convert to a non-error.

[jdonaldson]

This sounds reasonable. I could provide some kind of errorMap() function that just lets you map the error type and automatically rethrow. Another alternative is just to do a try/catch around the error handler methods as well, and let you manually rethrow within those.

[dionjwa]

That would be great. errorMap sounds clear, but it's not clear what the second approach would look like.

[jdonaldson]

Yeah, the second approach wouldn't work. The catchError function isn't designed to propagate errors, only handle them. I'll look into errorMap.