How to deal with exceptions in Nasal?

[eatdirt]

Hi there,
I've discovered Nasal last week for a little project and I needed to add some methods into geo.nas to calculate distances to great circles. That works, but I am trying to make it safe, and there is one error case I need to deal with, see below (TO BE FIXED). What are the standard way in nasal, and more precisely here, in geo.nas, to deal with exceptions/errors?

Thanks, and sorry for the rookie's question!
chris.

Code: Select all

# arc distance on an earth sphere to the great circle passing by A and B; doesn't consider altitude
   greatcircle_distance_to: func(destA, destB) {
      me._pupdate();
      destA._pupdate();
      destB._pupdate();

      # AB is not a circle but a point
      if (destA._lat == destB._lat and destA._lon == destB._lon)
          return me.distance_to(destA);

      # AB is undertermined; a great circle should be defined with non-collinear vectors
############################ TO BE FIXED #############################
      if (destA._lat == -destB._lat and destA._lon == math.pi - destB._lon) {
          return -1.0;
      }
###################################################################
      var ca1 = math.cos(destA._lon);
      var cd1 = math.cos(destA._lat);   
      var sa1 = math.sin(destA._lon);
      var sd1 = math.sin(destA._lat);   

      var ca2 = math.cos(destB._lon);
      var cd2 = math.cos(destB._lat);   
      var sa2 = math.sin(destB._lon);
      var sd2 = math.sin(destB._lat);   

      var sa12 = math.sin(destA._lon - destB._lon);
      
          var ca3 = math.cos(me._lon);
      var cd3 = math.cos(me._lat);   
      var sa3 = math.sin(me._lon);
      var sd3 = math.sin(me._lat);   

      # this is sin(greatcircle_dist) * sin(arcAB)
                var sDsAB = cd3 * sa3 * (ca2 * cd2 * sd1 - ca1 * cd1 * sd2 )
          + ca3 * cd3 * ( cd1 * sa1 * sd2 - cd2 * sa2 * sd1 )
          - cd1 * cd2 * sd3 * sa12;
      
      # direct calculation of sin(arcAB) to not call sin(arcsin(distance_to))   
      var a = math.sin((destA._lat - destB._lat) * 0.5);
      var o = math.sin((destA._lon - destB._lon) * 0.5);

      var hs12 = a * a + cd1 * cd2 * o * o;
      var hc12 = 1.0 - hs12;      
         
      return ERAD * math.abs( math.asin( 0.5 * sDsAB / math.sqrt( hs12 * hc12 ) ) );
   },
   # arc distance on an earth sphere to the horizon   
        horizon: func() {
           me._pupdate();
      return ERAD*math.acos(ERAD/(ERAD+me._alt));      
   },


[Hooray]

Yes, you would explicitly call the corresponding function using the call() API while passing an empty vector for any error handling, once the call() completes you can check the size() of the previously empty vector to see if any errors occurred.

http://wiki.flightgear.org/index.php?ti ... call.28.29

Basically (untested, see the wiki article for working examples):

Code: Select all


var exception = func() {
die("oops ...");
}

var errors = [];
call(exception, [],nil,nil,errors);

if (size(errors)) {
foreach (var e; errors) {
 debug.dump(e);
 }
}



[eatdirt]

Thanks perfect. Your answer made me discover the function die()... It is all I need here, and then, the error can be caught the way you show.

Thanks,
Chris.