absent optional parameter for method seemingly not passed through as 'null'

[DartBot]

This issue was originally filed by ross.m.s...@gmail.com

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Based on the forum thread: https://groups.google.com/a/dartlang.org/group/misc/browse_thread/thread/08270b5e76b467ae#

this will print 'NULL' but not stroke my rect:

void strokeRectangle(Rectangle r, [num lineWidth]) {
    if(lineWidth == null) print("NULL");
    _context2d.strokeRect(r.left, r.top, r.width, r.height, lineWidth);
  }

this will stroke my rect:

void strokeRectangle(Rectangle r, [num lineWidth]) {
    if(lineWidth == null) print("NULL");
    _context2d.strokeRect(r.left, r.top, r.width, r.height, null);
  }

What version of the product are you using? On what operating system?

Dart Editor build 5205 on x86_64

Please provide any additional information below.

"My uninformed guess is you're running into the distinction between
undefined and null in the underlying javascript.
Parameters that aren't passed to a JS function get the value
undefined, which is distinct from null but compares equal to it. "

[kasperl]

Thanks a lot for the bug report, Ross!

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Set owner to @kasperl.
Removed Priority-Medium label.
Added Priority-High, Area-Frog, Triaged labels.

[kasperl]

Yeah, it does look like you're running into the distinction between JS null and undefined. Our compiler currently uses both values to represent Dart null, but sometimes you end up calling external JS code or DOM functions that treat the two JS values differently.

The safest workaround is probably something like this:

  void strokeRectangle(Rectangle r, [num lineWidth]) {
    if (lineWidth == null) {
      _context2d.strokeRect(r.left, r.top, r.width, r.height);
    } else {
      _context2d.strokeRect(r.left, r.top, r.width, r.height, lineWidth);
    }
  }

which is admittedly a bit harder on the eyes.

[DartBot]

This comment was originally written by ross.m.smith...@gmail.com

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thanks for looking into this Kasper! I have the workaround in place(s) for now :)

[rakudrama]

This is not a distinction between null and undefined.

strokeRect takes four arguments.

Many JavaScript DOM functions behave differently depending on the number of arguments, for example, passing 5 when only 4 are accepted does not simply ignore the 5th like a JavaScript function would.

Frog uses both undefined and null as Dart 'null'.
As an optimization, it leaves off arguments that are known to be null, since the default 'undefined' is an acceptable null.
So
   _context2d.strokeRect(r.left, r.top, r.width, r.height, null);
compiles as
   _context2d.strokeRect(r.left, r.top, r.width, r.height);

and magically works.

It is unfortunate for Dart that the DOM functions dispatch on the number of arguments.
Sometimes there is no value of y to make f(x, y) behave exactly like f(x).

As mentioned above, trailing null arguments are removed as an optimization. When there are default arguments, these are added first, and then the nulls are removed. This usually 'accidentally' results in the compiled call having the same number of arguments as the source call:

// Dart:
void f(x, [y, z]);
f(1);
// JS:
-> f(1)
-> f(1, null, null) // Add default arguments
-> f(1) // remove nulls

What needs to be fixed is ensure a source call that may end up calling a DOM method compiles to a call or chain of calls that calls the DOM method with exactly the same number of arguments.
The code that adds the defaults is separate to the code the removes the nulls. Both need to be fixed to improve the situation.

[anders-sandholm]

Removed Area-Frog label.
Added Area-Dart2JS, OldAreaFrog labels.

[anders-sandholm]

Removed OldAreaFrog label.
Added FromAreaFrog label.

[kasperl]

We should make sure we have a passing test case for this.

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Removed Priority-High, FromAreaFrog labels.
Added Priority-Medium label.
Changed the title to: "absent optional parameter for method seemingly not passed through as 'null'".

[DartBot]

This comment was originally written by ross.m.s...@gmail.com

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Yes, please add test coverage for this :) This issue is still present in build 8365. I get it in dart2js as well as Dartium now, so perhaps the root cause is different than it used to be - but there does still seem to be a bug.

[kasperl]

Set owner to ngeoffray@google.com.
Added this to the Later milestone.

[kasperl]

Removed this from the Later milestone.

[kasperl]

Added this to the M2 milestone.

[DartBot]

This comment was originally written by ngeoffray@google.com

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Removed this from the M2 milestone.
Added this to the Later milestone.

[kasperl]

Added TriageForM5 label.

[kasperl]

Is this still a problem? Most problems don't go away without us actively fixing them, but I'm not sure if this has gone stale.

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Removed TriageForM5 label.
Added AssumedStale label.

[peter-ahe-google]

Kasper, you said: "We should make sure we have a passing test case for this." on Jun 8, 2012.

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Added Triaged label.

[DartBot]

This comment was originally written by ross.m....@gmail.com

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I did a few quick tests in SDK 0.5.11_r23200 and this issue (forwarding null for absent optional) appears to be fixed; I'd still encourage that test case for regression's sake ;)

In other news passing 'null' as 'lineWidth' to 'strokeRect' works in dart2js but in Dartium it now throws an error:

Exception: Dart_DoubleValue expects argument 'double_obj' to be non-null.

[DartBot]

This comment was originally written by ross.m....@gmail.com

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Interestingly, the optional [num lineWidth] seems to have been since removed from the CanvasRenderingContext2D spec and from the Blink implementation, so the sample code in this issue is no longer useful , which (I think) explains the observations in my last comment.

As mentioned in my last comment though, I believe the original issue to be fixed now.

cheers,

[kasperl]

Marking as fixed based on Ross' comment.

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Added Fixed label.