Allow re-setting of default values

[gravitystorm]

Some symbolizer properties have default values that can't be expressed in carto. For example, line-dasharray defaults to "none", but only numbers are accepted. This makes it impossible to turn off dashes.

#lines {
  line-width: 1;
  line-color: black;
  line-dasharray: 4,4;
  [zoom >= 15] {
    line-dasharray: none;
  }
}

Invalid value for line-dasharray, a valid numbers is expected. none was given.

text-spacing is another example, where it's impossible to re-establish the default value (no repeated labels) after a value is set.

Sometimes it's possible to bodge the results, like setting line-dasharray to be 100,0 or text-spacing to 10000000000. But these are just bodges!

[tmcw]

This has been on our radar for a while (#18 is related) but haven't had any good ideas here, like some normal or auto or reset property.

[migurski]

“none” would be the correct way to express this. There’s no way in CSS to express default values either, but there are simple ways to turn things off like in Andy’s line-dasharray example.

[springmeyer]

happy to provide a fix for this on the mapnik side too, I've just hesitated to do that thinking it would be easier to handle in carto.js