Layout property proposal: numeric-precision

[mourner]

An alternative solution for #3358. Introduces numeric-precision layout property that affects how numbers are interpolated in text fields. If set,
numbers in resolved text fields of a layer will have the specified precision (e.g. "numeric-precision": 2 will make 123.45678 appear s 123.46).

This is just a proof of concept; a proper implementation would require corresponding style-spec changes, including settling on a default value there.

cc @danpat @jfirebaugh

[lucaswoj]

I'm not sure that the layer's layout properties is the best place for this bit of metadata. Users would be unable to use different numeric precisions for different {tokens} within text-field (and maybe even {icon-image}). Whether or not this is a common use case, it brings up a few questions about our long term API design direction.

I'm excited about exploring string operations in the spirit of mapbox/mapbox-gl-function#28

[jfirebaugh]

Agree 100% with @lucaswoj.

[mourner]

I agree that arbitrary expression functions are much more powerful and may be better suited for this kind of task, although it may take us a very long time to get there.

I'm also interested to see how would such an arbitrary expression look for rounding a number. Would we introduce special keywords like round? What if the property's name is round?

[lucaswoj]

I'm also interested to see how would such an arbitrary expression look for rounding a number. Would we introduce special keywords like round? What if the property's name is round?

I think it'd look like a string formatting library in Javascript

... we create a syntax that is:

• a valid subset of an existing programming language (Javascript?)

Spitballing syntax here, supposing the feature had a property called foo and feature properties were available on this:

this.foo.toPrecision(2)

http://www.w3schools.com/jsref/jsref_toprecision.asp