Mapbox-gl-js: Expressions aren't executed within 'stops' functions

[muesliq]

It seems that neither Math expressions nor the type conversion expression number are being executed within stops functions.

I couldn't find an explanation for that in the Style Spec so I assume it must be a bug.

Steps to Trigger Behavior

Stylesheet (within Symbol layer and Layout property and using data that always comes with a "rank" value):

"text-size": { "stops": [
            [["-", ["number", ["get", "rank"]], 1], 0],
            [["number", ["get", "rank"]], 10]
        ]},

The purpose of this code is to use the rank values to display map features later than they appear in the data, based on their rank value.

Note: I also tried "{rank}" instead of ["get", "rank"] and I also tried without the number conversion.

Expected Behavior

The "rank" value is supposed to be converted to a number. So given the example of rank=16, the text-size should work like this:

"text-size": { "stops": [
            [["-",16, 1], 0],
            [16, 10]
        ]},

Also the above code doesn't work.

Which in return using the Math expression should be interpreted as:

"text-size": { "stops": [
            [15, 0],
            [16, 10]
        ]},

The above code obviously works.

Actual Behavior

Instead I get the error message: stop domain value must be a number, string, or boolean

[jfirebaugh]

A property value is _either_ a function or an expression -- you can't mix the two.

[muesliq]

Wouldn't it be really useful to make this possible, as an enhancement?

And: Couldn't find this information in the spec. Only the sentence "The value for any layout property, paint property, or filter may be specified as an expression. " - which together with your sentence is clear but without it wasn't.

[anandthakker]

@muesliq expressions are intended as a full replacement for stop functions. See "curve" for an analogue. (Note that "curve" will be replaced with "interpolate" and "step" in the next version, per #5528.)

[muesliq]

Thanks for clarification @anandthakker! Is it possible to use expressions within expressions?

[anandthakker]

In general, it is. However, note that the stop levels for curve expressions
(and, similarly, the keys for "match" expressions) are required to be
literal values rather than sub-expressions. This is a restriction we've
imposed to enable important performance optimizations.

On Thu, Nov 2, 2017 at 9:23 AM Helge Fahrnberger [e-mail]
wrote:

Thanks for clarification @anandthakker https://github.com/anandthakker!
Is it possible to use expressions within expressions?

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