Mapbox-gl-js: Coalesce expression doesn't work as expected

[muesliq]

mapbox-gl-js version: 0.42.2

Steps to Trigger Behavior

Use this code:

layout: {"text-field": ["coalesce", ["get", "fieldA"], ["get", "fieldB"]]}

where fieldA is null and fieldB is not null

Expected Behavior

Display fieldB.

Actual Behavior

Empty label. (It works though when fieldA is not null, displaying fieldA.)

Note:

A work-around with case did the job. However this seems to be a bug. Sorry if I somehow misunderstood the Style Spec and it isn't!

[anandthakker]

Thanks for the report @muesliq! Could you by any chance provide a live jsfiddle example reproducing this issue?

[jfirebaugh]

This is an outcome of type inference and automatic type assertions. What's happening is that since text-field must be a string, the result of coalesce must be a string, so it's inferred that each of the two gets must also be a string. So there is a string assertion automatically inserted, as if you had written:

["coalesce", ["string", ["get", "fieldA"]], ["string", ["get", "fieldB"]]]

Then when the expression is evaluated, it's found that the first get produces null rather than a string, which causes an error and aborts the evaluation. (The visual result is that you get the default value of the property, an empty string.)

This is _arguably_ working as intended, but I think the behavior is so surprising that we will need to revisit the design.

[anandthakker]

Nice analysis @jfirebaugh.

I think the best fix here is to treat coalesce as a special case when performing inference, not inserting assertions for its arguments, and instead inserting an assertion around the whole coalesce expression where necessary.