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.