only functions can be called, got null - related to std.prune?

[Jeroen Op 't Eynde]

I ran into this error and can't make sense of it yet. It seems to fail on std.prune but I can't find a way to reproduce this in smaller snippet.

Here's the stack trace:

only functions can be called, got null
    <std>:296:30-35:                                                                field <groups> access
    <std>:291:20-22:                                                                variable <a> access
    argument <v> evaluation
    <std>:291:8-23:                                                                 function <builtin_is_array> call
    <std>:291:8-291:9:                                                              if condition
    <std>:296:20-36:                                                                function <prune> call
    <std>:283:10-12:                                                                variable <b> access
    <std>:283:13-283:14:                                                            if condition
    <std>:296:10-37:                                                                function <isContent> call
    vendor/github.com/grafana/jsonnet-libs/prometheus/mixins.libsonnet:79:15-43:    function <prune> call
    vendor/github.com/grafana/jsonnet-libs/prometheus/mixins.libsonnet:79:12-69:    if condition
    vendor/github.com/grafana/jsonnet-libs/prometheus/mixins.libsonnet:79:12-69:    evaluating field name
    vendor/github.com/grafana/jsonnet-libs/prometheus/mixins.libsonnet:98:38-50:    field <files> access
    <std>:260:24-26:                                                                variable <o> access
    argument <obj> evaluation
    <std>:260:5-34:                                                                 function <builtin_object_fields_ex> call
    vendor/github.com/grafana/jsonnet-libs/prometheus/mixins.libsonnet:98:21-51:    function <objectFields> call
    argument <arr> evaluation
    vendor/github.com/grafana/jsonnet-libs/prometheus/mixins.libsonnet:95:9-95:10:  function <builtin_sort> call
    argument <arr> evaluation
    vendor/github.com/grafana/jsonnet-libs/prometheus/mixins.libsonnet:95:10-95:11: function <builtin_reverse> call
    vendor/github.com/grafana/jsonnet-libs/ksonnet-util/util.libsonnet:231:10-37:   function <builtin_manifest_yaml_doc> call
    vendor/github.com/grafana/jsonnet-libs/prometheus/ha-mixin.libsonnet:56:19-75:  function <manifestYaml> call
    field <prometheus-0.yml> manifestification
    field <data> manifestification
    field <prometheus_config_maps> manifestification
    field <main_prometheus> manifestification
    field <default> manifestification

[Yaroslav Bolyukin]

Does it fail in some public library? It works for me on the prometheus-operator/kube_prometheus repo, which this output looks like.

[Jeroen Op 't Eynde]

It's in https://github.com/grafana/jsonnet-libs/blob/master/prometheus/ but the code I'm calling goes through a few more layers. I'll try to come up with something reproducible when I find the time.

[Yaroslav Bolyukin]

Anyway, the issue here is code like this is run:

local v = null;
v(1)

std.prune should not cause this, as it is implemented in the same way as cpp-jsonnet one: https://github.com/netomi/jsonnet/blob/96aa98e305a108da5d38f06bb3894324ef50cec4/stdlib/std.jsonnet#L1603

For easier debugging, jrsonnet supports --trace-format option: jrsonnet file.jsonnet --trace-format=explaining