Using a defined helper as an iteration for a range - kubernetes-helm

Question

While recently writing a helm chart, I ran into an issue that I couldn't find a solution to.
Basically, I am trying to range off of a defined value (labels), but appear to be running into an issue because the defined value is a string and not a map. I tried to convert it with toYaml to no avail:
{{- range $key, $value := ( include "myChart.selectorLabels" . | toYaml ) }}
- key: {{ $key }}
operator: In
values:
- {{ $value }}
{{- end }}
Example error with toYaml:
COMBINED OUTPUT:
Error: Failed to render chart: exit status 1: Error: template: myApp/templates/deployment.yaml:103:77: executing "myApp/templates/deployment.yaml" at <toYaml>: range can't iterate over |-
app.kubernetes.io/name: myApp
app.kubernetes.io/instance: env-myApp
conf-sha: 0b92fad469486fedb6ad012c1b5f22c
Example error without toYaml (note lack of linebreak before first value):
COMBINED OUTPUT:
Error: Failed to render chart: exit status 1: Error: template: myApp/templates/deployment.yaml:103:73: executing "myApp/templates/deployment.yaml" at <.>: range can't iterate over app.kubernetes.io/name: myApp
app.kubernetes.io/instance: env-myApp
conf-sha: 0b92fad469486fedb6ad012c1b5f22c
Expected render:
- key: app.kubernetes.io/name
operator: In
values:
- myApp
- key: app.kubernetes.io/instance
operator: In
values:
- env-myApp
- key: conf-sha
operator: In
values:
- 0b92fad469486fedb6ad012c1b5f22c
I think there must be a more elegant solution to this in helm's templating that I am missing.

Answer 1

Helm has two extension functions to convert between strings and complex YAML structures. You're calling toYaml which takes an arbitrary object and serializes it to a YAML string. What you actually have is the string result from include which happens to be parseable YAML, and you need the opposite function, fromYaml.
{{- range $key, $value := ( include "myChart.selectorLabels" . | fromYaml ) }}
{{/*- not toYaml ^^^^ */}}
Neither function is especially well-documented. They are specific to Helm. There are corresponding toJson and fromJson that work similarly.