consul-template - merging maps from keys that are yaml docs

Question

I'm trying to come up with a setup for a customer that will produce values.yaml for a helm chart using consul-template.

The "brain" is that there are defaults per environment, defaults per service, and concrete values for a specific service in a specific environment, and all these defaults should be merged.

The project I inherited uses git2consul and expresses every key in the tree in it's own file, so that trees are explodeed into toYAML as follows:

{{- $service := env "SERVICE_NAME" -}}
{{- $env := env "ENV" -}}
{{ (tree            (print "_all-envs/_all-svcs"  ) | explode)
   | mergeMap (tree (print "_all-envs/" $service  ) | explode)
   | mergeMap (tree (print $env  "/_all-svcs"  ) | explode)
   | mergeMap (tree (print $env  "/" $service  ) | explode)
   | toYAML }}

But then, I get numbers and booleans represented as explicit strings, because toYAML is not type aware, and spits everything as a string - and if it's not a natural string toYAML forces it to be an explicit string.

So I started to fiddle with hacking around toYAML to format things back to num/bool and this feels like not the right direction because sometimes true needs to be a boolean and in other cases - the chart wants it to yet be a string (i.e "boolean").

So I'm trying to come with an alternative, where I try to keep entire YAML docs in keys (instead of in trees), and since YAML is type-aware - I should expect the types to be preserved.

However, I cannot find a way how I can merge yaml documents parsed from keys. My current thought was something in the spirit of:

{{ key "_all-envs/_all-svcs" | parseYAML 
   | mergeMap (print "_all-envs/" $service | key | parseYAML )) 
   | mergeMap (print $env  "/_all-svcs" | key | parseYAML )) 
   | mergeMap (print $env  "/" $service | key | parseYAML )) 
   | toYAML }}

However, this fails with:

wrong type for value; expected map[string]interface {}; got map[interface {}]interface {}

How should I solve this?

If it cannot be done with consul-template, I'll need to develop an inhouse code for that, that gets a list of keys that hold docs and deep-merges them in the right order. However, the customer would like to avoid that and stick to public tools...