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Compose YAML Functions in Templates with atmos.Resolve

· 2 min read
Erik Osterman
Erik Osterman
Founder @ Cloud Posse

Atmos adds a new template function, atmos.Resolve, that evaluates any Atmos YAML function — like !git.repository, !exec, !store, or !terraform.output — at template-render time. This lets you compose a YAML function's result with other strings and template variables in a single value.

What Changed​

A YAML tag owns the entire scalar, so you cannot write "!git.repository/{{ ... }}". And because Atmos evaluates Go templates before YAML functions, referencing a function through an intermediate variable in a plain template doesn't work either — at template time the value is still the unresolved tag string "!git.repository".

atmos.Resolve closes that gap. It runs the YAML-function processor on demand during template rendering, so the resolved value is available where you can concatenate it with other text:

key: '{{ atmos.Resolve "!git.repository" }}/something'

Why This Matters​

The motivating use case is prefixing the Terraform workspace_key_prefix (or any value) with the repository slug, without resorting to !exec and shell concatenation:

settings:
  context:
    repo: "!git.repository"

components:
  terraform:
    vpc:
      backend:
        s3:
          workspace_key_prefix: '{{ atmos.Resolve .settings.context.repo }}/{{ or .metadata.name .metadata.component }}'

In a repository whose origin remote is https://github.com/cloudposse/atmos.git, this resolves to cloudposse/atmos/vpc.

How to Use It​

atmos.Resolve works with every Atmos YAML function. A plain (untagged) string is returned unchanged, so it's safe to pass values that may or may not contain a YAML function:

vars:
  bucket: '{{ atmos.Resolve "!env BUCKET_PREFIX" }}-state'

It runs during the template phase, giving it the same evaluation semantics as atmos.Component. Reach for a plain YAML tag when you don't need composition, and use atmos.Resolve when you do.

Get Involved​

See the atmos.Resolve documentation for the full reference.