Readable Nesting

Readable Nesting

Nesting Intrinsic Functions Without Losing Readability

Functions nest, and the engine evaluates them from the inside out. Both values below build the same URL for an SSM parameter from a website bucket's WebsiteURL minus its http://; deployed on LocalStack 63,725 , both returned https://<bucket>.s3-website.localhost.localstack.cloud:4566/dev/books (with the container's port 4566, not the host port 31566):

The same value, nested with Join and flattened with SubYAML

      # Nested
      Value: !Join
        - ""
        - - "https://"
          - !Select [1, !Split ["//", !GetAtt FrontEndBucket.WebsiteURL]]
          - !Sub "/${Stage}/books"
      # Flattened
      Value: !Sub
        - https://${Host}/${Stage}/books
        - Host: !Select [1, !Split ["//", !GetAtt FrontEndBucket.WebsiteURL]]

How the engine evaluates the Sub version, innermost function first

Prefer Fn::Sub to Fn::Join with an empty delimiter: the string then looks like its result.
Name inner expressions in Fn::Sub's variable map, one expression per variable.
YAML cannot stack two short tags: Value: !Base64 !Sub "..." is a parse error (cfn-lint 2,641 E0000 did not find expected key); write Fn::Base64: !Sub "...".
A !Sub without variables is noise; cfn-lint reports W1020 'Fn::Sub' isn't needed.
Beyond three levels, move the value into a parameter, a mapping or another stack's output.

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