foo.el

(define-derived-mode cfn-mode yaml-mode
  "Cloudformation"
  "Cloudformation template mode.")

(add-to-list 'magic-mode-alist
              '("\\(---\n\\)?AWSTemplateFormatVersion:" . cfn-mode))

(when (featurep 'flycheck)
  (flycheck-define-checker cfn-lint
    "AWS CloudFormation linter using cfn-lint.

Install cfn-lint first: pip install cfn-lint

See `https://github.com/aws-cloudformation/cfn-python-lint'."

    :command ("cfn-lint" "-f" "parseable" source)
    :error-patterns ((warning line-start (file-name) ":" line ":" column
                              ":" (one-or-more digit) ":" (one-or-more digit) ":"
                              (id "W" (one-or-more digit)) ":" (message) line-end)
                      (error line-start (file-name) ":" line ":" column
                            ":" (one-or-more digit) ":" (one-or-more digit) ":"
                            (id "E" (one-or-more digit)) ":" (message) line-end))
    :modes (cfn-mode))

  (add-to-list 'flycheck-checkers 'cfn-lint)
  (add-hook 'cfn-mode-hook 'flycheck-mode))