Adding support for Fn::Sub.

[Noirbot]

Description

Cloudformation has the Fn:Sub function now, which provides functionality similar to Ruby's built-in string interpolation. We should add support for it.

Steps to Test or Reproduce

There are two forms of Fn::Sub, both of which I've added to the DSL, using the same function to produce each, depending on parameters provided.

There's the verbose version where you declare your own variables:

sub("${var1}::AWS::${var2}", {:var1 => 'Yes', :var2 => 'Working'})

This produces this CFN output:

{
  "Fn::Sub": [
    "${var1}::AWS::${var2}",
    {
      "var1": "Yes",
      "var2": "Working"
    }
  ]
}

There's also a shorthand version that you can use when you're only referencing template params or logical ids that only requires passing in the string:

sub("arn:aws:s3:::${ParamName}/*")

That produces this output:

{
  "Fn::Sub": "arn:aws:s3:::${ParamName}/*"
}

Environment

This should work for all modern templates.

Deploy Notes

I'm guessing this will require a release, since it's an added function, but probably not a minor/major version.

Todos

• [x] Pull Request
• [ ] Tests
• [ ] Documentation

  Request to Review

@jonaf/@temujin9

Adding support for both forms of Fn::Sub.

[jonaf]

Thanks @Noirbot ! I have two additional requests for this PR:

1. Would you mind also adding a simple example to the test template and update the test to validate the output, such that we can prevent any regressions with the syntax in the future?
2. Would you update the README.md to include the sub function in the list of supported AWS intrinsic functions?

[Noirbot]

@jonaf Updates have been made, should be better now!