CloudFormation Magic to Generate A List of ARNs from a List of Account Ids

Question

In my template, I am passing a CommaDelimitedList of account ids as a parameter.

I am hoping to do some Fn::Join and/or Fn::Sub magic to transform the list as follow:

 "Accounts" : {
     "Type" : "CommaDelimitedList",
     "Default" : "12222234,23333334,1122143234,..."
}

To be used in the template as a list `root` ARN's as :
 [
   "arn:aws:iam::12222234:root"
   "arn:aws:iam::23333334:root"
   "arn:aws:iam::1122143234:root"
 ]

Right now I am passing in the full ARNs, so it's working, but it is kluncky. However the CFN built in functions are proving very hard at doing this.

Any one have ready code for something like this?

Answer 1

I was able to adapt the existing answer by Sam Hammamy to work around the limitation of requiring special handling for the first and last items by using Fn::Sub. You can also combine two of the Joins.

In YAML:

AWS: !Split
  - ','
  - !Sub
    - 'arn:aws:iam::${inner}:root'
    - inner: !Join
      - ':root,arn:aws:iam::'
      - Ref: "Accounts"

And in JSON:

"Fn::Split": [
    ",", 
    {
        "Fn::Sub": [
            "arn:aws:iam::${rest}:root", 
            {
                "rest": {
                    "Fn::Join": [
                        ":root,arn:aws:iam::", 
                        { "Ref": "Accounts" }
                    ]
                }
            }
        ]
    }
]