Can you add support for Cloudformation replacements?

[ispyinternet]

When building state matchines using a Cloudformation resource and an external json file, there is support to replace Resource ARNs using DefinitionSubstitions see: https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-stepfunctions-statemachine.html

It would be great if the following file which uses this pattern could pass the validation!?:

{
    "Comment": "AWS Step Functions Example",
    "StartAt": "Start",
    "Version": "1.0",
    "States": {
        "Start": {
            "Type": "Task",
            "Resource": "${StartArn}",
            "Next": "Parallel State"
        },
        "Parallel State": {
            "Type": "Parallel",
            "Next": "End",
            "Branches": [{
                    "StartAt": "Wait3000",
                    "States": {
                        "Wait3000": {
                            "Type": "Task",
                            "Resource": "${Wait3000Arn}",
                            "End": true
                        }
                    }
                },
                {
                    "StartAt": "Wait500",
                    "States": {
                        "Wait500": {
                            "Type": "Task",
                            "Resource": "${Wait500Arn}",
                            "End": true
                        }
                    }
                }
            ]
        },
        "End": {
            "Type": "Task",
            "Resource": "${EndArn}",
            "End": true
        }
    }
}

Edit: asked at: awslabs/statelint#41

[jezhiggins]

This isn't something we'd use at WMFS - at least not anytime soon - so isn't something I could justify work time for. However, I'd be happy to help with a PR if you were able to work one up.

Statelint (both this version or Amazon's Ruby version) is a pretty thin wrapper around the J2119 validator library - https://github.com/wmfs/j2119/ or https://github.com/awslabs/j2119. The Statelint j2119 grammar defines the Resource field as

A Task State MUST have a URI field named "Resource".

To support the substitution, we'd need to introduce a new field type allowing a URI or a substitution pattern. This would involved extending the list of types (types.js/.rb) and defining the validation rule (constraints.js/.rb). Probably the trickiest bit is getting the regex right :)

WIth that change made, the change to statelint would be straightforward. Provide a new entry point, which loaded the cloudformation specific grammar, and everything should then just work.