cdk.FnJoin synth bug

[xdodocodex]

I'm introducing an output for CloudFormation stack as follow:

new cdk.Output(this, "ApiEndpoint", {
                value: new cdk.FnJoin("", ["https://","dummy.ref",".execute-api.",this.env.region,".amazonaws.com/","stage_name"]),
                description: "Endpoint for this stage of the api"
            })

What I expect from the command

cdk synth

is that a code like the following is created inside the CloudFormation template:

Outputs:
    ApiEndpoint:
        Description: 'Endpoint for this stage of the api'
        Value:
            'Fn::Join':
                - ""
                -
                    - 'https://'
                    - dummy.ref
                    - .execute-api.
                    - eu-central-1
                    - .amazonaws.com/
                    - stage_name
        Export:
            Name: 'PvideoSmartCF:ApiEndpoint'

Instead the generated code is like:

Outputs:
    ApiEndpoint:
        Description: 'Endpoint for this stage of the api'
        Value:
            'Fn::Join':
                - ""
                -
                    -
                        - 'https://'
                        - dummy.ref
                        - .execute-api.
                        - eu-central-1
                        - .amazonaws.com/
                        - stage_name
        Export:
            Name: 'PvideoSmartCF:ApiEndpoint'

Which result in an error during the deployment:

failed: ValidationError: Template error: every Fn::Join object requires two parameters, (1) a string delimiter and (2) a list of strings to be joined or a function that returns a list of strings (such as Fn::GetAZs) to be joined.
Template error: every Fn::Join object requires two parameters, (1) a string delimiter and (2) a list of strings to be joined or a function that returns a list of strings (such as Fn::GetAZs) to be joined.

Do anyone know how to solve this problem, or if it is a a bug or I am doing something wrong?

Thank you fro the support.

[eladb]

FnJoin accepts a variadic argument, so you would want to use it like so:

new cdk.FnJoin("", "https://","dummy.ref",".execute-api.",this.env.region,".amazonaws.com/","stage_name")

Alternatively, you can use the sugar FnConcat:

new cdk.FnConcat("https://","dummy.ref",".execute-api.",this.env.region,".amazonaws.com/","stage_name")

[xdodocodex]

Thank you this solved the problem

[eladb]

I wonder if we should change FnJoin to correspond 1:1 with CloudFormation. I am assuming you will not be the only person making that assumption...

[RomainMuller]

I think the FnJoin function in CloudFormation isn't ideal in most programming languages. The fact its variadic certainly makes it worse here (probably we should just get an array and call it quits).

But I'm thinking there may be value in providing a more fluent API instead... Something similar to what Google Guava does --- Joiner.on('; ').join(value, value, value);.

[RomainMuller]

Additionally, I guess the current typing of the arguments of FnJoin has listOfValues as any. This makes it extremely easy for people to pass incorrect things in (aka: an array). An option would be to flatten the argument (if the varargs contains a single value, and that is an array, just assume the user incorrectly passed in an array literal).

A type union would make things nicer here, but JSII won't support that (not transposable to most statically typed languages, such as Java, C#, ...).