Specify HostedZone NameServers as CloudFormation Outputs - aws-cloudformation

Question

I am creating a CFN stack for a number of domains. The domain are not with the AWS registry, but a third-party one.
I want to have the list of nameservers from the SOA as part of the stack Outputs. However, as they aren't returned as a string but, according to the docs, a "set", I can't figure out how to extract and return them.
Details:
According to the docs for AWS::Route53::HostedZone, you can obtain the list of nameservers with
Return Values
[...]
Fn::GetAtt
Fn::GetAtt returns a value for a specified attribute of this type. The
following are the available attributes and sample return values.
NameServers
Returns the **set** of name servers for the specific hosted zone. For example: ns1.example.com.
This attribute is not supported for private hosted zones.
So, I tried to do:
Resources:
MyZone:
Type: 'AWS::Route53::HostedZone'
Properties:
Name: my.domain.
...
Outputs:
MyZone:
Value: !Ref MyZone
MyZoneServers:
Value: !GetAtt MyZone.NameServers
but that gives:
An error occurred (ValidationError) when calling the UpdateStack operation: Template format error: The Value field of every Outputs member must evaluate to a String.
When I only output the zone ref, it works just fine and get the Z... string for the zone.
I've tried various other tricks and approaches, mostly with various intrinsic functions such as !Split, !Select, etc. Nowhere can I seem to find what this "set" is: a list? a comma-separated string? (in which case !Split should work)
I could retrieve the nameservers via the describe function of Route53 after the stack is created, but my feeling is that I'm missing something totally obvious so don't want to add that extra step.

Answer 1

The set of nameservers is an array of strings. In order to output it you need to use !Join like this:
Resources:
MyZone:
Type: 'AWS::Route53::HostedZone'
Properties:
Name: my.domain.
...
Outputs:
MyZone:
Value: !Ref MyZone
MyZoneServers:
Value: !Join [',', !GetAtt MyZone.NameServers] # or any other delimiter that suits you
You should see the following Outputs: