RFC for Fn::Length Support

[juegong2]

Description

We will support an intrinsic function called Fn::Length that will return the number of elements in a given list.

Checklist

• Tracking issue created (label: status/proposed)
• RFC pull request submitted
• Reviewers assigned (label: status/review)
• Reviewers signed-off (label approved applied to pull request)
• Final comments period (label: status/final-comments-period)
• Approved and merged (label: status/approved)
• Implementation started (label: status/implementing)
• Implementation complete (label: status/done)

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Author is responsible to progress the RFC according to this checklist, and
apply the relevant labels to this issue so that the RFC table in README gets
updated.

[josb]

Would it be out of scope for this intrinsic function to also return the number of characters | bytes | codepoints when passed a single string instead of a list?

[benkehoe]

@josb what's the use case for that in a CloudFormation template?

[josb]

Some resource identifiers are limited in length; this would allow logic to be written to mitigate situations where resources would otherwise produce errors (e. g. the logic could construct shorter names to work around length limitation issues).

[benkehoe]

Would something that does that operation more directly be a better solution?

[josb]

I'm not sure what a general-purpose solution to that problem would look like, sorry.

That said, it would appear that a way to do numeric size comparisons in !If (beyond !Equals) would also be needed to address the use case where I think !Length would be useful, so perhaps this is a non-starter for now anyway.

[benbridts]

I'm not sure what a general-purpose solution to that problem would look like, sorry.

It could be something similar to GenerateResourceIdentifier (which can be used by resource providers to generate logical ids),

Or there could be a cut/trim function

# this one would always add a random string
Fn::GenerateIdentifier [ $name, $maxlength ]

# this one would drop characters at the end
Fn::Cut [ $string, $maxlength ]

Edit:
In an ideal world, you wouldn't need that first one, but I there are resources that require a Name so I would find that useful as it would allow you to do things like

Fn::GenerateIdentifier [ !Ref AWS::StackName, 50]

I expect there might be a need for generating the same identifier twice (in the same stack - across stacks can be solved by Imports), so an optional Seed parameter might be useful too. CloudFormation would have to prevent regeneration on updates anyway (or a seed parameter could be required to prevent changes to the value)

Fn::GenerateIdentifier [ !Ref AWS::StackName, 50, "reusable"] 

[josb]

Good suggestions, @benbridts. This function, as I originally intended it to be used, would be more useful if (numeric) comparison operators were available besides !Equals. Sigh.

[MalikAtalla-AWS]

Hey @benbridts and @josb. We really appreciate all your feedback. You mention some interesting ideas about better support for numeric comparisons or new intrinsic functions like Fn::GenerateIdentifier/Fn::Cut. If you want to create new issues in this repo briefly describing your proposals then we would have a good way to gauge demand from the community (through likes and comments).