Ability to constrain a UUID to a specific version

[jesselang]

While there is a constraint for a valid UUID, there isn't a constraint for a particular version.

import (
"uuid"
)

id: uuid.Valid
id: "1c7b233b-4806-41a7-bff2-8feebef4870f"

Describe the solution you'd like
A clear and concise description of what you want to happen.

Something like:

import (
"uuid"
)

id: uuid.Valid & uuid.Version(4)
id: "1c7b233b-4806-41a7-bff2-8feebef4870f"

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

There is an operation in the uuid package that returns the version, but I couldn't find a good way to use this as a constraint.

Additional context
Add any other context or screenshots about the feature request here.

[mvdan]

How about: https://tip.cuelang.org/play/?id=0Yg5UD02ouh#cue@export@cue

Replace the == 4 with == 3 for example, and you'll see the failure.

With the must builtin per #575, you could rewrite _isV4: true & uuid.Version(id) == 4 into _isV4: must(uuid.Version(id) == 4, "id must be an UUID v4"), for example. A bit nicer for sure, although what you can do today still gets the job done.

[jesselang]

Yeah, using another boolean field will work, but the error message isn't going to help a user at all. It seems like the must builtin would provide the ability to specify a meaningful error message. Glad to know there is something in the works to wrap these functions into constraints. Thanks!

[mvdan]

I'm going to convert this into a discussion, because I feel like it's a valid question that others might find useful in the future, and I don't want to close it as a duplicate issue of the must built-in :)