attr: Added IsNull() and IsUnknown() methods to the Value interface type

[bflad]

Closes #193

[kmoe]

Needs review by @paddycarver but I'm on board with this as you know!

[paddycarver]

I still think I prefer a tfsdk.ValueIsNull and tfsdk.ValueIsUnknown to reimplementing this on every single type.

I think asking type developers to implement the logic themselves creates opportunities for hard to debug situations where the value produces a null/unknown value when converted into a Terraform type, but reports that it is not null.

I also think it's extra work for type developers that doesn't need to be done and is just repeating yourself.

I recall you had some reasons for preferring not to do that. The one that comes to mind is that the tfsdk package is getting large and unwieldy, which I agree with, but I think can be mitigated by refactoring our package hierarchy.

Can you remind me what the other reasons you brought up were?

[paddycarver]

Arguments for methods:

• Discoverability: because standalone functions aren't tied to the attr.Value type, they're not surfaced by the language server, godoc, etc. as things you can do.

[paddycarver]

We decided we wanted to wait on this momentarily until we can tackle #215, #172, and other refactoring issues, and refactor our package boundaries a little. Right now, it's a little hard to understand how large the discoverability tradeoff is going to be, just because we're guessing at what package architectures may be available in the future. I'm still hopeful we'll be able to have an attr package that wraps all this up neatly, but we can't today because we get import cycles. We'll return to this once we figure out what package boundaries we can draw after refactoring a bit.

[bflad]

I'm going to close this out for now given some other recent/upcoming changes. Can revisit this again in the future, if it makes sense.

[github-actions]

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