Introduce Value_Type.Null, fix edge cases for read_many

[radeusgd]

Pull Request Description

• Closes Introduce a Value_Type.Null. #6281

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Checklist

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[radeusgd]

Changes affect Snowflake so I'm running the Extra suite:

• https://github.com/enso-org/enso/actions/runs/12145121839
• https://github.com/enso-org/enso/actions/runs/12158622104

I expect it to fail because it is failing on develop. Yet I want to see if there are no more failures than what is expected.

[radeusgd]

Followup to fix Nothing * Nothing: #11751 and Nothing in expressions in DB #11757.

[GregoryTravis]

I find this logic hard to follow. The return value would be either Nothing or the return value of result.put expr -- but isn't the the return value of Ref.put the old value?

[radeusgd]

The return value of put is discarded.

The function returns ref.get at the very end.

I agree it is not ideal. This is a hack to get multiline if without adding a ton of indentation to the else branches. There's just too many branches here (and while sometimes too many branches is bad, here it is just intrinsic to this function) to make it readable.

Once we get multiline if (#6408) I will be more than happy to replace this with just:

if cond1 then
    ...
else if cond2 then
    ...
else
    Nothing

[radeusgd]

Do you think I should rewrite it with increasing indentation?

I'd rather keep it as is as IMO it is readable once the reasons are explained.

[GregoryTravis]

I see, it makes more sense now.

[radeusgd]

Followup to fix Nothing * Nothing: #11751 and Nothing in expressions in DB #11757.

I think we could merge this PR and then improve the missing bits from these 2 tickets in a follow-up PR. Unless there is any opposing views and we want to do it as a single change?

[radeusgd]

I've rebased on latest develop and noticed there are some test failures because of other changes. I will try to fix these soon, but it shouldn't change the shape of the overall changes that much.

[GregoryTravis]

I see, it makes more sense now.