Define constant for the invalid dyn.Value

[pietern]

Changes

The nil value is a real valid value that we need to represent. To accommodate this we introduced dyn.KindInvalid as the zero-value for dyn.Kind (see #904), but did not yet update the comments on dyn.NilValue or add tests for kind.go.

This also moves KindNil to be last in the definition order (least likely to care about it).

Tests

Tests pass.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8a1be76) 48.43% compared to head (8f3cd2e) 48.60%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1101      +/-   ##
==========================================
+ Coverage   48.43%   48.60%   +0.17%     
==========================================
  Files         268      268              
  Lines       10497    10499       +2     
==========================================
+ Hits         5084     5103      +19     
+ Misses       4859     4842      -17     
  Partials      554      554              

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[andrewnester]

Based on the purpose of it EmptyValue or ZeroValue might be a better name, WDYT?

[pietern]

That's what it is, essentially, though I'd like to convey the validity of a value.

Both EmptyValue and ZeroValue sound like they may be valid, but it shouldn't be.

It's also symmetric with the name of the Kind and the IsValid function:

func (v Value) IsValid() bool {
	return v.k != KindInvalid
}

Then when we do some operation (like a get) and verify it returned an actual node in the tree, we can do:

out, _ := v.Get("foo.bar")
if !out.IsValid() {
  // foo.bar doesn't exist
}