Changing the second line to: foo.txt text !diff would restore the default unset-ness for diff, while: foo.txt text diff will force diff to be set (both will presumably result in a diff, since Git has presumably not previously been detecting foo.txt as binary).

Unspecified No pattern matches the path, and nothing says if the path has or does not have the attribute, the attribute for the path is said to be Unspecified.

Unset The path has the attribute with special value "false"; this is specified by listing the name of the attribute prefixed with a dash - in the attribute list.

4.3.9 undefined value primitive value used when a variable has not been assigned a value 4.3.11 null value primitive value that represents the intentional absence of any object value

Check whether a value is defined (non-nullable), meaning it is neither `null` or `undefined`. This can be useful as a type guard, as for example, `[1, null].filter(Boolean)` does not always type-guard correctly.

Dry::Types::Undefined

There are times where it is useful to know whether a value was passed to run or the result of a filter default. In particular, it is useful when nil is an acceptable value.

main > h = {key_with_nil_value: nil}
=> {:key_with_nil_value=>nil}

main > h[:key_with_nil_value]
=> nil

main > h[:missing_key]  # this would be undefined in JavaScript (a useful distinction) rather than null, but in Ruby it's indistinguishable from the case where a nil value was actually explicitly _supplied_ by the caller/user
=> nil

# so we have to check for "missingness" ("undefinedness"?) differently in Ruby

main > h.key?(:key_with_nil_value)
=> true

main > h.key?(:missing_key)
=> false