# Is DV_INTERVAL missing invariants?

**Category:** [Specifications](https://discourse.openehr.org/c/specifications/6)
**Created:** 2021-12-18 01:53 UTC
**Views:** 471
**Replies:** 6
**URL:** https://discourse.openehr.org/t/is-dv-interval-missing-invariants/2210

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## Post #1 by @pablo

Reading the RM 1.1.0 data types spec, in DV_INTERVAL and Interval types I think we are missing an invariant, for instance, if lower_included = true and lower = NULL, is that valid? Same with upper.

I'm not seeing any constraint/invariant that says something about those cases. IMO lower/upper_included  should imply lower/upper is not NULL.

REFs:

- https://specifications.openehr.org/releases/RM/Release-1.1.0/data_types.html#_dv_interval_class
- https://specifications.openehr.org/releases/BASE/latest/foundation_types.html#_interval_class

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## Post #2 by @thomas.beale

DV_INTERVAL inherits the invariants from [Interval (BASE/Foundation Types)](https://specifications.openehr.org/releases/BASE/latest/foundation_types.html#_interval_class).

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## Post #3 by @pablo

Hi @thomas.beale I have considered the interval rules (see link above; though I don't see that case considered. That is why I'm asking :)

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## Post #4 by @thomas.beale

Sorry, read too fast - I thought I read 'missing invariants' ;)

Yes, we should add the following:

```
lower_included implies lower /= NULL
upper_included implies upper /= NULL
```

Do you mind adding a PR for this, containing this suggestion?

We will need to process your waiting list of PRs pretty soon ;)

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## Post #5 by @pablo

Thanks @thomas.beale rechecking this I believe it should be:

> !lower_unbounded <=> lower != NULL
> !upper_unbounded <=> upper != NULL

Note the <=> is stronger than the implies because this is an IFF (https://en.wikipedia.org/wiki/If_and_only_if)

Because for intervals that have boundaries the lower/upper should always be there, but the interval could or not include the boundaries, like:

- [1..10] includes both boundaries in the interval (lower_included = upper_included = true)
- (1..10] includes only the upper boundary (lower_included = false, upper_included = true)
- [1..10) includes only the lower boundary (lower_included = true, upper_included = false)

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## Post #6 by @thomas.beale

You could do this to be strict - the original invariant specification was designed with the sense that if lower_unbounded or upper_unbounded = True, it doesn't actually matter if any value is attached to lower or upper. The simpler way to state the stricter conditions is:
```
lower_unbounded xor lower /= Null
upper_unbounded xor upper /= Null
```

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## Post #7 by @pablo

Makes sense.

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**Canonical:** https://discourse.openehr.org/t/is-dv-interval-missing-invariants/2210
**Original content:** https://discourse.openehr.org/t/is-dv-interval-missing-invariants/2210