Henrik Tidefelt
(In Development)
It was concluded in #2211 that an MCP for Ternary (3-valued logic) in Modelica should be created, and here it is.
The proposal is based on the following principles:
- A new built-in type,
Ternary. - A new literal constant
unknownfor the third truth value. - Explicit as well as implicit conversion from
Boolean. - No implicit conversion to
Boolean. - No new built-in functions.
See design for details.
| Date | Description |
|---|---|
| 2019-11-22 | Henrik Tidefelt. Filling this document with initial content. |
| 2021-10-25 | Henrik Tidefelt. Introducing collection of examples. |
All authors of this MCP or their organizations have signed the "Modelica Contributor License Agreement".
See separate document.
The introduction of the keyword unknown introduces a backwards incompatibility with code making use of that name for identifiers.
A prototype has been implemented in a development version of Wolfram SystemModeler. In the prototype, the type is named __Wolfram_Ternary, and the third truth value is named __Wolfram_unknown.
The prototype is complemented with a collection of example models, provided in a separate repository: https://github.com/henrikt-ma/TernaryTest
Although introducing a new built-in type is a change that ammounts to a large number of smaller changes, finding the places in a code base that need attention is easy due to the similarity between Ternary and Boolean. In a similar way, the implicit conversion from Boolean to Ternary is a feature that can be implemented by glancing at the handling of implicit conversion from Integer to Real.
There are still a few examples in the collection that are not supported by the prototype. Fixing the the last examples is work in progress and none of them seem tricky.
Regarding the application of this MCP to a new attribute called visible in the Dialog annotation, the proposed choice of Kleene logic certainly gets the job done. The default value of unknown which means apply tool-specific rules for visibility, a user can easily write a ternary logical expression to override the default in either way, and the expression may also evaluate to unknown in cases where the user doesn't want to fall back on the tool logic. Thanks to the implicit cast from Boolean, most uses of visible would probably be in the simple forms visible = true or visible = false. However, upon closer acquaintance with the Kleene logic, which still appears as the most natural choice for Ternary, it has been questioned whether this is the best way to model the absence of information, see rationale.
To the best of our knowledge, there are no patents that would conflict with the incorporation of this MCP.
- Wikipedia, Three-valued logic, https://en.wikipedia.org/wiki/Three-valued_logic
- Wikipedia, Four-valued logic, https://en.wikipedia.org/wiki/Four-valued_logic