Rules that Give You Too Much Freedom
by Silvie Spreeuwenberg
Freedom should be removed by rules. In fact, a rule that does not remove freedom in some way is not a rule according to the SBVR standard. That being said, you may be confused by the title of this column. Well, given this restriction on what a rule is and what you expect from a rule statement, there are still many ways to express such a rule.
RuleSpeak and SBVR's Structured English impose some restrictions on our use of language (see the box below), but due to the flexible nature of natural language you still have a lot of freedom ... consequently, we rarely see the same rule written in exactly the same way by different people using natural language.
There is a tendency to ask for a more restricted rule language to improve:
- consistency in writing similar rules by different people,
- consistent interpretation (non-ambiguity),
- support for validation & verification,
- translation to software systems.
| RuleSpeak and SBVR restrictions
Every business rule statement must contain at least one business rule keyword.
The keywords in RuleSpeak are:
The keywords in SBVR Structured English are:
- 'It is obligatory that'
- 'It is necessary that'
Expressing rules as decision tables or decision trees greatly reduces your freedom of form but also your expressiveness. So, when I need the expressiveness of natural language, I introduce very specific rule patterns to restrict my freedom and gain rigor.
- ... must be calculated as ....
- A ... for ... must be calculated as ....
- A <data element> for a <timeframe> and a <location> must be calculated as <formula>.
Each pattern is more specific, provides more guidance, and (consequently) less freedom. The last pattern above is even specific to a particular domain.
Sentence patterns seem to be the fashion today. They are not only popular for rules but also used in the descriptions of user stories in agile projects: As a <role> I want <something> so that <benefit>.
Rules must decrease freedom and so must rule sentence patterns. I am curious about the sentence patterns you use or recognize in your organization. Please send me an e-mail with your examples at firstname.lastname@example.org .