"Callum Brown" [email protected] wrote:

Hi Arran (or anyone else who can clarify)

Regarding Q1 of the project:

Just wondering about the statement that the 'when-condition' and 'until-conditions' being optional, and if that requires further language to specify in BNF. I read a few forums and saw some people using square brackets eg [ ] but this wasn't accepted by the BNF editor.

Is it a necessary inclusion, and if yes, can you point me in the right direction as to how to represent optionality in BNF?

Thanks, Callum

Hi Callum,

Good questions. In relation to whether optionality requires additional metasymbols, beyond what BNF provides: just the vertical bar ("|") is already sufficient to expression optional elements of a language – can you see why?

In BNF, the vertical bar is the only metasymbol found on the right-hand side of a rule, and it's sufficient for expressing all context-free languages. All the other metasymbols we've seen – the asterisk ("*"), parentheses, plus symbol ("+"), et cetera, are not strictly needed – they just make writing grammars more concise and convenient, and are part of extended BNF (EBNF) rather than BNF. (The lab worksheet on syntax-based testing goes into more detail as to why the asterisk isn't needed.)

EBNF comes in many variants, so exactly what variant we're using is a good question. For this project, the spec states that we're using the version accepted by Paul Kline's "BNF playground". If you look at the playground webpage carefully, you should be able to find an explanation of what EBNF metasymbols it supports. Have a go, and if you're not able to find them, let me know.

Cheers

Arran