What's Irony's equivalent of Yacc's optional operator ("?")?

Question

I have a fragment of grammar in Yacc notation:

stylesheet
: [ CHARSET_SYM STRING ';' ]?
  [S|CDO|CDC]* [ import [ CDO S* | CDC S* ]* ]*
  [ [ ruleset | media | page ] [ CDO S* | CDC S* ]* ]*
;

How do I implement this fragment in Irony? I can't find any equivalent of ?, which means 0 or 1 occurrence in Yacc.

Ani · Accepted Answer · 2014-05-27T10:27:53.000

You can use the BnfTerm.Q method to represent '?' (0 or 1 occurrences). This was a reasonable design decision since C# does not let you write a custom implementation of the ? operator, unlike + and *.

From the Non Terminals page on the Irony Wikibook:

In traditional BNF notation, the "?", "+", and "*" characters are used to indicate "0 or 1 time", "1 or more times" and "0 or more times", respectively. In Irony, it's done slightly differently. You use the MakePlusRule and MakeStarRule methods from the base Grammar class for "+" and "*" or you can use the Q(), Plus(), and Star() methods directly on the term within the rule.

The Q, Plus and Star methods on terms were deprecated around 2010 and have been removed at some point since then. What is the alternative to Q? — Asad Saeeduddin, Jul 18 '14 at 14:57

score 1 · Answer 2 · answered Dec 19 '14 at 18:26

The author says an additional AST node is now required for this purpose. So for example you could substitute the following for an optional term in a rule:

new NonTerminal("OptionalTermName", Empty | TermThatShouldBeOptional)

http://irony.codeplex.com/discussions/550979

I wonder if it could be reduced to:

(Empty | TermThatShouldBeOptional)

The overload creates a BNF Term, rather than a NonTerminal node, which could lose some hierarchical information when joined directly with other BNF terms, depending on the implementation. I haven't investigated further.

What's Irony's equivalent of Yacc's optional operator ("?")?

2 Answers2