Building Parsers with Java™

14.2. A Logikus Grammar

The building blocks of Logikus programs are structures and variables. A typical structure has a string as its functor and has terms enclosed in parentheses. For now, let's sketch a grammar that will recognize a structure such as

starred(jamesCagney, "Mister Roberts", Year) 

The following grammar gives a first draft for recognizing Logikus structures and variables:

structure    = functor ('(' commaList(term) ')' | Empty); 
functor      = LowercaseWord | QuotedString;
term         = structure | variable;
variable     = UppercaseWord;
commaList(p) = p (',' p)*;

Note that the functor of a structure cannot be an uppercase word. Logikus shares this feature with Prolog; users need not and cannot declare variables. A string that begins with an ...