You have to think about EBNF and ABNF as extensions that help you just to be more concise and expressive while developing your grammars. For example think about an optional non-terminal symbol, in a BNF grammar you would define it by using intermediate symbols like: A ::= OPTIONAL OTHER OPTIONAL ::= opt_part | epsilon while … Read more
As of TypeScript 1.6 or so, you can now use either , or ; as a delimiter in interface declarations or anonymous object types! The team decided that the flexibility to use one or the other outweighed the concerns listed in the ‘old answer’ section. You’ll still need to use ; in class declarations to … Read more
The YACC syntax is in the Ruby source. Download it and run the bundled utiliy to get the readable syntax. wget ftp://ftp.ruby-lang.org/pub/ruby/2.0/ruby-2.0.0-p195.tar.gz tar xvzf ruby-2.0.0-p195.tar.gz cd ruby-2.0.0-p195 ruby sample/exyacc.rb < parse.y Output sample (total 918 lines for the v2.0.0-p195) program : top_compstmt ; top_compstmt : top_stmts opt_terms ; top_stmts : none | top_stmt | top_stmts … Read more
An important detail here is that grammars do not accept strings; they generate strings. Grammars are descriptions of languages that provide a means for generating all possible strings contained in the language. In order to tell if a particular string is contained in the language, you would use a recognizer, some sort of automaton that … Read more
Context Free is a concept from language theory that has important implications in parser implementation. A Context Free Language can be described by a Context Free Grammar, which is one in which all rules have a single non-terminal symbol at the left of the arrow: X→δ That simple restriction allows X to be substituted by … Read more
Whitespace between tokens Except at the beginning of a logical line or in string literals, the whitespace characters space, tab and formfeed can be used interchangeably to separate tokens. Whitespace is needed between two tokens only if their concatenation could otherwise be interpreted as a different token (e.g., ab is one token, but a b … Read more
How about LOLCODE? HAI CAN HAS STDIO? VISIBLE “HAI WORLD!” KTHXBYE Simplicity itself!
Examples from wikipedia LL(1) grammar S -> F S -> ( S + F ) F -> a input ( a + a ) parsing steps S -> “(” S “+” F “)” -> ( “F” + F ) -> ( “a” + F ) -> ( a + “a” ) LR(0) grammar (1) E … Read more
It’s not always easy (or even possible) to demonstrate that a grammar is ambiguous, but if there is a short ambiguous sentence, then it can be found with brute-force enumeration, which is what I believe that tool does. And the output is revealing; the shortest ambiguous sentence is the empty string. So it remains only … Read more
Being context free is first a property of generative grammars. It means that what a non-terminal can generate will not depend on the context in which the non-terminal appears (in non context-free generative grammar, the very notion of “string generated by a given non-terminal” is in general difficult to define). This doesn’t prevent the same … Read more