On Friday 11 February 2011 01:03:21 %u wrote: > Hi, > > I think I'm having a little trouble understanding what's meant by > context-free grammar. I've read that D is context-free, but is it really? > What about an expression like: > > int[U] s; > > ? You can't tell -- without looking at the context -- whether U is a data > type or a number, and so because associative arrays and regular arrays are > syntactically different elements of the language, the syntax of D is tied > in with its semantics, just like in C++. > > So is D really context-free? Or am I misunderstanding the meaning of the > term?
Yes. You're misunderstanding. It's actually quite difficult to parse a language if its grammar isn't context-free. You really should read up in context-free grammars if you want to understand it properly. Context-free means that it's unambiguous as to which grammar rule is to be applied at any point during the parsing process. In this case, U is a symbol. The parser probably doesn't care about much more than that. That symbol could end up being a number or a type. And actually, it probably isn't even that hard. U is a class, struct, alias, or template argument. The template argument and alias would be substituted with a real type or value before the compiler had to determine what type int[U] was. Not to mention, the context-free grammar is entirely for parsing tokens into an abstract syntax tree. The compiler then has to do whatever it does with the AST to actually generate assembly from it. So, as long as the parser can parse it into the AST, it could be the next stage's problem to determine what that really means. Regardless, I suggest that you read a compiler book if you really want to understand context-free grammars. I have "Compiler Construction: Principles and Practices" by Kenneth C. Louden, which is quite good, but the most popular one is the so-called "dragon" book. I'm not sure what it's actual title is though. - Jonathan M Davis
