On 2011-02-11 10:37, Jonathan M Davis wrote:
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
The title of the so called "dragon" book is "Compilers: Principles,
Techniques, and Tools" and can be found here:
http://www.amazon.com/Compilers-Principles-Techniques-Tools-2nd/dp/0321486811/ref=sr_1_1?s=books&ie=UTF8&qid=1297440057&sr=1-1
It's also listed on the bookshelf page on the DigitalMars site.
--
/Jacob Carlborg
