[antlr-interest] CommonTree & Tree grammar versus DIY

[Terence Parr]

On Aug 15, 2008, at 10:13 AM, Andy Tripp wrote:
> There are an awful lot of parser/lexer tools out there besides ANTLR.
> I would guess that of all the people doing AST manipulation, few of  
> them
> use ANTLR treewalkers to do it.

I don't recommend rewriting asts unless you stay in same tree structure.

>>> The "manipulating an AST" work of building a translator is quite a  
>>> different
>>> job than the "build a lexer/parser" part of it.
>> Yep.  i'm mostly a fan of walking not rewriting ASts by the  
>> way...that gets WAY too hard.
>
> Maybe I'm misunderstanding you, but walking vs. rewriting is like
> apples vs. oranges (or "the drive to and from work" vs. "going to
> work all day"). Walking is what you do when there's (almost) nothing
> to do.

Except the entire work of the semantic analyzer of a compiler  
etc... ;)  Compute symb tables, flow analysis, well, just about  
everything right? ;)

> Further, Andy has a nice approach of rewriting.
>> Expect ANTLRMorph, by Leon Su, this Fall!
>
> To pick an AST manipulation rule at random...consider that when  
> translating
> C to Java, you can assign a zero value to any C struct. The C struct  
> will become
> a Java class, and the zero needs to change to "null". Where in the AST
> do we need to check? Here are a few ways to "assign 0" in C:
>
> * simple assignment:   a = NULL;    (where we have #define NULL 0  
> somewhere)
> * function call arg:   f(0);        (where f's first arg is some  
> struct type)
> * ternary operator:    a = b ? 0 : 0; * function return:     a =  
> f();     (where f() returns struct type)

You mean pointer to struct not struct right?

> So if we have a checkForZeroAssignment() method, calls to it will  
> have to be inserted
> in at least four distinct places in our Ctree.g file. And  
> checkForZero() will
> have to "know" where it's being called from anyway (for example, in  
> the
> simple assignment case, it may have to search deep into nested curly  
> braces
> for array assignment).
>
> So which way is best to slice the problem? We can have a
> ZeroAssignmentRule class (along with a couple hundred others to do  
> other things)
> that searches the AST for these four cases. Or
> we can have a Ctree.g that, at the ASSIGN node calls  
> checkForZeroAssignment()
> and a bunch (20? 50?) of other checks to do other things.
>
> I'll be pretty impressed if ANTLRMorph makes this kind of work any  
> easier,
> as none of the other AST-rewrite systems seem to.

We don't do AST rewrite.  We go text-to-text.  You would list those  
rules above as you've specified.

assignment:
	"x = NULL;" -> "<x> = null"

expr:
	"f(0)" -> "<f>(null)"

the ternary could be handled a few ways...but it's worse, right?  You  
have a type conflict from int to ptr.  You have to decide what to do  
in that case in general.

ANTLRMorph rocks!

Ter