Why does the lexical analyzer drop comments ?

Peter Dalgaard wrote:

Duncan Murdoch wrote:

On 3/20/2009 2:56 PM, romain.francois at dbmail.com wrote:

It happens in the token function in gram.c:
? ? ?  c = SkipSpace();
? ? ?  if (c == '#') c = SkipComment();

and then SkipComment goes like that:
static int SkipComment(void)
{
? ? ?  int c;
? ? ?  while ((c = xxgetc()) != '\n' && c != R_EOF) ;
? ? ?  if (c == R_EOF) EndOfFile = 2;
? ? ?  return c;
}

which effectively drops comments.

Would it be possible to keep the information somewhere ?
The source code says this:
? *?  The function yylex() scans the input, breaking it into
? *?  tokens which are then passed to the parser.?  The lexical
? *?  analyser maintains a symbol table (in a very messy fashion).

so my question is could we use this symbol table to keep track of, 
say, COMMENT tokens.
Why would I even care about that ? I'm writing a package that will
perform syntax highlighting of R source code based on the output of 
the
parser, and it seems a waste to drop the comments.
An also, when you print a function to the R console, you don't get 
the comments, and some of them might be useful to the user.

Am I mad if I contemplate looking into this ? 

Comments are syntactically the same as whitespace.  You don't want 
them to affect the parsing.

Well, you might, but there is quite some madness lying that way.

Back in the bronze age, we did actually try to keep comments attached 
to (AFAIR) the preceding token. One problem is that the elements of 
the parse tree typically involve multiple tokens, and if comments 
after different tokens get stored in the same place something is not 
going back where it came from when deparsing. So we had problems with 
comments moving from one end of a loop the other and the like.

Ouch. That helps picturing the kind of madness ...

Another way could be to record comments separately (similarly to 
srcfile attribute for example) instead of dropping them entirely, but 
I guess this is the same as Duncan's idea, which is easier to set up.

You could try extending the scheme by encoding which part of a 
syntactic structure the comment belongs to, but consider for instance 
how many places in a function call you can stick in a comment.

f #here
( #here
a #here (possibly)
= #here
1 #this one belongs to the argument, though
) #but here as well

If you're doing syntax highlighting, you can determine the 
whitespace by
looking at the srcref records, and then parse that to determine what 
isn't being counted as tokens.  (I think you'll find a few things 
there besides whitespace, but it is a fairly limited set, so 
shouldn't be too hard to recognize.)

The Rd parser is different, because in an Rd file, whitespace is 
significant, so it gets kept.

Duncan Murdoch