[Rd] Why does the lexical analyzer drop comments ?

[Peter Dalgaard]

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:
>> Â Â Â  c = SkipSpace();
>> Â Â Â  if (c == '#') c = SkipComment();
>>
>> and then SkipComment goes like that:
>> static int SkipComment(void)
>> {
>> Â Â Â  int c;
>> Â Â Â  while ((c = xxgetc()) != '\n' && c != R_EOF) ;
>> Â Â Â  if (c == R_EOF) EndOfFile = 2;
>> Â Â Â  return c;
>> }
>>
>> which effectively drops comments.
>>
>> Would it be possible to keep the information somewhere ?
>> The source code says this:
>> Â *Â  The function yylex() scans the input, breaking it into
>>  *  tokens which are then passed to the parser.  The lexical
>> Â *Â  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.

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
> 
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