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Question R's parser : - parsing "x=\n 1"

2 messages · Saptarshi Guha, Simon Urbanek

Original
Saptarshi Guha
# 
Hello
I was reading the source main/src/gram.y and had one question, how does R parse

x =
  1

According the grammar:

prog	:	END_OF_INPUT			{ return 0; }
	|	'\n'				{ return xxvalue(NULL,2,NULL); }
	|	expr_or_assign '\n'			{ return xxvalue($1,3,&@1); }
	|	expr_or_assign ';'			{ return xxvalue($1,4,&@1); }
	|	error	 			{ YYABORT; }
	;


So this should be of the 3rd form.
Also, the expr_or_assign is of the 2nd form in

expr_or_assign  :    expr                       { $$ = $1; }
                |    equal_assign               { $$ = $1; }
                ;

where equal_assign is

equal_assign    :    expr EQ_ASSIGN expr_or_assign              { $$ =
xxbinary($2,$1,$3); }

When the parser sees 'x' and '=' it expects an expr_or_assign and we
know it will receive an expr. However, the expr cannot be a new
line(according to the defn of expr)

So instead of an expr, the parse gets a newline and should fail.
Q: So how does R parse this?
I think it fails with a Parse_Incomplete and keeps on reading till EOF
(or till an expression is complete).
But this is not really an incomplete expression but if my
interpretation is correct a syntax error, yet R parses it.
So i understood something else or R's engine manages to do things differently
Regards
Saptarshi
Simon Urbanek
#
On Nov 25, 2009, at 1:03 AM, Saptarshi Guha wrote:

            

      
      
      
    

        
It is. "x=" is incomplete -- try it in R and you'll see that you get a  
continuation prompt because it's incomplete:

 > x=
+ 1

Cheers,
S