On Nov 25, 2009, at 1:03 AM, Saptarshi Guha wrote:
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
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
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
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