Re: context dependencies

[Ricky Zhang]

Hi Ron,

I tried to simplify my production rule to present here. The non-terminal
expr can be derived from more non-terminal besides ID and NUMBER. But I can
safely assume that the boundary of expr is not SINGLE_SPACE.

SINGLE_SPACE should not be discarded in the following cases:
    1. Between SINGLE_QUOTE and SINGLE_QUOTE
    2. Between DOUBLE_QUOTE and DOUBLE_QUOTE
    3. Between expr and expr

To make it more user friendly, the case like:
        ID SINGLE_SPACE SINGLE_SPACE ID
should treat it as one SINGLE_SPACE without throwing syntax error.

I have tried the suggestion in Bison document Lexical Tie-ins section. But
it doesn't work. Here is my finding/experiment:

In Flex file,
...
SINGLE_SPACE    [ ]
%%
{SINGLE_SPACE}+                        { if(! should_discard_flag) return
SINGLE_SPACE_OP; }
...

In Bison file,

expr : expr                                       { should_discard_flag =
false;}
          SINGLE_SPACE_OP expr    { should_discard_flag = true;}
         | expr '+' expr ...
         ...
         ;

The should_discard_flag flag doesn't work because the stream of the first
expr in 'expr SINGLE_SPACE_OP expr' has not reduced to expr yet. It was
still ID by the time Flex scan SINGLE_SPACE. The suggestion from Bison
document Lexical Tie-ins section only works for the terminal case.


On Fri, Dec 2, 2016 at 1:25 AM, Ron Burk <address@hidden> wrote:

> Hi Ricky,
>
> Just curious: What actually terminates an 'expr' production? For example,
> if they were terminated by newlines and the parser encountered this token
> stream:
>   ID SINGLE_SPACE '\n'
> do you intend to treat that as a "missing right-hand-side of SINGLE_SPACE
> operator" error?
>
> Likewise, I guess you're saying that this token stream:
>     ID SINGLE_SPACE SINGLE_SPACE ID
> would generate a syntax error, penalizing the user who accidentally (or to
> make things line up) uses two spaces instead of one?
>
> Trying to figure out exactly where the context boundaries are within which
> you want white space to become significant.
>
> Ron
>
>
>
> On Thu, Dec 1, 2016 at 8:19 PM, Ricky Zhang <address@hidden>
> wrote:
>
> > I have read through 'handling context dependencies' section Bison manual.
> > But I still haven't figured out the proper solution to my problem.
> >
> > We have production rule:
> >
> > expr : expr SINGLE_SPACE expr { /* some operation */ }
> >         | expr + expr
> >         ...
> >         ;
> >
> > expr: NUMBER
> >        | ID
> >        ;
> >
> > and also for all white space [SINGLE_SPACE | \t]* in the language should
> be
> > ignored. Are there any easy way to solve it?
> >
> > Thanks,
> > Ricky
> > _______________________________________________
> > address@hidden https://lists.gnu.org/mailman/listinfo/help-bison
> >
> _______________________________________________
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