Graham Wideman wrote:
> Hi Sam,
>
> Thanks for your comments. More below on your questions:
>
>> I'm curious as to why you want to sometimes consider whitespace, though.
>> Is this a self-designed language, or a specification you're working from
>> that makes whitespace 'sometimes' significant?
>>
>> You example was a function call or declaration. You can always get help
>>from the lexer here if there are situations where there *must* be a
>> space, and situations where there *mustn't* be a space, and nothing
>> else... have tokens that include the lparen.
>
> Yes, I am considering the least-messy way to tackle a few of these issues in
> PHP. (And the function example I gave was just a simple example, not a
> problem in PHP.)
>
> One example that PHP has is the use of "$" as a prefix to identifiers,
> sometimes.
>
> An ordinary variable:
>
> $myvar = 'hello';
> $othervar = $myvar;
>
> Everywhere that such a variable appears, the dollar prefix is required, and
> no space is allowed. Now it's tempting to write the grammar as:
>
> variableName
> : Dollar Identifier ...
> ...
> Identifier
> : ('a'..'z' | 'A'..'Z' | '_') ('a'..'z' | 'A'..'Z' | '0'..'9' | '_')*
>
> This Identifier rule is good for all named things in PHP, but the parser rule
> would allow whitespace between $ and Identifier, which is not accepted by the
> actual PHP parser.
>
> So, maybe it's better to stick the "$" at the beginning of the lexer rule for
> Identifier (call it DollarIdentifier or something).
>
> But then you get to variables that are members of a class/object.
>
> class C {
> var $mymember = 'Hello';
> ...
> }
> $c = new C();
> print $c->mymember;
>
> Note how the declaration uses a $ prefix, but the usage does not (the only $
> is on the object variable, not the id of the member variable). But I'm
> somewhat loath to handle the $ sometimes in lexer rules, and sometimes in
> parser rules, as this seems apt to confuse later. (Maybe not... I haven't
> assessed how messy it gets going down this path.)
>
> I do indeed see ways to lex/parse this more strictly, I'm just exploring for
> the least messy way.
My limited experience has shown me that the more complex way usually
ends up less messy in the end...
I'd lex $id and id entirely separately, as they are syntactically
distinct entities. $blah is always a variable, a "true" variable, and
$c->member should be three tokens - a VARIABLEID ($c), a MEMBER (->) and
an ID (member). If PHP requires there be no space between those tokens,
then that might be a problem, but conceptually you'd parse it to a tree like
^(MEMBER VARIABLEID ID)
or, filling in values,
^(MEMBER $c member)
The point being that -> is a member operator. Your tree walker would see
the $member and give that class a member called member, perhaps, which
the MEMBER operator would fine. It's easy to trim/add a '$' from a string.
--
Sam Barnett-Cormack
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