On Fri, Jun 02, 2006 at 02:17:25PM +0800, Shu-chun Weng wrote:
> 1. Spaces at beginning and end of rule blocks should be ignored
> since space before and after current rule are most likely be
> defined in rules using current one.
> 1a. I'm not sure if it's "clear" to define as this, but the spaces
> around the rule-level alternative could also be ignored.
At one point I had been exploring along similar lines, but at the
moment I'd say we don't want to do this. See below for an example...
> For instance, look at the rule FunctionAppExpr defined in
> MiniPerl6 grammar file.
>
> rule FunctionAppExpr
> {<Variable>|<Constants>|<ArrayRef>|<FunctionName>[<?ws>?<'('><?ws>?<Parameters><')'>]?}
FWIW, I'd go ahead and write this as a token statement instead of
a rule:
token FunctionAppExpr {
| <Variable>
| <Constants>
| <ArrayRef>
| <FunctionName> [ <?ws> \( <?ws> <Parameters> \) ]?
}
In fact, now that I've written the above I'm more inclined to say
it's not a good idea to ignore some whitespace in rule definitions
but not others. Consider:
rule FunctionAppExpr {
| <Variable>
| <Constants>
| <ArrayRef>
| <FunctionName>[ \( <Parameters> \) ]?
}
Can we quickly determine where the <?ws> are being generated?
What if the [...] portion had an alternation in it?
(And, if we ignore leading/trailing whitespace in rule blocks, do
we also ignore leading/trailing whitespace in subpatterns?)
In a couple of grammars I've developed already (especially the
one used for pgc.pir), having whitespace at the beginning of rules
and around alternations become <?ws> is useful and important.
In these cases, ignoring such whitespace would mean adding explicit
<?ws> in the rule to get things to work. At that point it feels like
waterbed theory -- by "improving" things for the FunctionAppExpr
rule above we're pushing the complexity somewhere else.
In general I'd say that in a production such as FunctionAppExpr
where there are just a few places that need <?ws>, then it's
better to use 'token' and explicitly indicate the allowed
whitespace.
(Side observation: in ...|<FunctionName>[<?ws>?<'('><?ws>?<Parameters><')'>]?}
above, there's no whitespace between <Parameters> and the closing paren.
Why not?)
> 2. I am not sure the default rule of <ws>, I couldn't found it in
> S05. Currently the engine use :P5/\s+/ but I would like it to
> be :P/\s*/ when it's before or after non-words and remains
> the same (\s+) otherwise.
PGE does the "\s* when before or after non-words and \s+ otherwise"
explicitly in its <ws> rule, which is written in PIR. (Being able
to write subrules procedurally is I<really> nice.)
In P5 it'd probably be something like
(?:(?<!\w)|(?!\w))\s*|\s+
or maybe better is
(?:(?<!\w)|(?!\w)|\s)\s*
Pm
