Re: how to find out what variables an exp ression references?

Jonathan Revusky <revusky <at> gmail.com> writes:

> 
> On Tue, Oct 7, 2008 at 3:44 PM, Attila Szegedi <szegedia <at> gmail.com> wrote:
> > This should work (code lifted from "?eval" built-in's code):
> >
> >             SimpleCharStream scs = new SimpleCharStream(new
> > StringReader("(" + s + ")"), 1, 1, s.length() + 2);
> >             FMParserTokenManager token_source = new
> > FMParserTokenManager(scs);
> >             token_source.SwitchTo(FMParserConstants.FM_EXPRESSION);
> 
> The above line is really the key. If you just instantiate an FMParser
> with something like
> new FMParser(new StringReader(myString)...) it isn't in the
> appropriate lexical state to parse an expression. Of course, a silly
> workaround would also be to parse the string "${" + myString + "}" and
> then drill down to get the expression iobject nside the interpolation
> object, but that's a little silly. But anyway, it switches into the
> FM_EXPRESSION lexical state after hitting the ${ token. Obviously it's
> better and more efficient to simply instantiate the parser in the
> appropriate lexical state as in the above snippet. Just providing some
> needless detail. 
> 
> JR
> 
> >             FMParser parser = new FMParser(token_source);
> >             parser.template = getTemplate(); token.
> >             Expression exp = parser.Expression();
> >
> > Attila.
> >
> > --
> > weblog: http://constc.blogspot.com
> > twitter: http://twitter.com/szegedi
> >


Many thanks for the feedback, especially for that bit of 'needless' detail... ;-)

I tried using the FMParserTokenManager, but this type is not visible from
outside of the freemarker.core package.

Using the "${" + myString + "}" trick gets me one step further, but I don't see
how I can walk the parser tree (parser.getNextToken()) to filter out any
parameter tokens, since the Token type is also package local.

... it's a pity the parser implementation is not more accessible.

I could of course create some types in the freemarker.core package to circumvent
this, but I was kind of hoping there would be some higher level Freemarker API
functions that allows me to parse an expression and extract the variable
references (among other things) before actually evaluating the expression.


thnx,
Conrad.



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