Re: [xsl] Indirect referencing in XSLT/XPath?

["Dimitre Novatchev dnovatchev@xxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>]

>  Is there any way to do indirect referencing in XSLT/XPath?

Yes, but not assignments, as in XPath any created variable is immutable.

For your particular example:

let $aircraft := {"altitude": {"units" : "Flight_Level"} }
 return $aircraft?altitude?units

produces:

Flight_Level

And this can be executed in a suitable XPath processor, such as BaseX or
Oxygen:

Thanks,
Dimitre.

On Sat, Apr 5, 2025 at 7:35b/AM Roger L Costello costello@xxxxxxxxx <
xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx> wrote:

> Hi Folks,
>
> The SNOBOL language has a wicked cool indirect referencing capability. Is
> there an indirect referencing capability in XSLT/XPath?
>
> Here is a brief description of SNOBOL's indirect referencing capability:
>
> First, let's assign UNITS the string METERS
>
> UNITS = "METERS"
>
> In that statement UNITS is a name, and METERS is a data string.
>
> In this statement UNITS is a data string:
>
> ALTITUDE = "UNITS"
>
> Let's output the value of ALTITUDE:
>
> OUTPUT = ALTITUDE
> prints UNITS.
>
> [In SNOBOL, the word OUTPUT is a keyword; when it is assigned a value,
> that value is output. I think that is pure genius.]
>
> The value of UNITS hasn't changed, as we can see with this output:
>
> OUTPUT = UNITS
> prints METERS.
>
> SNOBOL has an indirect referencing operation to reference the value of a
> value. ["reference the value of a value" Let that sink into your brain; it
> is so cool.] The indirect referencing operation is indicated by the unary
> operator $ written in front of a name:
>
> OUTPUT =  $ALTITUDE
> prints METERS.
>
> Let's change the value of UNITS:
>
> UNITS = "FEET"
>
> Now do an indirect reference:
>
> OUTPUT =  $ALTITUDE
> prints FEET.
>
> Values may be assigned using indirect referencing:
>
> $ALTITUDE = "FLIGHT_LEVEL"
>
> We can see that the value of UNITS has changed:
>
> OUTPUT =  UNITS
> prints FLIGHT_LEVEL.
>
> More than one level of indirect referencing is allowed, using $$, $$$,
> $$$$, etc.
>
> We've been using ALTITUDE as a name; now let's use it as a data string:
>
> AIRCRAFT = "ALTITUDE"
>
> Here's the reference chain that we've created: AIRCRAFT references
> ALTITUDE which references UNITS which references FLIGHT_LEVEL. Stated
> another way, AIRCRAFT has a property, ALTITUDE, and ALTITUDE has a
> property, UNITS, and UNITS has a value, FLIGHT_LEVEL.
>
> Let's do some indirect referencing to show the reference chain:
>
> OUTPUT = AIRCRAFT
> prints ALTITUDE
>
> OUTPUT = $AIRCRAFT
> prints UNITS.
>
> OUTPUT = $$AIRCRAFT
> prints FLIGHT_LEVEL.
> ----------------------------------------------------
> Okay, that is a quick introduction to SNOBOL's indirect referencing
> capability. I think it's wonderful. I can see the utility of changing an
> item that is way down the reference chain--e.g., change the value of
> UNITS--without impacting items that are way up the reference chain--e.g.,
> no impact to ALTITUDE or AIRCRAFT.
>
> XML entities resemble indirect referencing:
>
> <!DOCTYPE Document [
> <!ENTITY UNITS "FLIGHT_LEVEL">
> <!ENTITY ALTITUDE "&UNITS;">
> <!ENTITY AIRCRAFT "&ALTITUDE;">
> ]>
> <Document>
>     &AIRCRAFT;
> </Document>
>
> I was delighted to see that SNOBOL uses the $ symbol. XSLT/XPath uses the
> $ symbol to identify a variable, e.g., $foo. In SNOBOL, the $ symbol is a
> unary operator. Does XSLT/XPath consider the $ symbol to be a unary
> operator? I don't think that XSLT/XPath permits two consecutive $ symbols,
> e.g., $$foo is illegal and $($foo) is illegal, right?
>
> Is there any way to do indirect referencing in XSLT/XPath?
>
> /Roger