[xsl] Indirect referencing in XSLT/XPath?

["Roger L Costello costello@xxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>]

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