[exprla-devel] [xpl] Adding up a list of numbers, check it out.

[reid_spencer <ras...@re...>]

--- In xpl-dev@y..., Jonathan Burns <saski@w...> wrote:
<xpl>

<comment> This line is mandatory. </comment>

<include content="xpl.dtd" version="x.x"/ >

<comment>
    This includes a pre-existing data type,
    an XML document with the structure
    containing a list of integers:

    <List>
        [ <int> string </int> ]*
    </List>

    Within the program, an element of this form
    will be referred to by the name "intList".
</comment>

<include intList="list-of-int.dtd">

<comment>
    Here we define a program variable to contain
    a list of integers.
<comment>

<variable type=intList>
    <name>
        mylist
    </name>
</variable>

<comment>

    We define an integer variable>

</comment>

<variable type=int>
    <name>
        total
    </name>
</variable>

<comment>

    We define a general-purpose element

</comment>

<variable type=element>
    <name>
        index
    </name>
</variable>

<comment>

    We define "abort" as a macro for an
    action to take on error.

</comment>

<set>
    abort

    <stdout>
        <data>
            <List>
                zero
            </List>
        </data>
     </stdout>
</set>

<comment>

    At last, the action commences!

    We read in the intList data.

</comment>

<stdin>
    mylist
</stdin>

<if>
    <false>
        <verify>
            mylist
        </verify>
    </false>

    abort        <comment> Handy things, macros. </comment>
</if>

<comment>

    We set the general element "index" to refer
    to mylist as a whole. But we can think of this
    as referring to the root of the mylist.
    (Compare directory commands: think of this as
    setting up "index" as a symbolic link to mylist.)

    N.B. "set" is used here as a full copy assignment,
    with all children. It may be wasteful in some cases.

</comment>

<set>
    index
    mylist
</set>

<comment>

    Check whether mylist is empty.

</comment>

<if>
    <equals>
        <number-of-children>
            mylist
        </number-of-children>
        zero
    <equals>

    abort

<else>

    <comment>

        Shift "index" down, to refer to "mylist"'s first child.
        (Compare change-directory command.)

    </comment>

    <set>
        index
        <first-child>
            index      <comment> This could as well be "mylist".
</comment>
        </first-child>
    </set>

    <comment>

        Initialize our "total" variable.

    </comment>

    <set>
        total

        <data>
            <int>
                zero
            </int>
        <data>
    </set>

    <comment>

        And this is what does the work:
        a Do-While loop which adds the list elements
        to our "total" variable.

        N.B.
            It doesn't make sense to add up strings,
            even strings like "9848001" and "0".
            But it does make sense to add up elements
            such as

                <int>
                    9848001
                </int>

    </comment>

    <do>

        <add>
            total
            index
        </add>

        <comment>

            Here's the While condition

        </comment>

        <not> <equals>
             index
             <last-child>
                mylist
             <last-child>
         </equals>
     </do>

</else>
</if>

<comment>

    And we're out, with the list elements
    added up in "total".
    So, output it.

</comment>

<stdout>
   <data>
       <List>
            total
        </List>
     </data>
</stdout>

<comment>

    And take the rest of the day off.

</comment>

</xpl>
--- End forwarded message ---