RE: Declarative programming requires a different mindset

Hi Folks,

Yesterday’s responses were outstanding and illuminating. Thank you.

I learned that a declarative program describes the relationship of the output to the input.

This raises new questions. Let’s take an example. Suppose that this is the input:

<XML-file>http://www.example.org/book.xml</XML-file>

I want the output to contain the URL and the filename separated:

<XML-file>

    <URL>http://www.example.org/</URL>

    <Filename>book.xml</Filename>

</XML-file>

This XSLT code describes the relationship of the output to the input:

<xsl:template match="XML-file">

    <xsl:variable name="url"

                  select="f:substring-before-last(., '/')" />

    <XML-file>

        <URL>

            <xsl:value-of select="$url" />

        </URL>

        <Filename>

            <xsl:value-of select="substring-after(., $url)" />

        </Filename>

    </XML-file>

</xsl:template>

I believe this code is declarative. Do you agree?

Note my use of the function, f:substring-before-last(). There is no such built-in function, I created it. Below is how I implemented it. The implementation doesn’t seem descriptive. It seems quite recipe-like:

   Get the substring before $delimiter and output it,

   then output $delimiter, and then recurse.

It seems quite imperative. Do you agree?

If I stuff a bunch of imperative code into functions, and the “main” code is declarative, do I still have a declarative program?

Here’s my implementation of f:substring-before-last():

<xsl:function name="f:substring-before-last" as="xs:string?">

    <xsl:param name="string" as="xs:string" />

    <xsl:param name="delimiter" as="xs:string" />

    <xsl:if test="contains( $string, $delimiter )">

        <xsl:variable name="url">

            <xsl:value-of

               select="substring-before( $string, $delimiter )"/>

            <xsl:value-of select="$delimiter"/>

            <xsl:value-of

               select="f:substring-before-last(substring-after

                        ( $string, $delimiter ), $delimiter)" />

        </xsl:variable>

        <xsl:value-of select="$url" />

    </xsl:if>         

</xsl:function>