<application>PL/TclU</application> in a particular database, use the
    <command>CREATE EXTENSION</command> command, for example
    <literal>CREATE EXTENSION pltcl</literal> or
    <literal>CREATE EXTENSION pltclu</literal>.
   </para>
  </sect1>

  <!-- **** PL/Tcl description **** -->

   <sect1 id="pltcl-functions">
    <title>PL/Tcl Functions and Arguments</title>

    <para>
     To create a function in the <application>PL/Tcl</application> language, use
     the standard <xref linkend="sql-createfunction"/> syntax:

<programlisting>
CREATE FUNCTION <replaceable>funcname</replaceable> (<replaceable>argument-types</replaceable>) RETURNS <replaceable>return-type</replaceable> AS $$
    # PL/Tcl function body
$$ LANGUAGE pltcl;
</programlisting>

     <application>PL/TclU</application> is the same, except that the language has to be specified as
     <literal>pltclu</literal>.
    </para>

    <para>
     The body of the function is simply a piece of Tcl script.
     When the function is called, the argument values are passed to the
     Tcl script as variables named <literal>1</literal>
     ... <literal><replaceable>n</replaceable></literal>.  The result is
     returned from the Tcl code in the usual way, with
     a <literal>return</literal> statement.  In a procedure, the return value
     from the Tcl code is ignored.
    </para>

    <para>
     For example, a function
     returning the greater of two integer values could be defined as:

<programlisting>
CREATE FUNCTION tcl_max(integer, integer) RETURNS integer AS $$
    if {$1 &gt; $2} {return $1}
    return $2
$$ LANGUAGE pltcl STRICT;
</programlisting>

     Note the clause <literal>STRICT</literal>, which saves us from
     having to think about null input values: if a null value is passed, the
     function will not be called at all, but will just return a null
     result automatically.
    </para>

    <para>
     In a nonstrict function,
     if the actual value of an argument is null, the corresponding
     <literal>$<replaceable>n</replaceable></literal> variable will be set to an empty string.
     To detect whether a particular argument is null, use the function
     <literal>argisnull</literal>.  For example, suppose that we wanted <function>tcl_max</function>
     with one null and one nonnull argument to return the nonnull
     argument, rather than null:

<programlisting>
CREATE FUNCTION tcl_max(integer, integer) RETURNS integer AS $$
    if {[argisnull 1]} {
        if {[argisnull 2]} { return_null }
        return $2
    }
    if {[argisnull 2]} { return $1 }
    if {$1 &gt; $2} {return $1}
    return $2
$$ LANGUAGE pltcl;
</programlisting>
    </para>

    <para>
     As shown above,
     to return a null value from a PL/Tcl function, execute
     <literal>return_null</literal>.  This can be done whether the
     function is strict or not.
    </para>

    <para>
     Composite-type arguments are passed to the function as Tcl
     arrays.  The element names of the array are the attribute names
     of the composite type. If an attribute in the passed row has the
     null value, it will not appear in the array. Here is an example:

<programlisting>
CREATE TABLE employee (
    name text,
    salary integer,
    age integer
);

CREATE FUNCTION overpaid(employee) RETURNS boolean AS $$
    if {200000.0 &lt; $1(salary)} {
        return "t"
    }
    if {$1(age) &lt; 30 &amp;&amp; 100000.0 &lt; $1(salary)} {
        return "t"
    }
    return "f"
$$ LANGUAGE pltcl;
</programlisting>
    </para>

    <para>
     PL/Tcl functions can return composite-type results, too.  To do this,
     the Tcl code must return a list of column name/value pairs matching
     the expected result type.  Any column names omitted from the list
     are returned as nulls, and an error is raised if there are unexpected
     column names.  Here is an example:

<programlisting>
CREATE FUNCTION square_cube(in int, out squared int, out cubed int) AS $$
    return [list squared [expr