> What will really be useful is a function with an argument of item(),
> that returns the real type of the argument.
> 
> This would be trivial to achieve if same-arity function 
> overloading was allowed.
> 
> Does anybody know of another way?

You can write

<xsl:choose>
<xsl:when test="$arg instanceof xs:boolean">boolean</xsl:when>
<xsl:when test="$arg instanceof xs:string">string</xsl:when>
etc.

> 
> Or shall we propose an exslt:type-of()  ?
>

The WGs decided not to provide such a function mainly because it inhibits
substitutability. For example, a function that is declared to return an
xs:decimal might actually (on some or all occasions) return an xs:integer,
or perhaps an oracle:binary-coded-decimal-integer. The result of an
xx:type-of() function would therefore differ from one implementation to
another. People would tend to write

if (exslt:type-of($x) = 'decimal')

which would fail on some implementations and succeed on others. By contrast,
writing "$x instance of xs:decimal" is portable, because it also matches
subtypes.

(Saxon does provide such a function, but I advise its use only for
diagnostics.)

Michael Kay
http://www.saxonica.com/