In XPath 4.0 this could be:

fn:castable-as  := function($in as xs:string, $typeName as xs:string)
                             {  fn:op("castable as")  ($in,  $typeName  )
  }

or just:

fn:castable-as  :=    fn:op("castable as")

and this function could be chained within a sequence of other functions.

but I don't think we at present allow  "castable as"  as a possible
argument value of fn:op

What would also be more desirable is to have a type-object (not a type-name
as xs:string) as the 2nd function parameter and in the language.

A type object can have such useful associated functions as parse() and
show() (print / serialize), that one wouldn't even need to have a special
function checking if a value is parsable to a given type. Just parse it and
act correspondingly to the result. Having a type object passed as parameter
one could specify a series of generic operations on any value of that type.

Thanks,
Dimitre

On Wed, May 31, 2023 at 4:57b/AM Christophe Marchand
cmarchand@xxxxxxxxxxxxxx
<xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx> wrote:

> Hello !
>
> I have to write a function taht take a xs:string as parameter, returns a
> xs:boolean, and says true() if string is a valid number, false() otherwise.
>
> Something like
>
> <xsl:function name="f:isNumber" as="xs:boolean">
>   <xsl:param name="expression" as="xs:string"/>
>   <xsl:sequence select="....."/>
> </xsl:function>
>
> '1' -> true()
> '2' -> true()
> 'foo' -> false()
> 'bar' -> false()
>
> Rules to be a valid number are the same as those used in number() function.
>
> Any help wil be much appreciated !
>
> Christophe
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