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Re: Some thoughts on function bodies and <sequence>

From: Erik Bruchez <erik@bruchez.org>
Date: Wed, 14 Mar 2012 10:50:55 -0700
Message-ID: <CAAc0PEVcdamvyoE=gn0dgh2eDYb1Q7tT0nh5bJxAFXAsHHu6rg@mail.gmail.com>
To: public-forms@w3.org
John,

I don't disagree. However the discussion is mainly about how to
integrate extension function declarations into XForms, and that is not
covered by XML or XPath proper, so we need a bit of thinking on how to
do it. To summarize, here is what we are talking about:

* enclosing <function> element (a no brainer so far)
* function signature following the lightweight XPath function
signature syntax (benefit: concision and standard syntax)
* function expression/body inside an attribute or within the <function> element
* the expression/body is plain XPath or JavaScript or whatever
language one wants to use

In my last email, I am trying to remove the <sequence> (or <result>)
element, and casting doubt as to whether we need even the <var>
element. So all in all I think the tendency here is towards minimalism
and less custom syntax rather than more.

-Erik

On Wed, Mar 14, 2012 at 10:40 AM, John Boyer <boyerj@ca.ibm.com> wrote:
> All,
>
> It would be better to avoid creating our own syntax if at all possible.
> Between XML and XPath, we have a lot of grammar to choose from already
> without having to write our own parser.
> Whether browser-based or java-based, sticking to XML and XPath would produce
> a much better chance that much of the processing can be expressed in terms
> of existing functionality.
>
> Cheers,
> John M. Boyer, Ph.D.
> Distinguished Engineer, IBM Forms and Smarter Web Applications
> IBM Canada Software Lab, Victoria
> E-Mail: boyerj@ca.ibm.com
>
> Twitter: http://twitter.com/johnboyerphd
> Blog: http://www.ibm.com/developerworks/blogs/page/JohnBoyer
> Blog RSS feed:
> http://www.ibm.com/developerworks/blogs/rss/JohnBoyer?flavor=rssdw
>
>
>
>
>
> From:        Erik Bruchez <erik@bruchez.org>
> To:        public-forms@w3.org
> Date:        14/03/2012 10:03 AM
> Subject:        Some thoughts on function bodies and <sequence>
> Sent by:        ebruchez@gmail.com
> ________________________________
>
>
>
> All,
>
> I dug up some emails and found Mike's reply in November 2009 on the topic:
>
> "I would recommend, if you want to keep things simple, only allowing
> xforms:sequence [as opposed to also supporting xforms:value-of]. Or
> naming it xforms:return - that's what it was in XSLT until we
> generalized it to allow sequence constructors, and in many ways I
> regret changing it."
>
> That sounds like a good suggestion to me, assuming we are going to
> support an element to specify the result.
>
> Now I am wondering if we can do away with nested elements altogether.
> I would like to see things like this, based on the examples in [1]:
>
> Example 1:
>
> <function signature="my:sum($p as xs:integer, $q as xs:integer) as
> xs:integer">
>    $p + $q
> </function>
>
> Or, using @value:
>
> <function signature="my:sum($p as xs:integer, $q as xs:integer) as
> xs:integer" value="$p + $q"/>
>
> Example 2 (XPath 3):
>
> <function signature="dollar-round($num as xs:decimal) as xs:string">
>    let $num100str := string(round($num * 100)),
>        $trunc := substring-before($num100str, '.'),
>        $intpart = substring($trunc, 0, string-length($trunc)-2),
>        $fracpart = substring($trunc, string-length($intpart), 2)
>
>    return concat($intpart, '.', $fracpart)
> </function>
>
> (I am sure you can write that function better in XPath 2 or XPath 3 BTW!)
>
> @value can also be used here, but it's a bit more clumsy visually.
>
> Example 3:
>
> <function signature="my:sumproduct($p as node()*, $q as node()*) as
> xs:double">
>   sum(for $i in 1 to count($p) return $p[$i] * $q[$i])
> </function>
>
> or:
>
> <function signature="my:sumproduct($p as node()*, $q as node()*) as
> xs:double"
>    value="sum(for $i in 1 to count($p) return $p[$i] * $q[$i])"/>
>
> This would work well for JavaScript too:
>
> Example 1 (JavaScript):
>
> <function signature="my:sum($p as xs:integer, $q as xs:integer) as
> xs:integer" type="javascript>
>    return p + q
> </function>
>
> <function signature="dollar-round($num as xs:decimal) as xs:string"
> type="javascript>
>    var num100str = ...
>    var trunc = ...
>    var intpart = ...
>    var fracpart = ...
>
>    return intpart + "." + fracpart
> </function>
>
> etc.
>
> Now unfortunately in XPath 2 we can't use `let`, but maybe we can
> still get away with the <sequence> or <return> element:
>
> <function signature="dollar-round($num as xs:decimal) as xs:string">
>    <var name="num100str" value="string(round($num * 100))"/>
>    <var name="trunc" value="substring-before($num100str, '.')"/>
>    <var name="intpart" value="substring($trunc, 0,
> string-length($trunc)-2)"/>
>    <var name="fracpart" value="substring($trunc, string-length($intpart),
> 2)"/>
>
>    concat($intpart, '.', $fracpart)
> </function>
>
> This allows functions that don't need variables in XPath 2 to keep the
> return value as an expression within the body of the <function>
> element, while still providing help to the poort XPath 2 or XPath 1
> users that want to use variables.
>
> Alternatively, we could completely get rid of the nested <var>
> element. This would mostly impact XPath 1 and XPath 2 implementations,
> with the idea that implementations would tend to migrate to XPath 3 as
> soon as possible anyway.
>
> So to summarize some ideas above:
>
> 1. We should consider using <return> instead of <sequence> if we do
> decide to use an element for return values.
> 2. However, we should consider *not* using an element for return values at
> all.
> 3. We should consider allowing function bodies simply as the content
> of <function>.
> 4. We should also support a shorthand with the @value attribute.
> 5. We might consider dropping support for nested <var> within
> <function> (not so sure about this one).
>
> Food for thoughts.
>
> -Erik
>
> [1] http://www.w3.org/MarkUp/Forms/wiki/Custom_XPath_functions
>
>
Received on Wednesday, 14 March 2012 17:51:44 UTC

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