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Re: [xsl] XPath 3.0 How to implement the function composition operator?


Subject: Re: [xsl] XPath 3.0 How to implement the function composition operator?
From: Dimitre Novatchev <dnovatchev@xxxxxxxxx>
Date: Mon, 15 Oct 2012 20:17:02 -0700

Roger and Michael,

Here is the definition of the compose() function I was after --
working with BaseX:

let  $apply := function($f as function( item()*) as item()*,
                        $x as item()*
                        ) as item()*
                {$f($x)},

     $_comp := function($f as function( item()*) as item()*,
                        $g as function( item()*)  as item()*,
                        $z as item()*
                      )
                      as item()*
             {
              $apply($f, $apply($g, $z))
             },

     $comp := function($f as function( item()*) as item()*,
                       $g as function( item()*)  as item()*
                      )
                     as function( item()*) as item()*
             {
               $_comp($f, $g, ?)
             },

     $double := function($m as item())  as item()
                { $m *2 },
     $incr   := function($x as item()) as item()
                { $x + 1}

 return
     $comp($double, $incr) (30)


This produces thecorrect, expected result:

62



On Mon, Oct 15, 2012 at 5:56 PM, Dimitre Novatchev <dnovatchev@xxxxxxxxx> wrote:
> I thought that using the argument placeholder "?" could be used to
> specify a more readable implementation.
>
> However it seems tht Saxon EE 9.3.05 (coming with oXygen) doesn't
> support argument place holders.
>
> For this query:
>
>          let $f := function($m as xs:integer, $n as xs:integer) as xs:integer
>                          {$m + $n}
>            return
>                $f(5, ?)(3)
>
> an error message is raised:
>
> Unexpected token "?" in path expression
> Start location: 24:0
> URL: http://www.w3.org/TR/xpath20/#ERRXPST0003
>
> Could someone, please, explain what is the issue with this expression?
>
>
> Cheers,
> Dimitre
>
>
>
> On Mon, Oct 15, 2012 at 4:02 PM, Michael Kay <mike@xxxxxxxxxxxx> wrote:
>> compose is a function that takes two functions as input and produces a third
>> function as output, so it looks like this:
>>
>> $compose   :=  function($a as function(item()*) as item()*,
>>
>>                         $b as function(item()*) as item()*)
>>                      as (function(item()*) as item()*)
>> {  function($c as item()*) as item()* { $b($a($c)) } }
>>
>> (Or the other way around. I don't know which way Haskell does it.)
>>
>> Michael Kay
>> Saxonica
>>
>>
>>
>> On 15/10/2012 23:08, Costello, Roger L. wrote:
>>>
>>> Hi Folks,
>>>
>>> How is function composition implemented in XPath 3.0?
>>>
>>> Example: Suppose I want to compose these two function:
>>>
>>>      1. increment: this function increases its argument by 1.
>>>
>>>      2. double: this function multiplies its argument by 2.
>>>
>>> In Haskell I can compose the two functions like so:
>>>
>>>      f = double . increment
>>>
>>> And then I can apply the composed functions to an argument:
>>>
>>>      f 2
>>>
>>> The result is 6.
>>>
>>> How is f implemented in XPath 3.0?
>>>
>>> Here is my attempt, which is not correct:
>>>
>>>              let $increment :=  function($x as xs:integer) {$x + 1},
>>>                   $double        :=  function($y as xs:integer) {$y * 2},
>>>                   $compose   :=  function(
>>>                                                                 $a as
>>> function(item()*) as item()*,
>>>                                                                 $b as
>>> function(item()*) as item()*
>>>                                                              )
>>>                                                             as item()*
>>>                                                 {$b($a)},
>>>                  $f  :=  $compose($double, $increment)
>>>              return $f(2)
>>>
>>> What is the correct way?
>>>
>>> /Roger
>>
>
>
>
> --
> Cheers,
> Dimitre Novatchev
> ---------------------------------------
> Truly great madness cannot be achieved without significant intelligence.
> ---------------------------------------
> To invent, you need a good imagination and a pile of junk
> -------------------------------------
> Never fight an inanimate object
> -------------------------------------
> To avoid situations in which you might make mistakes may be the
> biggest mistake of all
> ------------------------------------
> Quality means doing it right when no one is looking.
> -------------------------------------
> You've achieved success in your field when you don't know whether what
> you're doing is work or play
> -------------------------------------
> Facts do not cease to exist because they are ignored.
> -------------------------------------
> Typing monkeys will write all Shakespeare's works in 200yrs.Will they
> write all patents, too? :)
> -------------------------------------
> I finally figured out the only reason to be alive is to enjoy it.



-- 
Cheers,
Dimitre Novatchev
---------------------------------------
Truly great madness cannot be achieved without significant intelligence.
---------------------------------------
To invent, you need a good imagination and a pile of junk
-------------------------------------
Never fight an inanimate object
-------------------------------------
To avoid situations in which you might make mistakes may be the
biggest mistake of all
------------------------------------
Quality means doing it right when no one is looking.
-------------------------------------
You've achieved success in your field when you don't know whether what
you're doing is work or play
-------------------------------------
Facts do not cease to exist because they are ignored.
-------------------------------------
Typing monkeys will write all Shakespeare's works in 200yrs.Will they
write all patents, too? :)
-------------------------------------
I finally figured out the only reason to be alive is to enjoy it.


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