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(_at_)gmail(_dot_)com> 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(_at_)saxonica(_dot_)com>
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
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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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