Hi Rob, Hi Marc,
As Rob showed, HOF functions are a possible (and I think actually the
only way, although I am not sure) way to deal with functions like
concat, which do expect a variable number of arguments. This is very
valid and might be a valid solution. I would just like to add that you
should keep the performance in mind. fold-left() of course comes to a
price. A simple comparison using fold vs. using string-join
declare variable $args :=
let $a := ('a', 'b', 'c')
for $i in 1 to 50000
return $a[random:integer(3) + 1];
declare function local:apply($fn, $args) {
$fn($args)
};
declare function local:apply2($fn, $args) {
fold-left($args, '', $fn)
};
(
prof:time(local:apply2(concat#2, $args)),
prof:time(local:apply(fn:string-join#1, $args))
)
shows a huge performance impact (around 500ms for fold, 0.01ms for
string-join).
This is not me saying you should not use fold (in fact it is a very nice
and elegant function), but that you might want to consider the
performance impact.
Cheers,
Dirk
On 14/08/14 08:15, Rob Stapper wrote:
> Par example:
>
>
>
> declare variable $args := ('a', 'b', 'c');
>
> declare function local:apply
>
> ( $fn
>
> , $args
>
> )
>
> { fold-left( $args
>
> , ''
>
> , $fn
>
> )
>
> } ;
>
>
>
> local:apply( concat#2, $args)
>
>
>
> Van: basex-talk-boun...@mailman.uni-konstanz.de
> [mailto:basex-talk-boun...@mailman.uni-konstanz.de] Namens Marc van Grootel
> Verzonden: woensdag 13 augustus 2014 22:16
> Aan: BaseX
> Onderwerp: [basex-talk] Apply variable argument list to anonymous function
>
>
>
> Hi,
>
>
>
> I am trying to call an anonymous function with a variable argument list.
>
>
>
> Maybe I'm overlooking something within XQuery but I cannot figure out how to
> do this.
>
>
>
> This is the code I would like to get working.
>
>
>
> declare function local:apply($fn, $args) {
>
> $fn($args)
>
> };
>
>
>
> declare variable $concat := fn:concat#3;
>
> declare variable $args := ('a', 'b', 'c'); (: needs to handle any list of
> strings :)
>
> local:apply($concat, $args)
>
> (: => 'abc' :)
>
>
>
> What I want local:apply to do is similar to apply in some function languages
> such as Clojure (http://clojuredocs.org/clojure_core/clojure.core/apply)
>
>
>
> In the code above I have two problems. a) how to bind the fn:concat function
> to a variable. Arity? #1, #2 ... ? b) how to "apply" a variable argument list
> to the function passed in to local:apply.
>
>
>
> I figure I could do something maybe with some other higher order functions
> but I cannot see the solution. I don't mind rtfm responses, I may have missed
> it.
>
>
>
> I keep bombarding the list with questions so it is only fair to give a bit of
> context. I am porting parts of a few small Clojure libraries to XQuery, most
> important libraries are Ring and Compojure which are libraries for use in web
> frameworks. My implementation currently builds on top of RESTXQ but
> eventually can provide an alternative way of handling HTTP routing requests
> through function handlers (without function annotations). This is similar to
> WSGI in Python and Rack in Ruby. I would've liked to release this sooner but
> it is more work than anticipated and I want to provide something that does
> these ideas justice. So it's ready when it's ready ;-) but I'm pretty close.
> I'm currently finalizing the routing part and need to work some more on
> middleware handlers. Until then go look at https://github.com/ring-clojure to
> see what this is about.
>
>
>
> --Marc
>
>
>
> ---
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> Antivirus-bescherming actief is.
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>
--
Dirk Kirsten, BaseX GmbH, http://basex.org
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