How can I implement a user-defined function with a name in the xpath expression?

Question

I am using XSLT. I know the Inline Function Expressions, Is there some way to declare a named function in xpath expression? because I need the function name to implement a recursive call.

Using XSLT and `xsl:function` you can do it easily, in pure XPath use `let`. — Martin Honnen, Dec 27 '19 at 06:05
How do that in pure XPath ? Would you like to show me a code snippet? — cmf41013, Dec 27 '19 at 06:33
@cmf41013 Here is a more recent example: https://stackoverflow.com/questions/58101443/xpath-recursive-parent-selection-in-a-flat-structure/58174330#58174330 — Dimitre Novatchev, Dec 27 '19 at 22:57

score 3 · Accepted Answer · answered Dec 27 '19 at 07:07

3

In XSLT I would simply suggest to use xsl:function as that way your function has a name and you can call it recursively inside of the function body.

As for pure XPath 3, Dimitre explored that path some years ago in https://dnovatchev.wordpress.com/2012/10/15/recursion-with-anonymous-inline-functions-in-xpath-3-0-2/ using let and higher-order functions (a feature not supported unfortunately in Saxon 9 HE), I think his code there uses a function type syntax not quite aligned with the final spec so his example would need to be

let $f := 
    function($n as xs:integer,
             $f1 as function(xs:integer, function(*)) as xs:integer) as xs:integer {
        if ($n eq 0)
        then 1
        else $n * $f1($n -1, $f1)

    },
    $F := function($n as xs:integer) as xs:integer {
        $f($n, $f)
    }
return $F(5)

which could be shortened to

let $f := 
    function($n as xs:integer,
             $f1 as function(xs:integer, function(*)) as xs:integer) as xs:integer {
        if ($n eq 0)
        then 1
        else $n * $f1($n -1, $f1)

    },
    $F := $f(?, $f)
return $F(5)

I think given the latest allowed syntax.

answered Dec 27 '19 at 07:07

Martin Honnen

160,499
6
90
110

@cmf41013 For a more recent example see this answer: https://stackoverflow.com/questions/58101443/xpath-recursive-parent-selection-in-a-flat-structure/58174330#58174330 – Dimitre Novatchev Dec 27 '19 at 22:56
Martin, the expression `$F := $f(?, $f)` is brilliant! However, this makes the definition of `F()` unreadable -- it isn't obvious for the reader what are the types of the arguments of `F()` and he must study other functions in order to understand what this types are... -- something rather time-consuming and error-prone compared to the explicit definition.Even if we devise a tool that would generate the full signature of F() this will still be time-consuming and difficult to enforce -- in other words, inconvenient. – Dimitre Novatchev Dec 28 '19 at 03:36
@DimitreNovatchev, I see your point, but I simply wanted to present an option the latest XPath version offers, it was not my intention to favour that way over your code I cited. In general I think, it depends on the perspective, for instance in C#, when they introduced `var` to declare variables without typing them, my initial thoughts were that I don't need it and would never use it; but nowadays I consider it quite a help when for instance using Saxon in C# and digging through its complex APIs. – Martin Honnen Dec 28 '19 at 10:18
@DimitreNovatchev, and the placeholder syntax in XPath can certainly reduce the code you have to write when for instance importing an XQuery module to XSLT and trying to use it. – Martin Honnen Dec 28 '19 at 10:19
Yes, but I am writing a function library, where each function must have a well-defined signature -- how otherwise would we be able to explain to the intended audience what type of arguments a function takes and what is their meaning? The "inner" or "worker" functions, on the other side, are intended to stay invisible -- that is, the actual implementation of a published function should be hidden. – Dimitre Novatchev Dec 28 '19 at 17:56
I also noticed that in a course / tutorial it is more instructive and useful to use the full type name instead of just `var`. This empasizes the names of the types and helps the reader quickly get accustomed to the new types. – Dimitre Novatchev Dec 29 '19 at 06:02

score 1 · Answer 2 · answered Dec 27 '19 at 09:38

1

There is no way to declare a named function in XPath; XPath 3.1 only allows anonymous inline functions, and these cannot be recursive. I'm told that there's a way to achieve recursion in anonymous functions using a technique called Y-combinators, but it's fairly mind-boggling and I've never got my head around it. Your best approach, as Martin suggests, is to put this part of the logic at the XSLT level.

answered Dec 27 '19 at 09:38

Michael Kay

156,231
11
92
164

Re: "it's fairly mind-boggling and I've never got my head around it." ... To quote @martin_honnen 's comment above: "you can do it easily, in pure XPath" :) Are you both talking about the same thing? – Dimitre Novatchev Dec 28 '19 at 01:53
1

When I have tried to understand the Y combinator, I have certainly found it mind-boggling; I find Dimitre's method quite straightforward, and have never quite understood why the one is easy to follow and the other not. (So: I *think* MK and MH are talking about the same thing, but I could be wrong.) – C. M. Sperberg-McQueen Dec 28 '19 at 02:22

How can I implement a user-defined function with a name in the xpath expression?

2 Answers2