[xquery-talk] Arrow Operator to a Partially Applied Function

[Zachary N. Dean]

Ahh, I see what you mean now.

 

I guess everyone does it the same, and that is if a QName comes after the arrow, it is a function call and the argument can be placed right away. If a variable ref or parenthesis expression come after, they need to be resolved first, and then the argument set.

 

Wrapping the map:for-each call in parenthesis does the trick there by forcing the RHS to resolve before the Arrow.  

 

 

function($k, $v) {

    "$k=" || $k

} =>

(

  map:for-each(

      map {

          "a" : "1",

          "b" : "2",

          "c" : "3"

      },

      ?

  )

)()   

 

Makes me also wonder though if partial functions should be resolved first. Interesting!

 

 

 

From: talk-bounces at x-query.com <talk-bounces at x-query.com> On Behalf Of Adam Retter
Sent: Montag, 11. März 2019 07:45
To: Zachary N. Dean <contact at zadean.com>
Cc: talk at x-query.com; juri at existsolutions.com
Subject: Re: [xquery-talk] Arrow Operator to a Partially Applied Function

 

Thanks Zach. I suspect you are right. However I can't help wishing that the argument placeholders should be resolved before the arrow operator is applied...

 

For the average developer, the fact that my first query raises an error seems non intuitive, especally in light of the reformulation in my second query.

 

On Mon, 11 Mar 2019, 13:37 Zachary N. Dean, <contact at zadean.com <mailto:contact at zadean.com> > wrote:

Hello Adam,

I think the issue is that the Arrow Operator '=>' is simply "syntactic
sugar" for using the LHS as the FIRST argument of the function call on the
RHS.
The Argument Placeholder '?' has no direct relation to the Arrow Operator.

So, in the case of the first example:

function($k, $v) {
    "$k=" || $k
} =>
    map:for-each(
        map {
            "a" : "1",
            "b" : "2",
            "c" : "3"
        },
        ?
    )()

Simply becomes:

 map:for-each(
    function($k, $v) {
        "$k=" || $k
    },
     map {
         "a" : "1",
         "b" : "2",
         "c" : "3"
     },
     ?
 )()


So, the 3 argument map:for-each isn't found. (and the final call would have
the wrong arity if it was found)


Hope this helps,

Zack

-----Original Message-----
From: talk-bounces at x-query.com <mailto:talk-bounces at x-query.com>  <talk-bounces at x-query.com <mailto:talk-bounces at x-query.com> > On Behalf Of Adam
Retter
Sent: Montag, 11. März 2019 06:21
To: XQuery Talk ML <talk at x-query.com <mailto:talk at x-query.com> >
Cc: juri at existsolutions.com <mailto:juri at existsolutions.com> 
Subject: [xquery-talk] Arrow Operator to a Partially Applied Function

Without thinking too deeply, whilst writing some XQuery recently I was
initially surprised to discover that the following (simplified) query fails
with the static error - [XPST0017] map:for-each(map,function): 3 arguments
supplied, 2 expected.


xquery version "3.1";
declare namespace map = "http://www.w3.org/2005/xpath-functions/map";

function($k, $v) {
    "$k=" || $k
} =>
    map:for-each(
        map {
            "a" : "1",
            "b" : "2",
            "c" : "3"
        },
        ?
    )()


After having looked into the specs in more detail, I thought I was able to
derive a reason for this... i.e. the partial function application syntax is
not processed until the dynamic evaluation phase. Therefore when the arrow
operator is processed first during the static analysis phase, it tries to
find a map:for-each function that takes 3 parameters, the first being of the
function type on LHS of the arrow operator, and second being the map, and
the third being the unbound parameter `?`.

However, I then noticed that if I reformulatedby query with an indirection
through a variable binding `$x` then it compiles and executes just fine.
Presumably this indicates that my understanding above is incorrect, as
whilst the variable binding may change the evaluation order, the static
analysis and dynamic evaluation phases should still happen in the same
order.


xquery version "3.1";
declare namespace map = "http://www.w3.org/2005/xpath-functions/map";

let $x :=
    map:for-each(
        map {
            "a" : "1",
            "b" : "2",
            "c" : "3"
        },
        ?
    )
return

  function($k, $v) {
      "$k=" || $k
  } => $x()


I basically get the same results on eXist-db, Saxon, and BaseX. I would
appreciate if someone could help me understand what I am seeing here...

Thanks Adam.

--
Adam Retter

skype: adam.retter
tweet: adamretter
http://www.adamretter.org.uk
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