QT4

What's in it for me?

Declarative Amsterdam 2025

Juri Leino and John Lumley

Overview of QT4CG activities
Summaries of changed specs
A late breaking change

Agenda

〰 BREAK 〰

Continued sessions:
- XQuery
- XSLT
Question and Answers

Detailed sessions:
- XPath
- Functions and Operators

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Started 6th September 2022
140 weekly (Zoom) meetings thus far
~12 active members
>2000 issues and pull requests
~ 1 year to final feedback and completion

start

finish

QT4 Community Group

qt4cg.org — A Community Group under W3C

start

finish

Started 6th September 2022
140 weekly (Zoom) meetings thus far
~12 active members
>2000 issues and pull requests
~ 1 year to final feedback and completion

QT4 Community Group

qt4cg.org — A Community Group under W3C

QT4 Community Group

qt4cg.org — A Community Group under W3C

start

finish

Started 6th September 2022
140 weekly (Zoom) meetings thus far
~12 active members
>2000 issues and pull requests
~ 1 year to final feedback and completion

How to find changes?

Sections with significant changes in the TOC

New functions

Previous/ Next

Links to Issue and PR

How to find changes?

Process (nested) maps and arrays as easily as node trees
Full processing of (deep) JSON as a target Syntax and function additions to improve ease of use
Minimal backwards incompatibility

Moving from 3.1 to 4.0

XPath

Functions + 0perators

XQuery

XSLT

XPath

Better support for nested maps/arrays
- JNode – an array/map entry with ‘axis-relationships’
- Deep lookup operators
- Maps are now ordered
Structured Record (map) types
Improved ‘pipelines’ => -> =!>

Functions and Operators

mandatory higher-order functions
EXPath File and Binary modules adopted and refined
~90 new functions
~60 functions extended
consistency
options records

XQuery

Declare custom records and enums
Declare functions without prefix
Use Qname literals in computed element constructors
Optional function parameters

XSLT

Syntactic additions to make stylesheets cleaner
- Enclosing modes
- Declared item/record types
Better support for maps/arrays, including template matching
Potential freezing of streaming support

A late breaking change...

Nodes

XNodes

GNodes

JNodes

XNodes

GTree

JTree

XTree

JNodes

parent: a reference to the parent JNode in the tree.
content: the value of the array member or map entry.
selector: the map key or array index that distinguishes this child from its siblings.
position: needed to handle "mixed" content

JNodes

jtree()

jnode-content()

jnode-selector()

jnode-position()

JNodes

      [ "a", "b", "c"]
      /*[1]
      /..
      /*[last()]
      /string()

JNodes

jtree([ "a", "b", "c"])
      /*[1]
      /..
      /*[last()]
      /string()

JNodes

{
  "fr": {
    "capital": "Paris", 
    "languages": [ "French" ] }, 
  "de": {
     "capital": "Berlin",
     "languages": [ "German" ] }
}
//languages

JNodes

{
  "fr": {
    "capital": "Paris", 
    "languages": [ "French" ] }, 
  "de": {
     "capital": "Berlin",
     "languages": [ "German" ] }
}
//child::get("languages")

JNodes

{
  "fr": {
    "capital": "Paris", 
    "languages": [ "French" ] }, 
  "de": {
     "capital": "Berlin",
     "languages": [ "German" ] }
}
//languages/jnode-content()

JNodes

XNodes


<node id="something">
  content
</node>


{
  'key': [1,2,3]
}

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Fiddle

CODE

CONTEXT

RESULT

SELECT EXAMPLE

RUN

SELECT CONTEXT

TYPE

Fiddle

https://fiddle.basex.org

https://docs.basex.org/13/XQuery_4.0

XPath

The map-Keyword

   { "this": "is", "a": "map" }

map

XPath

The map-Keyword

   { "this": "is", "a": "map" }

XPath

Lookups with ?

{ "test 1": 10 }?("test 1")

{ 1: 10 }?1

{ "test": 10 }?test

XPath

Lookups with ?

{ "test 1": 10 }?("test 1")

XPath

Lookups with ?

{ "test 1": 10 }? "test 1"

XPath

Lookups with ?

{ "test 1": 10 }?($property)

XPath

Lookups with ?

{ "test 1": 10 }? $property

XPath

Arrow operator

recap

EXPR => my:f() => my:g()

my:g(my:f(EXPR))

XPath

Arrow operator

recap

1 to 3
=> sum()
=> math:pow(2)
=> function ($i) { 
     ``[The square of the sum is `{$i}`]`` 
   }()

XPath

1 to 3
=> for-each(math:pow(?,2))
=> sum()
=> function ($i) { 
     ``[The sum of squares is `{$i}`]`` 
   }()

Arrow operator

recap

XPath

ArrowBang

1 to 3
=!> math:pow(2)
=> sum()
=> function ($i) {
     ``[The sum of squares is `{$i}`]``
   }()

XPath

Arrow shortfalls

1 to 3 ! math:pow(2,.)

1 to 3

=!> fn($a){math:pow(2, $a)}()

function argument order

XPath

Arrow shortfalls

1 to 3 ! (. + 2)

1 to 3

=!> fn($a){ $a + 2 }()

RHS has to be a function call

XPath

Pipeline operator

3 -> math:pow(2, .)

EXPR -> EXPR

XPath

string templates

{ "number": 1, "name": "template" }
-> `This is {?number} string {?name}`

not a string constructor

XPath

fn-Keyword

let $g := fn() {}

let $f := function () {}

return ($f(), $g())

XPath

Focus Functions

let $id := fn{ . }

$id(1)

no arguments but the context value

XPath

Conditionals I

if (COND) { "COND was true" }

braced-if (without else)

XPath

Conditionals II

EXPR otherwise "fallback value"

otherwise

XPath

type checks I

fn ($e as element()) {
   (: do something with the element :)
}

element wildcards

XPath

type checks I

fn ($e as element(*)) {
   (: do something with the element :)
}

element wildcards

XPath

type checks II

fn ($e as element(*:div)) {
   (: work with all the divs :)
}

element wildcards

XPath

type checks II

declare default namespace ##any;

fn ($e as element(div)) {
   (: work with all the divs :)
}

element wildcards

XPath

type checks III

//element(div|section)
(: select divs and sections :)

UnionNodeTests

XPath

type checks IV

fn ($input as (map(*)|array(*))) {

    $input?1
}

ChoiceItemTypes

XPath

Destructuring I

let ($head, $tail) := (1 to 3)
return ($tail, $head)

Destructuring a sequence

XPath

Destructuring I

let ($p, $q) := (1 to 3)
return ($p, $q)

Destructuring a sequence

XPath

Destructuring II

let $m := { "yksi": 1, "kaksi": 2 }
let ${$yksi} := $m
return $yksi

Destructuring a map

XPath

Destructuring III

let $[$uno, $dos] := [ 23, 42 ]
return $uno * $dos

Destructuring an array

XPath

Methods

$my:pool =?> area()

record =?> function(record, *)

$my:pool?area($my:pool)

Functions and Operators

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XQuery

Context I

context item -> context value
set context default value(s)

declare context value as document() external :=
  document { (: the default :) }

Context II

declared namespaces: xs, fn, err

declare namespace array="???";

array:filter([1,-3,4,0], fn {
  math:abs(.) > 1
})

Context II

array:filter([1,-3,4,0], fn {
  math:abs(.) > 1
})

declared namespaces: xs, fn, err, map, array, math

Context II

declare option output:method "json";
array:filter([1,-3,4,0], fn {
  math:abs(.) > 1
})

declared namespaces: xs, fn, err, map, array, math, output

switch I

switch ($n) {
  case "one" return xs:double(1)

  case "two" return xs:double(2)

  default return xs:double('NaN')
}

switch with curly braces

switch II

switch ($n) {
  case "one", "un" return xs:double(1)

  case "two" return xs:double(2)

  default return xs:double("NaN")
}

multiple cases

Declare Custom Types

declare type my:binary
  as (xs:base64binary | xs:hexBinary);

enum

declare type my:color as enum(
  "red", "green", "blue"
)

Declare Custom Types

record

declare type my:rectangle as record(
  height as xs:decimal,
  width as xs:decimal
)

Declare Custom Types

record

declare record my:rectangle (
  height as xs:decimal,
  width as xs:decimal
)

Declare Custom Types

extendable record

declare record my:rectangle (
  height as xs:decimal,
  width as xs:decimal,
  *
)

Declare Custom Types

record with a method

declare record my:rectangle (
  height as xs:decimal,
  width as xs:decimal,
  area as fn(my:rectangle) as xs:decimal := fn ($self) {
    $self?height × $self?width
  }
)

Declare Custom Types

record with a method that is a focus function

declare record my:rectangle (
  height as xs:decimal,
  width as xs:decimal,
  area as fn(my:rectangle) as xs:decimal := fn {
    ?height × ?width
  }
)

Declare Custom Types

try-catch with error map

try {
    error(QName("local:error"), 
        "I can't let you do that, Dave.",
        { "custom": true() })
} catch * {
    trace($err:map)
}

Errors

{
  'code': 'local:error',
  'description': 'I can't let you do that, Dave.',
  'value': { "custom": true() },
  'lineNumber': 2,
  'columnNumber': 4,
  'module': 'my-module.xq',
  'stacktrace': …,
   ⋮
}

Errors

try-catch with finally

try {
    let $i := 0 return 1 div $i
} catch * {
    $err:description
} finally {

    message(`1 div {$i}`)
}