Building a URL query string from a map of parameters with XPath

Question

What would be the most readable way to build a URL query string from a { 'param': 'value' } map in XSLT/XPath 3.0?

Answer 1

The following function will work:

declare function local:build-uri($base-uri as xs:string, $params as map(xs:string, xs:string)) as xs:string {
  if (map:size($params) ne 0) then
    let $param-string := string-join(
        map:keys($params)[. ne ""] ! (encode-for-uri(.) || "=" || encode-for-uri($params?(.))),
        "&"
      )
    return $base-uri || "?" || $param-string
  else
    $base-uri
};

For example:

declare namespace map = "http://www.w3.org/2005/xpath-functions/map";
declare variable $params := map {
  "one": "1",
  "two": "2",
  "three": "3",
  "four": "4"
};

local:build-uri("http://www.example.com", map{}),
local:build-uri("http://www.example.com", $params),
local:build-uri("", $params),
()

returns:

http://www.example.com
http://www.example.com?four=4&one=1&two=2&three=3
?four=4&one=1&two=2&three=3

Edit: To support multi-value parameters (while keeping the function body compatible with XPath), something like this should work:

declare function local:build-uri(
  $base-uri as xs:string,
  $params as map(xs:string, xs:string*),
  $use-array-for-multivalue-params as xs:boolean (: param[]=value for PHP, etc. :)
) as xs:string {
  if (map:size($params) ne 0) then
    let $param-strings :=
      for $param in map:keys($params)[. ne '']
      return $params?($param) ! string-join((
        encode-for-uri($param),
        if ($use-array-for-multivalue-params and count($params?($param)) gt 1) then "[]" else "",
        "=",
        encode-for-uri(.)
      ), "")
    return $base-uri || "?" || string-join($param-strings, "&")
  else
    $base-uri
};

Answer 2

Not short, nor necessarily easy to understand.

BUT

• it handles null values (with csv you get key= the other to omit the key entirely)
• it handles xs:anyAtomicType (xs:dateTime, xs:decimal, xs:boolean, ...)
• adds a third, common way to serialize the query string parameters with multiple values separating the with a comma

xquery version "3.1";


declare namespace qs="http://line-o.de/ns/qs";


(:~
 : Append nothing to names of parameters with multiple values 
 : ?single=v1&multi=v2&multi=v3 
 :)
declare function qs:serialize-query-string($parameters as map(xs:string, xs:anyAtomicType*)) as xs:string? {
  qs:serialize(
    $parameters,
    qs:serialize-parameter(?, ?, ()))
};

(:~
 : Append [] to names of parameters with multiple values 
 : ?single=v1&multi[]=v2&multi[]=v3 
 :)
declare function qs:serialize-query-string-array($parameters as map(xs:string, xs:anyAtomicType*)) as xs:string? {
  qs:serialize(
    $parameters,
    qs:serialize-parameter(?, ?, '[]'))
};

(:~
 : Commma separated values for parameters with multiple values 
 : ?single=v1&multi=v2,v3 
 :)
declare function qs:serialize-query-string-csv($parameters as map(xs:string, xs:anyAtomicType*)) as xs:string? {
  qs:serialize(
    $parameters,
    qs:serialize-parameter-csv#2)
};

declare function qs:serialize(
   $parameters as map(xs:string, xs:anyAtomicType*),
   $serializer as function(xs:string, xs:anyAtomicType*) as xs:string*
) as xs:string? {
    if (map:size($parameters) eq 0)
    then ()
    else
      $parameters
        => map:for-each($serializer)
        => string-join('&')
        => qs:prepend-questionmark()
};

declare function qs:serialize-parameter (
  $raw-parameter-name as xs:string,
  $values as xs:anyAtomicType*,
  $appendix as xs:string?
) as xs:string* {
    let $parameter-name := concat(
        encode-for-uri($raw-parameter-name),
        if (exists($values) and count($values)) then $appendix else ()
    )

    return
        for-each($values, 
            qs:serialize-parameter-value($parameter-name, ?))
};

declare function qs:serialize-parameter-csv ($raw-parameter-name as xs:string, $values as xs:anyAtomicType*) as xs:string* {
  concat(
    encode-for-uri($raw-parameter-name), 
    '=',
    $values
      => for-each(function ($value) { encode-for-uri(xs:string($value)) })
      => string-join(',')
  )
};

declare function qs:serialize-parameter-value (
  $parameter as xs:string, $value as xs:anyAtomicType
) as xs:string {
    ``[`{$parameter}`=`{encode-for-uri($value)}`]``
};

declare function qs:prepend-questionmark ($query-string as xs:string) { 
  concat('?', $query-string)
};

qs:serialize-query-string(map{}),
qs:serialize-query-string-array(map{}),
qs:serialize-query-string-csv(map{}),
qs:serialize-query-string(map{ "a": ("b0","b1"), "b": "$=@#'" }),
qs:serialize-query-string-array(map{ "a": (xs:date("1970-01-01"),"b1"), "b": "$=@#'" }),
qs:serialize-query-string-csv(map{ "a": ("b0",3.14), "b": "$=@#'" }),
qs:serialize-query-string(map{ "a": ("b0","b1"), "c": () }),
qs:serialize-query-string-array(map{ "a": ("b0","b1"), "c": () }),
qs:serialize-query-string-csv(map{ "a": ("b0","b1"), "c": () })

Here is a gist with the above split into a module and tests:

https://gist.github.com/line-o/e492401494a4e003bb01b7a2f884b027

EDIT: less code duplication

Answer 3

let 
   $encode-parameters-for-uri:= function($parameters as map(*)) as xs:string? {
      let 
         (: serialize each map entry :)
         $encoded-parameters:= map:for-each(
            $parameters,
            function ($key, $values) {
               (: serialize the sequence of values for this key :)
               for $value in $values return 
                  encode-for-uri($key) || '=' || encode-for-uri($value)
            }
         ),
         (: join the URI parameters with ampersands :)
         $parameters-string:= string-join(
            $encoded-parameters,
            codepoints-to-string(38)
         )
      return
         (: prepend '?' if parameters exist :)
         if ($parameters-string) then
            '?' || $parameters-string
         else
            ()
   }
return
   $encode-parameters-for-uri(
      map{
         'size': 'large',
         'flavour': ('chocolate', 'strawberry')
      }
   )

result: ?flavour=chocolate&flavour=strawberry&size=large

A more concise version, also differing in that it converts an empty map into a zero-length string rather than an empty sequence of strings:

let 
   $encode-parameters-for-uri:= function($parameters as map(*)) as xs:string {
      if (map:size($parameters)) then
         '?' || string-join(
            map:for-each(
               $parameters,
               function ($key, $values) {
                  for $value in $values return 
                     encode-for-uri($key) || '=' || encode-for-uri($value)
               }
            ),
            codepoints-to-string(38)
         )
      else
         ''
   }
return
   $encode-parameters-for-uri(
      map{
      'foo': ('bar', 'baz'), 'direction': 'north'
      }
   )

result ?direction=north&foo=bar&foo=baz