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# Array Constructors

Array constructors come in two flavors: square and curly constructors. The square constructor will look familiar to most people:

`xquery let $array := [1, (), (3, 4)] return $array(3) `

Within the square constructor, the "," is just a separator (like in a function call), so the resulting array will correspond to the comma-separated members. In the example above we're retrieving the third member using $array(3), which is the sequence containing numbers 3 and 4. Getting the second member with $array(2) will return the empty sequence accordingly.

The curly constructor behaves slightly different: it takes a sequence of items and creates an array member from each of them:

```xquery let $array := array 1, (), (3, 4) return $array(3) ```

Above query returns "4"! See the difference? The "," in this case is the XQuery sequence constructor, so the sequence from which the array is built is 1, 3, 4.

As already announced, you can arbitrarily mix sequences, arrays and maps, resulting e.g. in:

```xquery let $books := [ map "eXist", "author": [ [ "Adam", "Retter" ], ["Erik", "Siegel" ] ], "language": "English" , map "XQuery", "author": [ [ " Priscilla", "Walmsley" ] ], "language": "English" ] ```

# Array Lookups

Just like a map, an array is also a function, which accepts a single integer parameter corresponding to the position of the member to retrieve (as always in XQuery, counting starts at 1). We have seen simple examples above. For nested data structures, just chain the function calls, e.g.:

`xquery $books(2)("title") `

Alternatively, there's a lookup operator, which is often a bit easier to read. It works on arrays as well as maps (but not on other data types):

`xquery $books?2?title `

The lookup may also appear inside a predicate. In this case, the left hand argument (the array or map) is often skipped and defaults to the context item:

`xquery $books?*[?title = "eXist"]?language `

The operator expects an integer, name, parenthesized expression or a wildcard as its right hand argument. So to use e.g. a variable for the lookup, wrap it into parens:

`xquery let $field := "title" return $books?2?($field) `

Note: because $books is an array, the lookup argument must evaluate to a sequence of integers or you'll see an error. It is possible to look up more than one array item at a time, e.g.: $books?(1 to 2)?title.

The wildcard returns the keys or members of a map or array. When used on a map, it results in a sequence of keys, whereas on an array, you get a sequence of members:

`xquery ["Hello", "world", "!"]?* `

Use the wildcard as a quick way to iterate an array:

`xquery for $book in $books?* return $book?title `

# Function Library

The XQuery 3.1 functions spec also includes a huge library of functions to process and modify arrays. All functions use the prefix array.

As all data types in XQuery, arrays are immutable and cannot be modified. The functions above will thus always return a new array. eXist tries to implement this in an efficient way for functions like array:tail, array:append, array:subarray, array:remove without creating redundant copies.

Please note that I did not implement array:sort yet. It will be added later.

Many of the functions mirror other functions already available in the standard function library, but take an array instead of a sequence as input. For example, array:fold-left works like fn:fold-left.

```xquery declare function local:price($hoursPerTask as array(xs:integer), $rate as xs:double) as xs:double fold-left($hoursPerTask, 0.0, function($sum, $hours) { $sum + $hours * $rate ) };

local:price([3, 8, 6, 5, 2], 96.0) ```

In this example we multiply the hours required for some task by our hourly rate and return the sum.

# JSON Support

Obviously, representing JSON data within an XQuery has become straightforward using maps and arrays. The function fn:parse-json takes a string of JSON data and returns either a map (for a JSON object), an array, an atomic value (xs:string, xs:double for numbers or xs:boolean), or the empty sequence (corresponding to null in JSON):

```xquery let $json := '"bob", "id": 10, "valid": true' let $user := parse-json($json) return $user?id ```

Note that by default parse-json is rather strict about the JSON syntax. For example, strings must use double quotes and duplicate keys generate an error. You can tell the function to be more relaxed about the JSON syntax by passing in a map of options:

```xquery let $json := "'bob', 'id': 10, 'valid': true" let $options := map true(), "duplicates": "use-last" let $user := parse-json($json, $options) return $user?id ```

To see the function in action on a real-world example, assume we would like to retrieve a list of commits from a git repository, using the HTTP/JSON API provided by github:

```xquery xquery version "3.1";

import module namespace http="http://expath.org/ns/http-client";

declare function local:log($json as array(*)) = $entry?commit return <tr> <td>{$commit?committer?date</td> <td>$commit?committer?name</td> <td>$commit?message</td> </tr> } </table> };

let $url := "https://api.github.com/repos/eXist-db/exist/commits?since=2015-01-01T00:00:00Z" let $request := <http:request method="GET" href="$url" timeout="30"/> let $response := http:send-request($request) return if ($response[1]/@status = "200") then local:log(parse-json(util:binary-to-string($response[2]))) else () ```

Here we're using the httpclient module to talk to the github API, which gives us more control over the communication. But there's also a simpler approach, using the fn:json-docfunction:

`xquery let $url := "https://api.github.com/repos/eXist-db/exist/commits?since=2015-01-01T00:00:00Z" return local:log(json-doc($url)) `

fn:json-doc retrieves the contents of the given URI and parses them using fn:parse-json. It works with external resources as well as binary documents stored in eXist. To access stored resources, just use a local path, e.g. /db/test/data.json.

# Serialization

eXistdb has supported serialization to JSON for several years, but the old serializer was based on mapping an XML query result to JSON, which caused some difficulties at times, e.g. if you had to produce an array for a certain property, even if it was empty. Contrary to this, the new JSON output method defined by the XQuery 3.1 Serialization spec is straightforward: it takes an array, map, atomic value or empty sequence and produces valid JSON.

The JSON serializer is selected if you set the serialization option method to json. This applies to both, the old and the new serializer. To distinguish between the two while preserving backwards compatibility, we use the following convention:

• if the sequence to serialize is a single XML element node, the old serializer is used
• if the sequence contains more than one item or the single item is not an XML element, it will be passed to the new serializer

This convention allows us to run all the old code unchanged without violating the 3.1 specification too much (according to the specs, a single XML element would be serialized to a JSON string).

To see the serializer in action, use the fn:serialize function:

```xquery xquery version "3.1";

declare namespace output="http://www.w3.org/2010/xslt-xquery-serialization";

let $array := map array { "v1", "v2" , "k2": "v3" } return serialize($array, <output:serialization-parameters> <output:method>json</output:method> </output:serialization-parameters>) ```

or save the query and define the serialization method as an output option:

```xquery xquery version "3.1";

declare namespace output="http://www.w3.org/2010/xslt-xquery-serialization"; declare option output:method "json"; declare option output:media-type "application/json";

map array { "v1", "v2" , "k2": "v3" } ```

# Availability

The new features will be available in the eXistdb 2.3 release, but we encourage users to help us testing. We tried to preserve backwards compatibility with existing XQuery code, so most, if not all, apps should work as before.

To experiment with arrays and JSON, just build from source or use a nightly. You may look through the test cases for some inspiration.

Finally, I also recommend watching Dannes' presentation on Mongrel: the MongoDB extension driver for eXistdb, which will rely on the features described in this article.

## What is ModelValidator?

'ModelValidator' is a new facility becoming available in eXistdb very soon that improves this situation. It automatically generates a server-side XForms model that reflects the constraints you've put onto your HTML controls.

Once the user submits the form ModelValidator will intercept the request and generate an XForms model that uses the original HTML document as input and will setup all constraints expressed in the HTML for server-side revalidation. Only in case the incoming data conform to the constraints the submission is forwarded to the action url expressed in the HTML form.

In the context of eXistdb you likely want to send the data to some XQuery script. As its tedious to read all the params one by one ModelValidator does more - it will transform the key/value pairs into an XML structure that you can use as input to your XQuery. Now that you can be sure that your data have been validated before reaching your script you can directly work with the XML.

But ModelValidator can do even more if you are willing to dive into the XForms Model syntax - every time an XForms model is generated for a form it will be stored into the database. When a new request comes in it will check if a model has already been generated and will use it if that's the case. You can now edit the generated model to establish even more rigid constraints that HTML5 does not allow to express.

Last but not least the current prototype does not even need any JavaScript to work - just plain HTML.

## What comes next?

The current prototype does not support all HTML5 datatypes yet and there's still some polishing and testing to do before we can roll it out. Further we have to work out a decent deployment into eXistdb. Hopefully we can publish it with the next version of eXistdb.

ModelValidator is just the first step in our redesign of betterFORM. To provide instant validation once the user has entered a value and to offer more fancy ways of alerting the next iteration will add JavaScript again to speak to the server under the hood. We have decided for modernizing the client-side technologies we're using. Instead of Dojo we'll go for Web Components (likely Polymer) and for the transport we'll exchange DWR (Direct Web Remoting) with Websockets.

## What will happen to 'full' XForms support?

Though we decided to move away from full XForms support (see betterFORM Blog) in betterFORM 6 we made some effort to further support betterFORM 5 as the fully XForms 1.1 compliant alternative. Thus you will be able to use betterFORM 5 and 6 (once it's there) side-by-side in eXistdb. There won't be any change for those that are using betterFORM 5 in their applications. It will just work as before.