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<p>A map is an additional kind of item.</p>
<p>A map comprises a collation and a set of entries. Each entry comprises a key which is an arbitrary atomic value, and an arbitrary sequence called the associated value. Within a map, no two entries have the same key, when compared using the <code>eq</code> operator under the map's collation. It is not necessary that all the keys should be mutually comparable (for example, they can include a mixture of integers and strings). Key values will never be of type <code>xs:untypedAtomic</code>, and they will never be the <code>xs:float</code> or <code>xs:double</code> value <code>NaN</code>.</p> <p>The function call <code>map:get($map, $key)</code> can be used to retrieve the value associated with a given key.</p>
<p>A <em>map</em> can also be viewed as a function from keys to associated values. To achieve this, a map is also a function item. The function corresponding to the map has the signature <code>function($key as xs:anyAtomicValueanyAtomicType) as item()*</code>. Calling the function has the same effect as calling the <code>get</code> function: the expression <code>$map($key)</code> returns the same result as <code>get($map, $key)</code>. For example, if <code>$books-by-isbn</code> is a map whose keys are ISBNs and whose assocated values are <code>book</code> elements, then the expression <code>$books-by-isbn("0470192747")</code> returns the <code>book</code> element with the given ISBN. The fact that a map is a function item allows it to be passed as an argument to higher-order functions that expect a function item as one of their arguments.</p>
<p>Like all other values, <em>maps</em> are immutable. For example, the <code>map:remove</code> function creates a new map by removing an entry from an existing map, but the existing map is not changed by the operation.</p>
<p>Like sequences, <em>maps</em> have no identity. It is meaningful to compare the contents of two maps, but there is no way of asking whether they are "the same map": two maps with the same content are indistinguishable.</p>
