doc: Add maps to reference manual

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+<?xml version="1.0" encoding="utf-8" ?>
+<!DOCTYPE chapter SYSTEM "chapter.dtd">
+
+<chapter>
+  <header>
+    <copyright>
+      <year>2014</year>
+      <holder>Ericsson AB. All Rights Reserved.</holder>
+    </copyright>
+    <legalnotice>
+      The contents of this file are subject to the Erlang Public License,
+      Version 1.1, (the "License"); you may not use this file except in
+      compliance with the License. You should have received a copy of the
+      Erlang Public License along with this software. If not, it can be
+      retrieved online at http://www.erlang.org/.
+
+      Software distributed under the License is distributed on an "AS IS"
+      basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
+      the License for the specific language governing rights and limitations
+      under the License.
+    </legalnotice>
+
+    <title>Maps</title>
+    <prepared></prepared>
+    <docno></docno>
+    <date></date>
+    <rev></rev>
+    <file>maps.xml</file>
+  </header>
+
+  <note>
+	  <p>Maps are considered experimental during OTP 17 and may be subject to change.</p>
+	  <p>The documentation below describes it being possible to use arbitrary
+		  expressions or variables as keys, this is <em>NOT</em> implemented in the current
+		  version of Erlang/OTP.</p>
+	  <p>Exceptions returns <c>badarg</c> instead of <c>badmap</c>, this will change in
+		  the future releases.</p>
+  </note>
+
+  <section>
+	  <title>Creating Maps</title>
+	  <p>
+		  Constructing a new map is done by letting an expression <c>K</c> be associated with
+		  another expression <c>V</c>:
+	  </p>
+	  <code>#{ K => V }</code>
+	  <p>
+		  New maps may include multiple associations at construction by listing every
+		  association:
+	  </p>
+	  <code>#{ K1 => V1, .., Kn => Vn }</code>
+	  <p>
+		  An empty map is constructed by not associating any terms with each other:
+	  </p>
+	  <code>#{}</code>
+	  <p>
+		  All keys and values in the map are terms. Any expression is first evaluated and
+		  then the resulting terms are used as <em>key</em> and <em>value</em> respectively.
+	  </p>
+	  <p>
+		  Keys and values are separated by the <c>=></c> arrow and associations are
+		  separated by <c>,</c>.
+	  </p>
+
+	  <p>
+		  Examples:
+	  </p>
+	  <code>
+M0 = #{},                 % empty map
+M1 = #{a => &lt;&lt;"hello"&gt;&gt;}, % single association with literals
+M2 = #{1 => 2, b => b},   % multiple associations with literals
+M3 = #{k => {A,B}},       % single association with variables
+M4 = #{{"w", 1} => f()}.  % compound key associated with an evaluated expression</code>
+	  <p>
+		  where, <c>A</c> and <c>B</c> are any expressions and <c>M0</c> through <c>M4</c>
+		  are the resulting map terms.
+	  </p>
+	  <p>
+		  If two matching keys are declared, the latter key will take precedence.
+	  </p>
+	  <p>
+		  Example:
+	  </p>
+
+<pre>
+1> <input>#{1 => a, 1 => b}.</input>
+#{1 => b }
+2> <input>#{1.0 => a, 1 => b}.</input>
+#{1 => b, 1.0 => a}
+</pre>
+	  <p>
+		  The order in which the expressions constructing the keys and their
+		  associated values are evaluated is not defined. The syntactic order of
+		  the key-value pairs in the construction is of no relevance, except in
+		  the above mentioned case of two matching keys.
+	  </p>
+  </section>
+
+  <section>
+	  <title>Updating Maps</title>
+	  <p>
+		  Updating a map has similar syntax as constructing it.
+	  </p>
+	  <p>
+		  An expression defining the map to be updated is put in front of the expression
+		  defining the keys to be updated and their respective values.
+	  </p>
+	  <code>M#{ K => V }</code>
+	  <p>
+		  where <c>M</c> is a term of type map and <c>K</c> and <c>V</c> are any expression.
+	  </p>
+	  <p>
+		  If key <c>K</c> does not match any existing key in the map, a new association
+		  will be created from key <c>K</c> to value <c>V</c>.  If key <c>K</c> matches
+		  an existing key in map <c>M</c> its associated value will be replaced by the
+		  new value <c>V</c>. In both cases the evaluated map expression will return a new map.
+	  </p>
+	  <p>
+		  If <c>M</c> is not of type map an exception of type <c>badmap</c> is thrown.
+	  </p>
+	  <p>
+		  To only update an existing value, the following syntax is used,
+	  </p>
+	  <code>M#{ K := V } </code>
+	  <p>
+		  where <c>M</c> is an term of type map, <c>V</c> is an expression and <c>K</c>
+		  is an expression which evaluates to an existing key in <c>M</c>.
+	  </p>
+	  <p>
+		  If key <c>K</c> does not match any existing keys in map <c>M</c> an exception
+		  of type <c>badarg</c> will be triggered at runtime. If a matching key <c>K</c>
+		  is present in map <c>M</c> its associated value will be replaced by the new
+		  value <c>V</c> and the evaluated map expression returns a new map.
+	  </p>
+	  <p>
+		  If <c>M</c> is not of type map an exception of type <c>badmap</c> is thrown.
+	  </p>
+	  <p>
+		  Examples:
+	  </p>
+	  <code>
+M0 = #{},
+M1 = M0#{a => 0},
+M2 = M1#{a => 1, b => 2},
+M3 = M2#{"function" => fun() -> f() end},
+M4 = M3#{a := 2, b := 3}.  % 'a' and 'b' was added in `M1` and `M2`.</code>
+	  <p>
+		  where <c>M0</c> is any map. It follows that <c>M1 .. M4</c> are maps as well.
+	  </p>
+	  <p>
+		  More Examples:
+	  </p>
+<pre>
+1> <input>M = #{1 => a}.</input>
+#{1 => a }
+2> <input>M#{1.0 => b}.</input>
+#{1 => a, 1.0 => b}.
+3> <input>M#{1 := b}.</input>
+#{1 => b}
+4> <input>M#{1.0 := b}.</input>
+** exception error: bad argument
+</pre>
+	  <p>
+		  As in construction, the order in which the key and value expressions
+		  are evaluated is not defined. The
+		  syntactic order of the key-value pairs in the update is of no
+		  relevance, except in the case where two keys match, in which
+		  case the latter value is used.
+	  </p>
+  </section>
+
+  <section>
+	  <title>Maps in Patterns</title>
+	  <p>
+		  Matching of key-value associations from maps is done in the following way:
+	  </p>
+
+	  <code>#{ K := V } = M</code>
+	  <p>
+		  where <c>M</c> is any map. The key <c>K</c> has to be an expression with bound
+		  variables or a literals, and <c>V</c> can be any pattern with either bound or
+		  unbound variables.
+	  </p>
+	  <p>
+		  If the variable <c>V</c> is unbound, it will be bound to the value associated
+		  with the key <c>K</c>, which has to exist in the map <c>M</c>. If the variable
+		  <c>V</c> is bound, it has to match the value associated with <c>K</c> in <c>M</c>.
+	  </p>
+	  <p> Example: </p>
+<code>
+1> <input>M = #{"tuple" => {1,2}}.</input>
+#{"tuple" => {1,2}}
+2> <input>#{"tuple" := {1,B}} = M.</input>
+#{"tuple" => {1,2}}
+3> <input>B.</input>
+2.</code>
+	  <p>
+		  This will bind variable <c>B</c> to integer <c>2</c>.
+	  </p>
+	  <p>
+		  Similarly, multiple values from the map may be matched:
+	  </p>
+	  <code>#{ K1 := V1, .., Kn := Vn } = M</code>
+	  <p>
+		  where keys <c>K1 .. Kn</c> are any expressions with literals or bound variables. If all
+		  keys exist in map <c>M</c> all variables in <c>V1 .. Vn</c> will be matched to the
+		  associated values of their respective keys.
+	  </p>
+	  <p>
+		  If the matching conditions are not met, the match will fail, either with
+	  </p>
+	  <list>
+		  <item>
+			  a <c>badmatch</c> exception, if used in the context of the matching operator
+			  as in the example, 
+		  </item>
+		  <item>
+			  or resulting in the next clause being tested in function heads and
+			  case expressions.
+		  </item>
+	  </list>
+	  <p>
+		  Matching in maps only allows for <c>:=</c> as delimiters of associations.
+		  The order in which keys are declared in matching has no relevance.
+	  </p>
+	  <p>
+		  Duplicate keys are allowed in matching and will match each pattern associated
+		  to the keys.
+	  </p>
+	  <code>#{ K := V1, K := V2 } = M</code>
+	  <p>
+		  Matching an expression against an empty map literal will match its type but
+		  no variables will be bound:
+	  </p>
+	  <code>#{} = Expr</code>
+	  <p>
+		  This expression will match if the expression <c>Expr</c> is of type map, otherwise
+		  it will fail with an exception <c>badmatch</c>.
+	  </p>
+	  <section>
+		  <title>Matching syntax: Example with literals in function heads</title>
+		  <p>
+			  Matching of literals as keys are allowed in function heads.
+		  </p>
+		  <code>
+%% only start if not_started
+handle_call(start, From, #{ state := not_started } = S) ->
+...
+    {reply, ok, S#{ state := start }};
+
+%% only change if started
+handle_call(change, From, #{ state := start } = S) ->
+...
+    {reply, ok, S#{ state := changed }};</code>
+	  </section>
+  </section>
+  <section>
+	  <title>Maps in Guards</title>
+	  <p>
+		  Maps are allowed in guards as long as all sub-expressions are valid guard expressions.
+	  </p>
+	  <p>
+		  Two guard BIFs handles maps:
+	  </p>
+	  <list>
+		  <item>
+			  <seealso marker="erts:erlang#is_map/1">is_map/1</seealso>
+		  </item>
+		  <item>
+			  <seealso marker="erts:erlang#map_size/1">map_size/1</seealso>
+		  </item>
+	  </list>
+  </section>
+</chapter>