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diff --git a/system/doc/reference_manual/maps.xml b/system/doc/reference_manual/maps.xml new file mode 100644 index 0000000000..78808ce4a2 --- /dev/null +++ b/system/doc/reference_manual/maps.xml @@ -0,0 +1,274 @@ +<?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 => <<"hello">>}, % 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> |