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