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authorBjörn-Egil Dahlberg <[email protected]>2014-05-14 17:07:38 +0200
committerBjörn-Egil Dahlberg <[email protected]>2014-05-14 17:07:38 +0200
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doc: Move Maps reference documentation to expressions
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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>