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File 2141-Update-the-documentation-for-comprehensions.patch of Package erlang (Revision 44321c08784c0bcdc6b5f65725943eda)

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From a26949a291183e6e50cf01cab7435b2aa8a1e4d7 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B6rn=20Gustavsson?= <bjorn@erlang.org>
Date: Mon, 23 Jan 2023 06:43:09 +0100
Subject: [PATCH 11/12] Update the documentation for comprehensions

The sections for List Comprehensions and Bit String Comprehensions
have been combined to a single Comprehensions section, which now cover
all three types of comprehensions and generators.
---
 system/doc/reference_manual/expressions.xml | 219 ++++++++++++--------
 1 file changed, 135 insertions(+), 84 deletions(-)

diff --git a/system/doc/reference_manual/expressions.xml b/system/doc/reference_manual/expressions.xml
index b24003e0e6..92506b91a3 100644
--- a/system/doc/reference_manual/expressions.xml
+++ b/system/doc/reference_manual/expressions.xml
@@ -1808,106 +1808,157 @@ end</pre>
 
 <section>
 <marker id="lcs"></marker>
- <title>List Comprehensions</title>
- List comprehensions is a feature of many modern functional
- programming languages. Subject to certain rules, they provide a
- succinct notation for generating elements in a list.
- List comprehensions are analogous to set comprehensions in
- Zermelo-Frankel set theory and are called ZF expressions in
- Miranda. They are analogous to the <c>setof</c> and
- <c>findall</c> predicates in Prolog.
- List comprehensions are written with the following syntax:
- <pre>
-[Expr || Qualifier1,...,QualifierN]</pre>
+ <title>Comprehensions</title>
+ Comprehensions provide a succinct notation for iterating over
+ one or more terms and constructing a new term. Comprehensions come
+ in three different flavors, depending on the type of term they
+ build.
+ List comprehensions construct lists. They have the following syntax:
+ <pre>
+[Expr || Qualifier1, . . ., QualifierN]</pre>
 Here, <c>Expr</c> is an arbitrary expression, and each
- <c>Qualifier</c> is either a generator or a filter.
- <list type="bulleted">
- <item>A generator is written as: 
-
- &nbsp;&nbsp;<c><![CDATA[Pattern <- ListExpr]]></c>. 
-<c>ListExpr</c> must be an expression, which evaluates to a
- list of terms.</item>
-<item>A bit string generator is written as: 
-
- &nbsp;&nbsp;<c><![CDATA[BitstringPattern <= BitStringExpr]]></c>. 
-<c>BitStringExpr</c> must be an expression, which evaluates to a
- bitstring.</item>
- <item>A filter is an expression, which evaluates to
- <c>true</c> or <c>false</c>, or a
- <seeguide marker="#guard_expressions">guard expression</seeguide>.
- If the filter is not a guard expression and evaluates
- to a non-Boolean value <c>Val</c>, an exception
- <c>{bad_filter, Val}</c> is triggered at runtime.</item>
- </list>
+ <c>Qualifier</c> is either a generator or a
+ filter.
+
+ Bit string comprehensions construct bit strings or binaries.
+ They have the following syntax:
+ <pre>
+&lt;&lt; BitStringExpr || Qualifier1, . . ., QualifierN &gt;&gt;</pre>
+
+ <c>BitStringExpr</c> is an expression that evaluates to a bit string.
+ If <c>BitStringExpr</c> is a function call, it must be
+ enclosed in parentheses. Each <c>Qualifier</c> is either a
+ generator or a filter.
+
+ Map comprehensions construct maps. They have the following syntax:
+ <pre>
+#{KeyExpr => ValueExpr || Qualifier1, . . ., QualifierN}</pre>
+ Here, <c>KeyExpr</c> and <c>ValueExpr</c> are arbitrary
+ expressions, and each <c>Qualifier</c> is either a
+ generator or a filter.
+
+ <change>Map comprehensions and map generators were introduced
+ in Erlang/OTP 26.</change>
+
+ There are three kinds of generators.
+
+ A list generator has the following syntax:
+
+<pre>
+Pattern &lt;- ListExpr</pre>
+
+ where <c>ListExpr</c> is an expression that evaluates to a
+ list of terms.
+
+ A bit string generator has the following syntax:
+
+ <pre>
+BitstringPattern &lt;= BitStringExpr</pre>
+
+ where <c>BitStringExpr</c> is an expression that evaluates to a
+ bit string.
+
+ A map generator has the following syntax:
+
+ <pre>
+KeyPattern := ValuePattern &lt;- MapExpression</pre>
+
+ where <c>MapExpr</c> is an expression that evaluates to a map,
+ or a map iterator obtained by calling <seemfa
+ marker="stdlib:maps#iterator/1">maps:iterator/1</seemfa> or
+ <seemfa
+ marker="stdlib:maps#iterator/2">maps:iterator/2</seemfa>.
+
+ A filter is an expression that evaluates to
+ <c>true</c> or <c>false</c>, or a
+ <seeguide marker="#guard_expressions">guard expression</seeguide>.
+ If the filter is not a guard expression and evaluates
+ to a non-Boolean value <c>Val</c>, an exception
+ <c>{bad_filter, Val}</c> is triggered at runtime.
+
 The variables in the generator patterns shadow previously bound variables,
 including variables bound in a previous generator pattern.
- A list comprehension returns a list, where the elements are the
+
+ A list comprehension returns a list, where the lists elements are the
 result of evaluating <c>Expr</c> for each combination of generator
- list elements and bit string generator elements, for which all
+ elements for which all filters are true.
+
+ A bit string comprehension returns a bit string, which is
+ created by concatenating the results of evaluating <c>BitStringExpr</c> for
+ each combination of bit string generator elements for which all
 filters are true.
- Example:
+
+ A map comprehension returns a map, where the map elements are
+ the result of evaluating <c>KeyExpr</c> and <c>ValueExpr</c> for
+ each combination of generator elements for which all filters are
+ true. If the key expressions are not unique, the last occurrence
+ is stored in the map.
+
+ Examples:
+
+ Multiplying each element in a list by two:
 <pre>
 1> <input>[X*2 || X &lt;- [1,2,3]].</input>
 [2,4,6]</pre>
- When there are no generators or bit string generators, a list comprehension
- returns either a list with one element (the result of evaluating <c>Expr</c>)
- if all filters are true or an empty list otherwise.
+
+ Multiplying each byte in a binary by two, returning a list:
+ <pre>
+1> <input>[X*2 || &lt;&lt;X&gt;&gt; &lt;= &lt;&lt;1,2,3&gt;&gt; &gt;&gt;.</input>
+[2,4,6]</pre>
+
+ Multiplying each byte in a binary by two:
+
+ <pre>
+1> <input>&lt;&lt; &lt;&lt;(X*2)&gt;&gt; || &lt;&lt;X&gt;&gt; &lt;= &lt;&lt;1,2,3&gt;&gt; &gt;&gt;.</input>
+&lt;&lt;2,4,6&gt;&gt;</pre>
+
+ Multiplying each element in a list by two, returning a binary:
+
+ <pre>
+1> <input>&lt;&lt; &lt;&lt;(X*2)&gt;&gt; || X &lt;- [1,2,3]].</input>
+&lt;&lt;2,4,6&gt;&gt;</pre>
+
+ Creating a mapping from an integer to its square:
+ <pre>
+1> <input>#{X => X*X || X &lt;- [1,2,3]}.</input>
+#{1 => 1,2 => 4,3 => 9}</pre>
+
+ Multiplying the value of each element in a map by two:
+ <pre>
+1> <input>#{K => 2*V || K := V &lt;- #{a => 1,b => 2,c => 3}}.</input>
+#{a => 2,b => 4,c => 6}</pre>
+
+ Filtering a list, keeping odd numbers:
+ <pre>
+1> <input>[X || X &lt;- [1,2,3,4,5], X rem 2 =:= 1].</input>
+[1,3,5]</pre>
+
+ Filtering a list, keeping only elements that match:
+ <pre>
+1> <input>[X || {_,_}=X &lt;- [{a,b}, [a], {x,y,z}, {1,2}]].</input>
+[{a,b},{1,2}]</pre>
+
+ Combining elements from two list generators:
+ <pre>
+1> <input>[{P,Q} || P &lt;- [a,b,c], Q &lt;- [1,2]].</input>
+[{a,1},{a,2},{b,1},{b,2},{c,1},{c,2}]
+</pre>
+
+ When there are no generators, a comprehension returns either a
+ term constructed from a single element (the result of evaluating
+ <c>Expr</c>) if all filters are true, or a term constructed from
+ no elements (that is, <c>[]</c> for list comprehension,
+ <c>&lt;&lt;&gt;&gt;</c> for a bit string comprehension, and
+ <c>#{}</c> for a map comprehension).
 Example:
 <pre>
 1> <input>[2 || is_integer(2)].</input>
 [2]
 2> <input>[x || is_integer(x)].</input>
 []</pre>
- More examples are provided in
- <seeguide marker="system/programming_examples:list_comprehensions">
- Programming Examples.</seeguide>
 
- </section>
-
-<section>
- <title>Bit String Comprehensions</title> 
- 
- Bit string comprehensions are
- analogous to List Comprehensions. They are used to generate bit strings
- efficiently and succinctly. 
- Bit string comprehensions are written with
- the following syntax:
- <pre>
-&lt;&lt; BitStringExpr || Qualifier1,...,QualifierN &gt;&gt;</pre>
- <c>BitStringExpr</c> is an expression that evaluates to a bit
- string. If <c>BitStringExpr</c> is a function call, it must be
- enclosed in parentheses. Each <c>Qualifier</c> is either a
- generator, a bit string generator or a filter.
- <list type="bulleted">
- <item>A generator is written as: 
- &nbsp;&nbsp;<c><![CDATA[Pattern <- ListExpr]]></c>. 
- <c>ListExpr</c> must be an expression that evaluates to a
- list of terms.</item>
- <item>A bit string generator is written as: 
-
- &nbsp;&nbsp;<c><![CDATA[BitstringPattern <= BitStringExpr]]></c>. 
-<c>BitStringExpr</c> must be an expression that evaluates to a
- bitstring.</item>
- <item>A filter is an expression, which evaluates to
- <c>true</c> or <c>false</c>, or a
- <seeguide marker="#guard_expressions">guard expression</seeguide>.
- If the filter is not a guard expression and evaluates
- to a non-Boolean value <c>Val</c>, an exception
- <c>{bad_filter, Val}</c> is triggered at runtime.</item>
- </list>
- The variables in the generator patterns shadow previously bound variables,
- including variables bound in a previous generator pattern.
- A bit string comprehension returns a bit string, which is 
- created by concatenating the results of evaluating <c>BitString</c> 
- for each combination of bit string generator elements, for which all
- filters are true.
- Example:
- <pre>
-1> <input>&lt;&lt; &lt;&lt; (X*2) &gt;&gt; ||
-&lt;&lt;X&gt;&gt; &lt;= &lt;&lt; 1,2,3 &gt;&gt; &gt;&gt;.</input>
-&lt;&lt;2,4,6&gt;&gt;</pre>
 More examples are provided in
- <seeguide marker="system/programming_examples:bit_syntax">
+ <seeguide marker="system/programming_examples:list_comprehensions">
 Programming Examples.</seeguide>
 </section>
 
-- 
2.35.3

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