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diff --git a/system/doc/extensions/misc.xml b/system/doc/extensions/misc.xml new file mode 100644 index 0000000000..576f705278 --- /dev/null +++ b/system/doc/extensions/misc.xml @@ -0,0 +1,310 @@ +<?xml version="1.0" encoding="latin1" ?> +<!DOCTYPE chapter SYSTEM "chapter.dtd"> + +<chapter> + <header> + <copyright> + <year>1999</year><year>2009</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>Miscellaneous</title> + <prepared>Arndt Jonasson</prepared> + <docno>1</docno> + <date>99-01-25</date> + <rev>PA1</rev> + <file>misc.sgml</file> + </header> + <p>In this chapter, a number of miscellaneous features of Erlang + are described.</p> + + <section> + <title>Token Syntax</title> + <p>In Erlang 4.8 (OTP R5A) the syntax of Erlang tokens have been + extended to allow the use of the full ISO-8859-1 (Latin-1) character + set. This is noticeable in the following ways:</p> + <list type="bulleted"> + <item>All the Latin-1 printable characters can be used and are shown without + the escape backslash convention.</item> + <item>Atoms and variables can use all Latin-1 letters.</item> + </list> + <p>The new characters from Latin-1 have the following + classifications in Erlang:</p> + <table> + <row> + <cell align="left" valign="middle"><em>Octal</em></cell> + <cell align="left" valign="middle"><em>Decimal</em></cell> + <cell align="left" valign="middle"> </cell> + <cell align="left" valign="middle"><em>Class</em></cell> + </row> + <row> + <cell align="left" valign="middle">200 - 237</cell> + <cell align="left" valign="middle">128 - 159</cell> + <cell align="left" valign="middle"> </cell> + <cell align="left" valign="middle">Control characters</cell> + </row> + <row> + <cell align="left" valign="middle">240 - 277</cell> + <cell align="left" valign="middle">160 - 191</cell> + <cell align="right" valign="middle">- ¿</cell> + <cell align="left" valign="middle">Punctuation characters</cell> + </row> + <row> + <cell align="left" valign="middle">300 - 326</cell> + <cell align="left" valign="middle">192 - 214</cell> + <cell align="center" valign="middle">À - Ö</cell> + <cell align="left" valign="middle">Uppercase letters</cell> + </row> + <row> + <cell align="center" valign="middle">327</cell> + <cell align="center" valign="middle">215</cell> + <cell align="center" valign="middle">×</cell> + <cell align="left" valign="middle">Punctuation character</cell> + </row> + <row> + <cell align="left" valign="middle">330 - 336</cell> + <cell align="left" valign="middle">216 - 222</cell> + <cell align="center" valign="middle">Ø - Þ</cell> + <cell align="left" valign="middle">Uppercase letters</cell> + </row> + <row> + <cell align="left" valign="middle">337 - 366</cell> + <cell align="left" valign="middle">223 - 246</cell> + <cell align="center" valign="middle">ß - ö</cell> + <cell align="left" valign="middle">Lowercase letters</cell> + </row> + <row> + <cell align="center" valign="middle">367</cell> + <cell align="center" valign="middle">247</cell> + <cell align="center" valign="middle">÷</cell> + <cell align="left" valign="middle">Punctuation character</cell> + </row> + <row> + <cell align="left" valign="middle">370 - 377</cell> + <cell align="left" valign="middle">248 - 255</cell> + <cell align="center" valign="middle">ø - ÿ</cell> + <cell align="left" valign="middle">Lowercase letters</cell> + </row> + <tcaption>Character classes</tcaption> + </table> + </section> + + <section> + <title>String Concatenation</title> + <p>Two adjacent string literals are concatenated into one. This is done already + at compile-time, and doesn't incur any runtime overhead. Example:</p> + <code type="none"> + "string" "42" </code> + <p>is equivalent to</p> + <code type="none"> + "string42" </code> + <p>This feature is convenient in at least two situations:</p> + <list type="bulleted"> + <item>when one of the + strings is the result of a macro expansion;</item> + <item>when a string is very + long, and would otherwise either have to wrap, making the source code + harder to read, or force the use of some runtime append operation.</item> + </list> + </section> + + <section> + <title>The ++ list Concatenation Operator</title> + <p>Since list concatenation is a very common operation, it is convenient + to have a terse way of expressing it. The ++ operator appends its second + argument to its first. Example: + </p> + <code type="none"> + X = [1,2,3], + Y = [4,5], + X ++ Y. </code> + <p>results in <c>[1,2,3,4,5]</c>.</p> + <p>The ++ operator has precedence between the binary '+' operator and + the comparison operators. + </p> + </section> + + <section> + <title>The - - list Subtraction Operator</title> + <p>The - - operator produces a list which is a copy of the first + argument, subjected to the following procedure: for each element in + the second argument, its first occurrence in the first argument is + removed.</p> + <code type="none"> + X = [1,2,3,2,1,2], + Y = [2,1,2], + X -- Y. </code> + <p>results in <c>[3,1,2]</c>.</p> + <p>The - - operator has precedence between the binary '+' operator and + the comparison operators. + </p> + </section> + + <section> + <title>Bitwise Operator bnot</title> + <p>Apart from the binary bitwise operators <c>band</c>, <c>bor</c> + and <c>bxor</c>, there is a unary operator <c>bnot</c> with the same + precedence as the other unary operators + and -, i.e., higher than + the binary operators. Example:</p> + <code type="none"> + bnot 7. </code> + <p>returns -8. + </p> + </section> + + <section> + <title>Logical Operators</title> + <p>The atoms <c>true</c> and <c>false</c> are usually used for representing + Boolean values. With the binary operators <c>and</c>, <c>or</c> and + <c>xor</c>, and the unary operator <c>not</c>, Boolean values can be + combined. Example:</p> + <code type="none"> + + M1 = lists:member(A, List1), + M2 = lists:member(A, List2), + M1 and M2.</code> + <p>Note that the operators are strict, i.e., they always evaluate both + their arguments.</p> + <p><c>not</c> has the same priority as the other unary operators. The + binary logical operators have precedence between the <c>=</c> operator + and the comparison operators, the <c>and</c> operator having higher + precedence than <c>or</c> and <c>xor</c>. + </p> + </section> + + <section> + <title>Match Operator = In Patterns</title> + <p>This extension was added in Erlang 4.8 (OTP R5A).</p> + <p>The = operator is also called the `match' operator. The match operator + can now be used in a pattern, so that <c>P1 = P2</c> is a valid pattern, + where both <c>P1</c> and <c>P2</c> are patterns. This compound pattern + when matched against a term causes the term to be matched against both + <c>P1</c> and <c>P2</c>.</p> + <p>One use for this construction is to avoid reconstructing a term which + was part of an argument to a function. Example:</p> + <code type="none"> + f({'+',X,Y}=T) -> {X+Y,T}.</code> + <p>It also makes it possible to rewrite the construction</p> + <code type="none"> + f(X) when X == #rec{x=1, y=a} -> ... </code> + <p>as</p> + <code type="none"> + f(#rec{x=1, y=a} = X) -> ... </code> + <p>In the absence of optimization for the former case, the + latter case is more efficient. + </p> + </section> + + <section> + <title>Literal String Prefix in Patterns</title> + <p>This extension was added in Erlang 4.8 (OTP R5A).</p> + <p>A new construction is allowed in patterns, namely a literal + string as the first operand of the ++ operator. Example:</p> + <code type="none"> + f("prefix" ++ L) -> ... </code> + <p>This is syntactic sugar for the equivalent, but harder to read</p> + <code type="none"> + f([$p,$r,$e,$f,$i,$x | L]) -> ... </code> + </section> + + <section> + <title>Disjunctions in Guards</title> + <p>This extension was added in Erlang 4.9 (OTP R6A).</p> + <p>A new construction is allowed in guards, the disjunction operator + ';'. The construction is syntactic sugar which removes the bother of + writing the same body after several guards.</p> + <code type="none"> + f(X) when xxx ; yyy ; zzz -> + pop(X).</code> + <p>This is syntactic sugar for the equivalent</p> + <code type="none"> + f(X) when xxx -> + pop(X); + f(X) when yyy -> + pop(X); + f(X) when zzz -> + pop(X). </code> + <p>The abstract format has been changed accordingly to contain a list of + (conjunctive) guards where there was previously only one guard. + </p> + </section> + + <section> + <title>Expressions in Patterns</title> + <p>This extension was added in Erlang 5.0 (OTP R7A).</p> + <p>An arithmetic expression can be used within a pattern, if it uses + only numeric or bitwise operators, and if its value can be evaluated + to a constant at compile-time. This is especially useful when the + expression is defined by a macro. + </p> + <p>Example:</p> + <code type="none"> + case X of + {1+2, T} -> T + end.</code> + </section> + + <section> + <title>Boolean expresions in guards</title> + <p>This extension was added in Erlang 5.1 (OTP R8).</p> + <p>In guards, the use of <c>and</c>, <c>or</c> and <c>not</c> is + now allowed. Guard expressions can combine these with + parenthesis. This allows for more elaborate guards than what + may be given with <c>,</c> and <c>;</c>.</p> + <note> + <p>The guard expressions written with these operators are boolean + expressions, and the boolean functions <c>is_list</c>, + <c>is_integer</c> etc. should be used, rather than + <c>list</c>, <c>integer</c> etc.</p> + </note> + <p>Example 1:</p> + <code type="none"> + f(X) when not (is_tuple(X) or is_list(X)) -> + ... </code> + <p>Example 2:</p> + <code type="none"><![CDATA[ + g(A, B) when (A > 0) and (B > 0) and not (A*A < B*B) -> + ... ]]></code> + </section> + + <section> + <title>Short-circuit boolean expressions</title> + <p>This extension was added in Erlang 5.1 (OTP R8).</p> + <p>In a boolean expression it is unnecessary to always evaluate all + terms. If the first term gives a result that determines the + result, the second term is not needed. In Erlang two new + keywords handles boolean expressions without evaluating both + terms, if it's unnecessary. (This is called short-curcuit + boolean evaluation.)</p> + <p>The keyword <c>andalso</c> is a short-curcuit version of + <c>and</c>. The keyword <c>orelse</c> is a short-curcuit version + of <c>or</c>. They can be used in boolean expressions (not + guards) instead of <c>and</c> and <c>or</c>.</p> + <p>Example 1:</p> + <code type="none"> + case A >= -1.0 andalso math:sqrt(A+1) > B of </code> + <p>This will work even if <c>A</c> is less than <c>-1.0</c>, since + in that case, the second term (after <c>andalso</c>) is never + evaluated. (Of course, the same effects could have been done + using guards. In guards, evaluation is always short-circuited, + since guard tests are known to be free of side-effects.)</p> + <p>Example 2:</p> + <code type="none"> + OnlyOne = is_atom(L) orelse + (is_list(L) andalso length(L) == 1), </code> + </section> +</chapter> + |