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<chapter>
  <header>
    <copyright>
      <year>2003</year><year>2020</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at
 
          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
      limitations under the License.

    </legalnotice>

    <title>Expressions</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
    <file>expressions.xml</file>
  </header>
  <p>In this section, all valid Erlang expressions are listed.
    When writing Erlang programs, it is also allowed to use macro-
    and record expressions. However, these expressions are expanded
    during compilation and are in that sense not true Erlang
    expressions. Macro- and record expressions are covered in
    separate sections:
   </p>
  <list type="bulleted">
    <item><p><seeguide marker="macros">Preprocessor</seeguide></p></item>
    <item><p><seeguide marker="records">Records</seeguide></p></item>
  </list>

  <section>
    <title>Expression Evaluation</title>
    <p>All subexpressions are evaluated before an expression itself is
      evaluated, unless explicitly stated otherwise. For example,
      consider the expression:</p>
    <code type="none">
Expr1 + Expr2</code>
    <p><c>Expr1</c> and <c>Expr2</c>, which are also expressions, are
      evaluated first - in any order - before the addition is
      performed.</p>
    <p>Many of the operators can only be applied to arguments of a
      certain type. For example, arithmetic operators can only be
      applied to numbers. An argument of the wrong type causes
      a <c>badarg</c> runtime error.</p>
  </section>

  <section>
    <marker id="term"></marker>
    <title>Terms</title>
    <p>The simplest form of expression is a term, that is an integer,
      float, atom, string, list, map, or tuple.
      The return value is the term itself.</p>
  </section>

  <section>
    <title>Variables</title>
    <p>A variable is an expression. If a variable is bound to a value,
      the return value is this value. Unbound variables are only
      allowed in patterns.</p>
    <p>Variables start with an uppercase letter or underscore (_).
      Variables can contain alphanumeric characters, underscore and <c>@</c>.
     </p>
     <p><em>Examples:</em></p>
    <pre>
X
Name1
PhoneNumber
Phone_number
_
_Height</pre>
    <p>Variables are bound to values using
      <seeguide marker="patterns">pattern matching</seeguide>. Erlang
      uses <em>single assignment</em>, that is, a variable can only be bound
      once.</p>
    <p>The <em>anonymous variable</em> is denoted by underscore (_) and
      can be used when a variable is required but its value can be
      ignored.</p>
      <p><em>Example:</em></p>
    <pre>
[H|_] = [1,2,3]</pre>
    <p>Variables starting with underscore (_), for example,
      <c>_Height</c>, are normal variables, not anonymous. They are
      however ignored by the compiler in the sense that they do not
      generate any warnings for unused variables.</p>
      <p><em>Example:</em></p>
      <p>The following code:</p>
    <pre>
member(_, []) ->
    [].</pre>
    <p>can be rewritten to be more readable:</p>
    <pre>
member(Elem, []) ->
    [].</pre>
    <p>This causes a warning for an unused variable,
      <c>Elem</c>, if the code is compiled with the flag
      <c>warn_unused_vars</c> set. Instead, the code can be rewritten
      to:</p>
    <pre>
member(_Elem, []) ->
    [].</pre>
    <p>Notice that since variables starting with an underscore are
      not anonymous, this matches:</p>
    <pre>
{_,_} = {1,2}</pre>
    <p>But this fails:</p>
    <pre>
{_N,_N} = {1,2}</pre>
    <p>The scope for a variable is its function clause.
      Variables bound in a branch of an <c>if</c>, <c>case</c>, 
      or <c>receive</c> expression must be bound in all branches 
      to have a value outside the expression. Otherwise they
      are regarded as 'unsafe' outside the expression.</p>
    <p>For the <c>try</c> expression variable scoping is limited so that
      variables bound in the expression are always 'unsafe' outside 
      the expression.</p>
  </section>

  <section>
    <marker id="pattern"></marker>
    <title>Patterns</title>
    <p>A pattern has the same structure as a term but can contain
      unbound variables.</p>
      <p><em>Example:</em></p>
    <pre>
Name1
[H|T]
{error,Reason}</pre>
    <p>Patterns are allowed in clause heads, <c>case</c> and
      <c>receive</c> expressions, and match expressions.</p>

    <section>
      <title>Match Operator = in Patterns</title>
      <p>If <c>Pattern1</c> and <c>Pattern2</c> are valid patterns,
        the following is also a valid pattern:</p>
      <pre>
Pattern1 = Pattern2</pre>
      <p>When matched against a term, both <c>Pattern1</c> and
        <c>Pattern2</c> are  matched against the term. The idea
        behind this feature is to avoid reconstruction of terms.</p>
       <p><em>Example:</em></p>
      <pre>
f({connect,From,To,Number,Options}, To) ->
    Signal = {connect,From,To,Number,Options},
    ...;
f(Signal, To) ->
    ignore.</pre>
      <p>can instead be written as</p>
      <pre>
f({connect,_,To,_,_} = Signal, To) ->
    ...;
f(Signal, To) ->
    ignore.</pre>
    </section>

    <section>
      <title>String Prefix in Patterns</title>
      <p>When matching strings, the following is a valid pattern:</p>
      <pre>
f("prefix" ++ Str) -> ...</pre>
      <p>This is syntactic sugar for the equivalent, but harder to
        read:</p>
      <pre>
f([$p,$r,$e,$f,$i,$x | Str]) -> ...</pre>
    </section>

    <section>
      <title>Expressions in Patterns</title>
      <p>An arithmetic expression can be used within a pattern if
        it meets both of the following two conditions:</p>
      <list type="bulleted">
        <item>It uses only numeric or bitwise operators.</item>
        <item>Its value can be evaluated to a constant when complied.</item>
      </list>
      <p><em>Example:</em></p>
      <pre>
case {Value, Result} of
    {?THRESHOLD+1, ok} -> ...</pre>
    </section>
  </section>

  <section>
    <title>Match</title>
    <p>The following matches <c>Expr1</c>, a pattern, against
      <c>Expr2</c>:</p>
    <pre>
Expr1 = Expr2</pre>
    <p>If the matching succeeds, any unbound variable in the pattern
      becomes bound and the value of <c>Expr2</c> is returned.</p>
    <p>If the matching fails, a <c>badmatch</c> run-time error occurs.</p>
    <p><em>Examples:</em></p>
    <pre>
1> <input>{A, B} = {answer, 42}.</input>
{answer,42}
2> <input>A.</input>
answer
3> <input>{C, D} = [1, 2].</input>
** exception error: no match of right-hand side value [1,2]</pre>
  </section>

  <section>
    <marker id="calls"></marker>
    <title>Function Calls</title>
    <pre>
ExprF(Expr1,...,ExprN)
ExprM:ExprF(Expr1,...,ExprN)</pre>
    <p>In the first form of function calls,
      <c>ExprM:ExprF(Expr1,...,ExprN)</c>, each of <c>ExprM</c> and
      <c>ExprF</c> must be an atom or an expression that evaluates to
      an atom. The function is said to be called by using the
      <em>fully qualified function name</em>. This is often referred
      to as a <em>remote</em> or <em>external function call</em>.</p>
      <p><em>Example:</em></p>
    <code type="none">
lists:keysearch(Name, 1, List)</code>
    <p>In the second form of function calls,
      <c>ExprF(Expr1,...,ExprN)</c>, <c>ExprF</c> must be an atom or
      evaluate to a fun.</p>

    <p>If <c>ExprF</c> is an atom, the function is said to be called by
      using the <em>implicitly qualified function name</em>.  If the
      function <c>ExprF</c> is locally defined, it is called.
      Alternatively, if <c>ExprF</c> is explicitly imported from the
      <c>M</c> module, <c>M:ExprF(Expr1,...,ExprN)</c> is called. If
      <c>ExprF</c> is neither declared locally nor explicitly
      imported, <c>ExprF</c> must be the name of an automatically
      imported BIF. </p>
      <p><em>Examples:</em></p>

    <code type="none">
handle(Msg, State)
spawn(m, init, [])</code>
    <p><em>Examples</em> where <c>ExprF</c> is a fun:</p>
    <pre>
1> <input>Fun1 = fun(X) -> X+1 end,</input>
<input>Fun1(3).</input>
4
2> <input>fun lists:append/2([1,2], [3,4]).</input>
[1,2,3,4]
3> </pre>

    <p>Notice that when calling a local function, there is a difference
    between using the implicitly or fully qualified function name.
    The latter always refers to the latest version of the module.
    See <seeguide marker="code_loading">Compilation and Code Loading
    </seeguide> and <seeguide marker="functions#eval">
    Function Evaluation</seeguide>.</p>

    <section>
      <title>Local Function Names Clashing With  Auto-Imported BIFs</title>
    <p>If a local function has the same name as an auto-imported BIF,
    the semantics is that implicitly qualified function calls are
    directed to the locally defined function, not to the BIF. To avoid
    confusion, there is a compiler directive available,
    <c>-compile({no_auto_import,[F/A]})</c>, that makes a BIF not
    being auto-imported. In certain situations, such a compile-directive
    is mandatory.</p>

    <warning><p>Before OTP R14A (ERTS version 5.8), an implicitly
    qualified function call to a function having the same name as an
    auto-imported BIF always resulted in the BIF being called. In
    newer versions of the compiler, the local function is called instead.
    This is to avoid that future additions to the
    set of auto-imported BIFs do not silently change the behavior
    of old code.</p>

    <p>However, to avoid that old (pre R14) code changed its
    behavior when compiled with OTP version R14A or later, the
    following restriction applies: If you override the name of a BIF
    that was auto-imported in OTP versions prior to R14A (ERTS version
    5.8) and have an implicitly qualified call to that function in
    your code, you either need to explicitly remove the auto-import
    using a compiler directive, or replace the call with a fully
    qualified function call. Otherwise you get a compilation
    error. See the following example:</p> </warning>

    <code type="none">
-export([length/1,f/1]).

-compile({no_auto_import,[length/1]}). % erlang:length/1 no longer autoimported

length([]) ->
    0;
length([H|T]) ->
    1 + length(T). %% Calls the local function length/1

f(X) when erlang:length(X) > 3 -> %% Calls erlang:length/1,
                                  %% which is allowed in guards
    long.</code>

    <p>The same logic applies to explicitly imported functions from
    other modules, as to locally defined functions.
    It is not allowed to both import a
    function from another module and have the function declared in the
    module at the same time:</p>

    <code type="none">
-export([f/1]).

-compile({no_auto_import,[length/1]}). % erlang:length/1 no longer autoimported

-import(mod,[length/1]).

f(X) when erlang:length(X) > 33 -> %% Calls erlang:length/1,
                                   %% which is allowed in guards

    erlang:length(X);              %% Explicit call to erlang:length in body

f(X) ->
    length(X).                     %% mod:length/1 is called</code>


    <p>For auto-imported BIFs added in Erlang/OTP R14A and thereafter,
    overriding the name with a local function or explicit import is always
    allowed. However, if the <c>-compile({no_auto_import,[F/A])</c>
    directive is not used, the compiler issues a warning whenever
    the function is called in the module using the implicitly qualified
    function name.</p>
    </section>
  </section>

  <section>
    <title>If</title>
    <pre>
if
    GuardSeq1 ->
        Body1;
    ...;
    GuardSeqN ->
        BodyN
end</pre>
    <p>The branches of an <c>if</c>-expression are scanned sequentially
      until a guard sequence <c>GuardSeq</c> that evaluates to true is
      found. Then the corresponding <c>Body</c> (sequence of expressions
      separated by ',') is evaluated.</p>
    <p>The return value of <c>Body</c> is the return value of
      the <c>if</c> expression.</p>
    <p>If no guard sequence is evaluated as true,
      an <c>if_clause</c> run-time error
      occurs. If necessary, the guard expression <c>true</c> can be
      used in the last branch, as that guard sequence is always true.</p>
    <p><em>Example:</em></p>
    <pre>
is_greater_than(X, Y) ->
    if
        X>Y ->
            true;
        true -> % works as an 'else' branch
            false
    end</pre>
  </section>

  <section>
    <marker id="case"></marker>
    <title>Case</title>
    <pre>
case Expr of
    Pattern1 [when GuardSeq1] ->
        Body1;
    ...;
    PatternN [when GuardSeqN] ->
        BodyN
end</pre>
    <p>The expression <c>Expr</c> is evaluated and the patterns
      <c>Pattern</c> are sequentially matched against the result. If a
      match succeeds and the optional guard sequence <c>GuardSeq</c> is
      true, the corresponding <c>Body</c> is evaluated.</p>
    <p>The return value of <c>Body</c> is the return value of
      the <c>case</c> expression.</p>
    <p>If there is no matching pattern with a true guard sequence,
      a <c>case_clause</c> run-time error occurs.</p>
    <p><em>Example:</em></p>
    <pre>
is_valid_signal(Signal) ->
    case Signal of
        {signal, _What, _From, _To} ->
            true;
        {signal, _What, _To} ->
            true;
        _Else ->
            false
    end.</pre>
  </section>

  <section>
    <marker id="send"></marker>
    <title>Send</title>
    <pre>
Expr1 ! Expr2</pre>
    <p>Sends the value of <c>Expr2</c> as a message to the process
      specified by <c>Expr1</c>. The value of <c>Expr2</c> is also
      the return value of the expression.</p>
    <p><c>Expr1</c> must evaluate to a pid, a registered name (atom), or
      a tuple <c>{Name,Node}</c>. <c>Name</c> is an atom and
      <c>Node</c> is a node name, also an atom.</p>
    <list type="bulleted">
      <item>If <c>Expr1</c> evaluates to a name, but this name is not
       registered, a <c>badarg</c> run-time error occurs.</item>
      <item>Sending a message to a pid never fails, even if the pid
       identifies a non-existing process.</item>
      <item>Distributed message sending, that is, if <c>Expr1</c>
       evaluates to a tuple <c>{Name,Node}</c> (or a pid located at
       another node), also never fails.</item>
    </list>
  </section>

  <section>
    <marker id="receive"></marker>
    <title>Receive</title>
    <pre>
receive
    Pattern1 [when GuardSeq1] ->
        Body1;
    ...;
    PatternN [when GuardSeqN] ->
        BodyN
end</pre>
    <p>Receives messages sent to the process using the send operator
      (!). The patterns <c>Pattern</c> are sequentially matched
      against the first message in time order in the mailbox, then
      the second, and so on. If a match succeeds and the optional
      guard sequence <c>GuardSeq</c> is true, the corresponding
      <c>Body</c> is evaluated. The matching message is consumed, that
      is, removed from the mailbox, while any other messages in
      the mailbox remain unchanged.</p>
    <p>The return value of <c>Body</c> is the return value of
      the <c>receive</c> expression.</p>
    <p><c>receive</c> never fails. The execution is suspended, possibly
      indefinitely, until a message arrives that matches one of
      the patterns and with a true guard sequence. </p>
    <p><em>Example:</em></p>
    <pre>
wait_for_onhook() ->
    receive
        onhook ->
            disconnect(),
            idle();
        {connect, B} ->
            B ! {busy, self()},
            wait_for_onhook()
    end.</pre>
    <p>The <c>receive</c> expression can be augmented with a
      timeout:</p>
    <pre>
receive
    Pattern1 [when GuardSeq1] ->
        Body1;
    ...;
    PatternN [when GuardSeqN] ->
        BodyN
after
    ExprT ->
        BodyT
end</pre>
    <p><c>ExprT</c> is to evaluate to an integer. The highest allowed
      value is 16#FFFFFFFF, that is, the value must fit in 32 bits.
      <c>receive..after</c> works exactly as <c>receive</c>, except
      that if no matching message has arrived within <c>ExprT</c>
      milliseconds, then <c>BodyT</c> is evaluated instead. The
      return value of <c>BodyT</c> then becomes the return value
      of the <c>receive..after</c> expression.</p>
    <p><em>Example:</em></p>
    <pre>
wait_for_onhook() ->
    receive
        onhook ->
            disconnect(),
            idle();
        {connect, B} ->
            B ! {busy, self()},
            wait_for_onhook()
    after
        60000 ->
            disconnect(),
            error()
    end.</pre>
    <p>It is legal to use a <c>receive..after</c> expression with no
      branches:</p>
    <pre>
receive
after
    ExprT ->
        BodyT
end</pre>
    <p>This construction does not consume any messages, only suspends
      execution in the process for <c>ExprT</c> milliseconds. This can be
      used to implement simple timers.</p>
    <p><em>Example:</em></p>
    <pre>
timer() ->
    spawn(m, timer, [self()]).

timer(Pid) ->
    receive
    after
        5000 ->
            Pid ! timeout
    end.</pre>
    <p>There are two special cases for the timeout value <c>ExprT</c>:</p>
    <taglist>
      <tag><c>infinity</c></tag>
      <item>The process is to wait indefinitely for a matching message;
       this is the same as not using a timeout. This can be
       useful for timeout values that are calculated at runtime.</item>
      <tag>0</tag>
      <item>If there is no matching message in the mailbox, the timeout
       occurs immediately.</item>
    </taglist>
  </section>

  <section>
    <title>Term Comparisons</title>
    <pre>
Expr1 <input>op</input> Expr2</pre>
    <table>
      <row>
        <cell align="left" valign="middle"><em>op</em></cell>
        <cell align="left" valign="middle"><em>Description</em></cell>
      </row>
      <row>
        <cell align="left" valign="middle">==</cell>
        <cell align="left" valign="middle">Equal to</cell>
      </row>
      <row>
        <cell align="left" valign="middle">/=</cell>
        <cell align="left" valign="middle">Not equal to</cell>
      </row>
      <row>
        <cell align="left" valign="middle">=&lt;</cell>
        <cell align="left" valign="middle">Less than or equal to</cell>
      </row>
      <row>
        <cell align="left" valign="middle">&lt;</cell>
        <cell align="left" valign="middle">Less than</cell>
      </row>
      <row>
        <cell align="left" valign="middle">&gt;=</cell>
        <cell align="left" valign="middle">Greater than or equal to</cell>
      </row>
      <row>
        <cell align="left" valign="middle">&gt;</cell>
        <cell align="left" valign="middle">Greater than</cell>
      </row>
      <row>
        <cell align="left" valign="middle">=:=</cell>
        <cell align="left" valign="middle">Exactly equal to</cell>
      </row>
      <row>
        <cell align="left" valign="middle">=/=</cell>
        <cell align="left" valign="middle">Exactly not equal to</cell>
      </row>
      <tcaption>Term Comparison Operators.</tcaption>
    </table>
    <p>The arguments can be of different data types. The following
      order is defined:</p>
    <pre>
number &lt; atom &lt; reference &lt; fun &lt; port &lt; pid &lt; tuple &lt; map &lt; nil &lt; list &lt; bit string</pre>
    <p><c>nil</c> in the previous expression represents the empty list
      (<c>[]</c>), which is regarded as a separate type from
      <c>list/0</c>. That is why <c>nil &lt; list</c>.
    </p>
    <p>Lists are compared element by element. Tuples are ordered by
      size, two tuples with the same size are compared element by
      element.</p>
    <p>Bit strings are compared bit by bit. If one bit string is a
      prefix of the other, the shorter bit string is considered smaller.</p>
    <p>Maps are ordered by size, two maps with the same size are compared by keys in
        ascending term order and then by values in key order.
        In maps key order integers types are considered less than floats types.
    </p>
    <p>Atoms are compared using their string value, codepoint by codepoint.</p>
      <p>When comparing an integer to a float, the term with the lesser
      precision is converted into the type of the other term, unless the
      operator is one of <c>=:=</c> or <c>=/=</c>. A float is more precise than
      an integer until all significant figures of the float are to the left of
      the decimal point. This happens when the float is larger/smaller than
      +/-9007199254740992.0. The conversion strategy is changed
      depending on the size of the float because otherwise comparison of large
      floats and integers would lose their transitivity.</p>

    <p>Term comparison operators return the Boolean value of the
      expression, <c>true</c> or <c>false</c>.</p>

    <p><em>Examples:</em></p>
    <pre>
1> <input>1==1.0.</input>
true
2> <input>1=:=1.0.</input>
false
3> <input>1 > a.</input>
false
4> <input>#{c => 3} > #{a => 1, b => 2}.</input>
false
5> <input>#{a => 1, b => 2} == #{a => 1.0, b => 2.0}.</input>
true
6> <input>&lt;&lt;2:2>> &lt; &lt;&lt;128>>.</input>
true
7> <input>&lt;&lt;3:2>> &lt; &lt;&lt;128>>.</input>
false
</pre>
  </section>

  <section>
    <title>Arithmetic Expressions</title>
    <pre>
<input>op</input> Expr
Expr1 <input>op</input> Expr2</pre>
    <table>
      <row>
        <cell align="left" valign="middle"><em>Operator</em></cell>
        <cell align="left" valign="middle"><em>Description</em></cell>
        <cell align="left" valign="middle"><em>Argument Type</em></cell>
      </row>
      <row>
        <cell align="left" valign="middle">+</cell>
        <cell align="left" valign="middle">Unary +</cell>
        <cell align="left" valign="middle">Number</cell>
      </row>
      <row>
        <cell align="left" valign="middle">-</cell>
        <cell align="left" valign="middle">Unary -</cell>
        <cell align="left" valign="middle">Number</cell>
      </row>
      <row>
        <cell align="left" valign="middle">+</cell>
        <cell align="left" valign="middle">&nbsp;</cell>
        <cell align="left" valign="middle">number</cell>
      </row>
      <row>
        <cell align="left" valign="middle">-</cell>
        <cell align="left" valign="middle">&nbsp;</cell>
        <cell align="left" valign="middle">Number</cell>
      </row>
      <row>
        <cell align="left" valign="middle">*</cell>
        <cell align="left" valign="middle">&nbsp;</cell>
        <cell align="left" valign="middle">Number</cell>
      </row>
      <row>
        <cell align="left" valign="middle">/</cell>
        <cell align="left" valign="middle">Floating point division</cell>
        <cell align="left" valign="middle">Number</cell>
      </row>
      <row>
        <cell align="left" valign="middle">bnot</cell>
        <cell align="left" valign="middle">Unary bitwise NOT</cell>
        <cell align="left" valign="middle">Integer</cell>
      </row>
      <row>
        <cell align="left" valign="middle">div</cell>
        <cell align="left" valign="middle">Integer division</cell>
        <cell align="left" valign="middle">Integer</cell>
      </row>
      <row>
        <cell align="left" valign="middle">rem</cell>
        <cell align="left" valign="middle">Integer remainder of X/Y</cell>
        <cell align="left" valign="middle">Integer</cell>
      </row>
      <row>
        <cell align="left" valign="middle">band</cell>
        <cell align="left" valign="middle">Bitwise AND</cell>
        <cell align="left" valign="middle">Integer</cell>
      </row>
      <row>
        <cell align="left" valign="middle">bor</cell>
        <cell align="left" valign="middle">Bitwise OR</cell>
        <cell align="left" valign="middle">Integer</cell>
      </row>
      <row>
        <cell align="left" valign="middle">bxor</cell>
        <cell align="left" valign="middle">Arithmetic bitwise XOR</cell>
        <cell align="left" valign="middle">Integer</cell>
      </row>
      <row>
        <cell align="left" valign="middle">bsl</cell>
        <cell align="left" valign="middle">Arithmetic bitshift left</cell>
        <cell align="left" valign="middle">Integer</cell>
      </row>
      <row>
        <cell align="left" valign="middle">bsr</cell>
        <cell align="left" valign="middle">Bitshift right</cell>
        <cell align="left" valign="middle">Integer</cell>
      </row>
      <tcaption>Arithmetic Operators.</tcaption>
    </table>

    <p><em>Examples:</em></p>
    <pre>
1> <input>+1.</input>
1
2> <input>-1.</input>
-1
3> <input>1+1.</input>
2
4> <input>4/2.</input>
2.0
5> <input>5 div 2.</input>
2
6> <input>5 rem 2.</input>
1
7> <input>2#10 band 2#01.</input>
0
8> <input>2#10 bor 2#01.</input>
3
9> <input>a + 10.</input>
** exception error: an error occurred when evaluating an arithmetic expression
     in operator  +/2
        called as a + 10
10> <input>1 bsl (1 bsl 64).</input>
** exception error: a system limit has been reached
     in operator  bsl/2
        called as 1 bsl 18446744073709551616</pre>
  </section>

  <section>
    <title>Boolean Expressions</title>
    <pre>
<input>op</input> Expr
Expr1 <input>op</input> Expr2</pre>
    <table>
      <row>
        <cell align="left" valign="middle"><em>Operator</em></cell>
        <cell align="left" valign="middle"><em>Description</em></cell>
      </row>
      <row>
        <cell align="left" valign="middle">not</cell>
        <cell align="left" valign="middle">Unary logical NOT</cell>
      </row>
      <row>
        <cell align="left" valign="middle">and</cell>
        <cell align="left" valign="middle">Logical AND</cell>
      </row>
      <row>
        <cell align="left" valign="middle">or</cell>
        <cell align="left" valign="middle">Logical OR</cell>
      </row>
      <row>
        <cell align="left" valign="middle">xor</cell>
        <cell align="left" valign="middle">Logical XOR</cell>
      </row>
      <tcaption>Logical Operators.</tcaption>
    </table>
    <p><em>Examples:</em></p>
    <pre>
1> <input>not true.</input>
false
2> <input>true and false.</input>
false
3> <input>true xor false.</input>
true
4> <input>true or garbage.</input>
** exception error: bad argument
     in operator  or/2
        called as true or garbage</pre>
  </section>

  <section>
    <title>Short-Circuit Expressions</title>
    <pre>
Expr1 orelse Expr2
Expr1 andalso Expr2</pre>
    <p><c>Expr2</c> is evaluated only if
      necessary. That is, <c>Expr2</c> is evaluated only if:</p>
    <list type="bulleted">
      <item><p><c>Expr1</c> evaluates to <c>false</c> in an
      <c>orelse</c> expression.</p>
      </item>
    </list>
      <p>or</p>
    <list type="bulleted">
      <item><p><c>Expr1</c> evaluates to <c>true</c> in an
      <c>andalso</c> expression.</p>
      </item>
    </list>
    <p>Returns either the value of <c>Expr1</c> (that is,
      <c>true</c> or <c>false</c>) or the value of <c>Expr2</c>
      (if <c>Expr2</c> is evaluated).</p>

    <p><em>Example 1:</em></p>
    <pre>
case A >= -1.0 andalso math:sqrt(A+1) > B of</pre>
    <p>This works even if <c>A</c> is less than <c>-1.0</c>,
      since in that case, <c>math:sqrt/1</c> is never evaluated.</p>
    <p><em>Example 2:</em></p>
    <pre>
OnlyOne = is_atom(L) orelse
         (is_list(L) andalso length(L) == 1),</pre>

    <p>From Erlang/OTP R13A, <c>Expr2</c> is no longer required to evaluate to a
    Boolean value. As a consequence, <c>andalso</c> and <c>orelse</c>
    are now tail-recursive.  For instance, the following function is
    tail-recursive in Erlang/OTP R13A and later:</p>

    <pre>
all(Pred, [Hd|Tail]) ->
    Pred(Hd) andalso all(Pred, Tail);
all(_, []) ->
    true.</pre>
  </section>

  <section>
    <title>List Operations</title>
    <pre>
Expr1 ++ Expr2
Expr1 -- Expr2</pre>
    <p>The list concatenation operator <c>++</c> appends its second
      argument to its first and returns the resulting list.</p>
    <p>The list subtraction operator <c>--</c> produces a list that
      is a copy of the first argument. The procedure is a follows:
      for each element in the second argument, the first
      occurrence of this element (if any) is removed.</p>
    <p><em>Example:</em></p>
    <pre>
1> <input>[1,2,3]++[4,5].</input>
[1,2,3,4,5]
2> <input>[1,2,3,2,1,2]--[2,1,2].</input>
[3,1,2]</pre>

     <warning><p>The complexity of <c>A -- B</c> is
     proportional to <c>length(A)*length(B)</c>. That is, it
     becomes very slow if both <c>A</c> and <c>B</c> are
     long lists.</p></warning>
   </section>

  <section>
    <marker id="map_expressions"></marker>
	<title>Map Expressions</title>
	  <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 can 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 a comma <c>,</c>.
	  </p>

	  <p>
		  <em>Examples:</em>
	  </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>
		  Here, <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 takes precedence.
	  </p>
	  <p>
		  <em>Example:</em>
	  </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 recently mentioned case of two matching keys.
	  </p>
  </section>

  <section>
	  <title>Updating Maps</title>
	  <p>
		  Updating a map has a 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>
		  Here <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
		  is created from key <c>K</c> to value <c>V</c>.
	  </p>
	  <p>     If key <c>K</c> matches an existing key in map <c>M</c>,
                  its associated value
	          is replaced by the new value <c>V</c>. In both cases, 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>
		  To only update an existing value, the following syntax is used:
	  </p>
	  <code>M#{ K := V } </code>
	  <p>
		  Here <c>M</c> is a term of type map, <c>V</c> is an expression and <c>K</c>
		  is an expression that 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> is triggered at runtime. If a matching key <c>K</c>
		  is present in map <c>M</c>, its associated value is 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>
		  <em>Examples:</em>
	  </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>
		  Here <c>M0</c> is any map. It follows that <c>M1 .. M4</c> are maps as well.
	  </p>
	  <p>
		  <em>More examples:</em>
	  </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 that case, the latter value is used.
	  </p>
  </section>

  <section>
	  <title>Maps in Patterns</title>
	  <p>
		  Matching of key-value associations from maps is done as follows:
	  </p>

	  <code>#{ K := V } = M</code>
	  <p>
	    Here <c>M</c> is any map. The key <c>K</c> must be a
	    <seeguide marker="#guard_expressions">guard
	    expression</seeguide>, with all variables already bound.
	    <c>V</c> can be any pattern with either bound or unbound
	    variables.
	  </p>
	  <p>
		  If the variable <c>V</c> is unbound, it becomes bound to the value associated
		  with the key <c>K</c>, which must exist in the map <c>M</c>. If the variable
		  <c>V</c> is bound, it must match the value associated with <c>K</c> in <c>M</c>.
	  </p>
	  <note><p>Before OTP 23, the expression defining the key
	  <c>K</c> was restricted to be either a single variable or a
	  literal.</p></note>
	  <p><em>Example:</em></p>
<pre>
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.</pre>
	  <p>
		  This binds variable <c>B</c> to integer <c>2</c>.
	  </p>
	  <p>
		  Similarly, multiple values from the map can be matched:
	  </p>
	  <code>#{ K1 := V1, .., Kn := Vn } = M</code>
	  <p>
		  Here keys <c>K1 .. Kn</c> are any expressions with
		  literals or bound variables. If all key expressions
		  evalute successfully and all keys exist in map
		  <c>M</c>, all variables in <c>V1 .. Vn</c> is
		  matched to the associated values of their respective
		  keys.
	  </p>
	  <p>
		  If the matching conditions are not met, the match fails, either with:
	  </p>
	  <list>
		  <item><p>A <c>badmatch</c> exception.</p>
		        <p>This is if it is used in the context of the match operator
			  as in the example.</p>
		  </item>
		  <item><p>Or resulting in the next clause being tested in function heads and
			  case expressions.</p>
		  </item>
	  </list>
	  <p>
		  Matching in maps only allows for <c>:=</c> as delimiters of associations.
	  </p>
	   <p>
		  The order in which keys are declared in matching has no relevance.
	  </p>
	  <p>
		  Duplicate keys are allowed in matching and match each pattern associated
		  to the keys:
	  </p>
	  <code>#{ K := V1, K := V2 } = M</code>
	  <p>
		  Matching an expression against an empty map literal, matches its type but
		  no variables are bound:
	  </p>
	  <code>#{} = Expr</code>
	  <p>
		  This expression matches if the expression <c>Expr</c> is of type map, otherwise
		  it fails with an exception <c>badmatch</c>.
	  </p>
	  <p>Here the key to be retrieved is constructed from an expression:</p>
	  <code>#{{tag,length(List)} := V} = Map</code>
	  <p><c>List</c> must be an already bound variable.</p>
	  <section>
		  <title>Matching Syntax</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 subexpressions are valid guard expressions.
	  </p>
	  <p>
		  The following guard BIFs handle maps:
	  </p>
	  <list>
		  <item>
			  <seemfa marker="erts:erlang#is_map/1">is_map/1</seemfa>
			   in the <c>erlang</c> module
		  </item>
		  <item>
			  <seemfa marker="erts:erlang#map_get/2">map_get/2</seemfa>
			   in the <c>erlang</c> module
		  </item>
		  <item>
			  <seemfa marker="erts:erlang#map_size/1">map_size/1</seemfa>
			   in the <c>erlang</c> module
		  </item>
	  </list>
	</section>
  </section>

  <section>
    <marker id="bit_syntax"></marker>
    <title>Bit Syntax Expressions</title>
    <code type="none"><![CDATA[<<>>
<<E1,...,En>>]]></code>
    <p>Each element <c>Ei</c> specifies a <em>segment</em> of
      the bit string. Each element <c>Ei</c> is a value, followed by an
      optional <em>size expression</em> and an optional <em>type specifier list</em>.</p>
    <pre>
Ei = Value |
     Value:Size |
     Value/TypeSpecifierList |
     Value:Size/TypeSpecifierList</pre>
    <p>Used in a bit string construction, <c>Value</c> is an expression
    that is to evaluate to an integer, float, or bit string.  If the
    expression is not a single literal or variable, it
    is to be enclosed in parentheses.</p>

    <p>Used in a bit string matching, <c>Value</c> must be a variable,
    or an integer, float, or string.</p>

    <p>Notice that, for example, using a string literal as in
    <c><![CDATA[<<"abc">>]]></c> is syntactic sugar for
    <c><![CDATA[<<$a,$b,$c>>]]></c>.</p>

    <p>Used in a bit string construction, <c>Size</c> is an expression
    that is to evaluate to an integer.</p>
    
    <p>Used in a bit string matching, <c>Size</c> must be a
    <seeguide marker="#guard_expressions">guard expression</seeguide>
    that evaluates to an integer. All variables in the guard expression
    must be already bound.</p>

    <note><p>Before OTP 23, <c>Size</c> was restricted to be an
    integer or a variable bound to an integer.</p></note>

    <p>The value of <c>Size</c> specifies the size of the segment in
    units (see below). The default value depends on the type (see
    below):</p>
    <list type="bulleted">
      <item>For <c>integer</c> it is 8.</item>
      <item>For <c>float</c> it is 64.</item>
      <item>For <c>binary</c> and <c>bitstring</c> it is
      the whole binary or bit string.</item>
    </list>
    <p>In matching, this default value is only
    valid for the last element. All other bit string or binary
    elements in the matching must have a size specification.</p>

    <p>For the <c>utf8</c>, <c>utf16</c>, and <c>utf32</c> types,
    <c>Size</c> must not be given. The size of the segment is implicitly
    determined by the type and value itself.</p>
    
    <p><c>TypeSpecifierList</c> is a list of type specifiers, in any
    order, separated by hyphens (-). Default values are used for any
    omitted type specifiers.</p>
    <taglist>
      <tag><c>Type</c>= <c>integer</c> | <c>float</c> | <c>binary</c> |
             <c>bytes</c> | <c>bitstring</c> | <c>bits</c> |
	     <c>utf8</c> | <c>utf16</c> | <c>utf32</c> </tag>
      <item>The default is <c>integer</c>. <c>bytes</c> is a shorthand for 
      <c>binary</c> and <c>bits</c> is a shorthand for <c>bitstring</c>.
      See below for more information about the <c>utf</c> types.
      </item>

      <tag><c>Signedness</c>= <c>signed</c> | <c>unsigned</c></tag>
      <item>Only matters for matching and when the type is <c>integer</c>. 
      The default is <c>unsigned</c>.</item>

      <tag><c>Endianness</c>= <c>big</c> | <c>little</c> | <c>native</c></tag>
      <item>Native-endian means that the endianness is resolved at load
       time to be either big-endian or little-endian, depending on
       what is native for the CPU that the Erlang machine is run on.
       Endianness only matters when the Type is either <c>integer</c>,
       <c>utf16</c>, <c>utf32</c>, or <c>float</c>. The default is <c>big</c>.
       </item>

      <tag><c>Unit</c>= <c>unit:IntegerLiteral</c></tag>
      <item>The allowed range is 1..256. Defaults to 1 for <c>integer</c>,
       <c>float</c>, and <c>bitstring</c>, and to 8 for <c>binary</c>.
       No unit specifier must be given for the types 
       <c>utf8</c>, <c>utf16</c>, and <c>utf32</c>.
       </item>
    </taglist>
    <p>The value of <c>Size</c> multiplied with the unit gives
      the number of bits. A segment of type <c>binary</c> must have 
      a size that is evenly divisible by 8. For a segment of type <c>float</c>
      the size must be either 64 or 32.</p>

    <note><p>When constructing binaries, if the size <c>N</c> of an integer
    segment is too small to contain the given integer, the most significant
    bits of the integer are silently discarded and only the <c>N</c> least
    significant bits are put into the binary.</p></note>

    <p>The types <c>utf8</c>, <c>utf16</c>, and <c>utf32</c> specifies
    encoding/decoding of the <em>Unicode Transformation Format</em>s UTF-8, UTF-16,
    and UTF-32, respectively.</p>

    <p>When constructing a segment of a <c>utf</c> type, <c>Value</c>
    must be an integer in the range 0..16#D7FF or
    16#E000....16#10FFFF. Construction
    fails with a <c>badarg</c> exception if <c>Value</c> is
    outside the allowed ranges. The size of the resulting binary
    segment depends on the type or <c>Value</c>, or both:</p>
     <list type="bulleted">
      <item>For <c>utf8</c>, <c>Value</c> is encoded in 1-4 bytes.</item>
      <item>For <c>utf16</c>, <c>Value</c> is encoded in 2 or 4 bytes.</item>
      <item>For <c>utf32</c>, <c>Value</c> is always be encoded in 4 bytes.</item>
    </list>

    <p>When constructing, a literal string can be given followed
    by one of the UTF types, for example: <c><![CDATA[<<"abc"/utf8>>]]></c>
    which is syntactic sugar for
    <c><![CDATA[<<$a/utf8,$b/utf8,$c/utf8>>]]></c>.</p>

    <p>A successful match of a segment of a <c>utf</c> type, results
    in an integer in the range 0..16#D7FF or  16#E000..16#10FFFF.
    The match fails if the returned value falls outside those ranges.</p>

    <p>A segment of type <c>utf8</c> matches 1-4 bytes in the binary,
    if the binary at the match position contains a valid UTF-8 sequence.
    (See RFC-3629 or the Unicode standard.)</p>

    <p>A segment of type <c>utf16</c> can match 2 or 4 bytes in the binary.
    The match fails if the binary at the match position does not contain
    a legal UTF-16 encoding of a Unicode code point. (See RFC-2781 or
    the Unicode standard.)</p>

    <p>A segment of type <c>utf32</c> can match 4 bytes in the binary in the
    same way as an <c>integer</c> segment matches 32 bits.
    The match fails if the resulting integer is outside the legal ranges
    mentioned above.</p>

    <p><em>Examples:</em></p>
    <pre>
1> <input>Bin1 = &lt;&lt;1,17,42&gt;&gt;.</input>
&lt;&lt;1,17,42&gt;&gt;
2> <input>Bin2 = &lt;&lt;"abc"&gt;&gt;.</input>
&lt;&lt;97,98,99&gt;&gt;
3> <input>Bin3 = &lt;&lt;1,17,42:16&gt;&gt;.</input>
&lt;&lt;1,17,0,42&gt;&gt;
4> <input>&lt;&lt;A,B,C:16&gt;&gt; = &lt;&lt;1,17,42:16&gt;&gt;.</input>
&lt;&lt;1,17,0,42&gt;&gt;
5> <input>C.</input>
42
6> <input>&lt;&lt;D:16,E,F&gt;&gt; = &lt;&lt;1,17,42:16&gt;&gt;.</input>
&lt;&lt;1,17,0,42&gt;&gt;
7> <input>D.</input>
273
8> <input>F.</input>
42
9> <input>&lt;&lt;G,H/binary&gt;&gt; = &lt;&lt;1,17,42:16&gt;&gt;.</input>
&lt;&lt;1,17,0,42&gt;&gt;
10> <input>H.</input>
&lt;&lt;17,0,42&gt;&gt;
11> <input>&lt;&lt;G,J/bitstring&gt;&gt; = &lt;&lt;1,17,42:12&gt;&gt;.</input>
&lt;&lt;1,17,2,10:4&gt;&gt;
12> <input>J.</input>
&lt;&lt;17,2,10:4&gt;&gt;
13> <input>&lt;&lt;1024/utf8&gt;&gt;.</input>
&lt;&lt;208,128&gt;&gt;
</pre>
    <p>Notice that bit string patterns cannot be nested.</p>
    <p>Notice also that "<c><![CDATA[B=<<1>>]]></c>" is interpreted as
      "<c><![CDATA[B =<<1>>]]></c>" which is a syntax error. The correct way is
      to write a space after '=': "<c><![CDATA[B= <<1>>]]></c>.</p>
    <p>More examples are provided in
    <seeguide marker="system/programming_examples:bit_syntax">
    Programming Examples</seeguide>.</p>
  </section>

  <section>
    <marker id="funs"></marker>
    <title>Fun Expressions</title>
    <pre>
fun
    [Name](Pattern11,...,Pattern1N) [when GuardSeq1] ->
              Body1;
    ...;
    [Name](PatternK1,...,PatternKN) [when GuardSeqK] ->
              BodyK
end</pre>
    <p>A fun expression begins with the keyword <c>fun</c> and ends
      with the keyword <c>end</c>. Between them is to be a function
      declaration, similar to a
      <seeguide marker="functions#syntax">regular function declaration</seeguide>,
      except that the function name is optional and is to be a variable, if
      any.</p>
    <p>Variables in a fun head shadow the function name and both shadow
      variables in the function clause surrounding the fun expression.
      Variables bound in a fun body are local to the fun body.</p>
    <p>The return value of the expression is the resulting fun.</p>
    <p><em>Examples:</em></p>
    <pre>
1> <input>Fun1 = fun (X) -> X+1 end.</input>
#Fun&lt;erl_eval.6.39074546&gt;
2> <input>Fun1(2).</input>
3
3> <input>Fun2 = fun (X) when X>=5 -> gt; (X) -> lt end.</input>
#Fun&lt;erl_eval.6.39074546&gt;
4> <input>Fun2(7).</input>
gt
5> <input>Fun3 = fun Fact(1) -> 1; Fact(X) when X > 1 -> X * Fact(X - 1) end.</input>
#Fun&lt;erl_eval.6.39074546&gt;
6> <input>Fun3(4).</input>
24</pre>
    <p>The following fun expressions are also allowed:</p>
    <pre>
fun Name/Arity
fun Module:Name/Arity</pre>
    <p>In <c>Name/Arity</c>, <c>Name</c> is an atom and <c>Arity</c> is an integer.
      <c>Name/Arity</c> must specify an existing local function. The expression is
      syntactic sugar for:</p>
    <pre>
fun (Arg1,...,ArgN) -> Name(Arg1,...,ArgN) end</pre>
    <p>In <c>Module:Name/Arity</c>, <c>Module</c>, and <c>Name</c> are atoms
      and <c>Arity</c> is an integer. Starting from Erlang/OTP R15,
      <c>Module</c>, <c>Name</c>, and <c>Arity</c> can also be variables.
      A fun defined in this way refers to the function <c>Name</c>
      with arity <c>Arity</c> in the <em>latest</em> version of module
      <c>Module</c>. A fun defined in this way is not dependent on
      the code for the module in which it is defined.
      </p>
    <p>More examples are provided in
      <seeguide marker="system/programming_examples:funs">
      Programming Examples</seeguide>.</p>
  </section>

  <section>
    <marker id="catch"></marker>
    <title>Catch and Throw</title>
    <code type="none">
catch Expr</code>
    <p>Returns the value of <c>Expr</c> unless an exception
      occurs during the evaluation. In that case, the exception is
      caught.</p>
    <p>For exceptions of class <c>error</c>, that is,
      run-time errors,
      <c>{'EXIT',{Reason,Stack}}</c> is returned.</p>
    <p>For exceptions of class <c>exit</c>, that is,
      the code called <c>exit(Term)</c>,
      <c>{'EXIT',Term}</c> is returned.</p>
    <p>For exceptions of class <c>throw</c>, that is
      the code called <c>throw(Term)</c>,
      <c>Term</c> is returned.</p>
    <p><c>Reason</c> depends on the type of error that occurred, and
      <c>Stack</c> is the stack of recent function calls, see
      <seeguide marker="errors#exit_reasons">Exit Reasons</seeguide>.</p>
    <p><em>Examples:</em></p>
    <pre>
1> <input>catch 1+2.</input>
3
2> <input>catch 1+a.</input>
{'EXIT',{badarith,[...]}}</pre>
    <p>Notice that <c>catch</c> has low precedence and catch
      subexpressions often needs to be enclosed in a block
      expression or in parentheses:</p>
    <pre>
3> <input>A = catch 1+2.</input>
** 1: syntax error before: 'catch' **
4> <input>A = (catch 1+2).</input>
3</pre>
    <p>The BIF <c>throw(Any)</c> can be used for non-local return from
      a function. It must be evaluated within a <c>catch</c>, which
      returns the value <c>Any</c>.</p>
      <p><em>Example:</em></p>
    <pre>
5> <input>catch throw(hello).</input>
hello</pre>
    <p>If <c>throw/1</c> is not evaluated within a catch, a
      <c>nocatch</c> run-time error occurs.</p>
  </section>

  <section>
    <marker id="try"></marker>
    <title>Try</title>
    <code type="none">
try Exprs
catch
    Class1:ExceptionPattern1[:Stacktrace] [when ExceptionGuardSeq1] ->
        ExceptionBody1;
    ClassN:ExceptionPatternN[:Stacktrace] [when ExceptionGuardSeqN] ->
        ExceptionBodyN
end</code>
    <p>This is an enhancement of
      <seeguide marker="#catch">catch</seeguide>.
      It gives the possibility to:</p>
    <list type="bulleted">
      <item>Distinguish between different exception classes.</item>
      <item>Choose to handle only the desired ones.</item>
      <item>Passing the others on to an enclosing
      <c>try</c> or <c>catch</c>, or to default error handling.</item>
    </list>
    <p>Notice that although the keyword <c>catch</c> is used in
      the <c>try</c> expression, there is not a <c>catch</c> expression
      within the <c>try</c> expression.</p>
    <p>It returns the value of <c>Exprs</c> (a sequence of expressions
      <c>Expr1, ..., ExprN</c>) unless an exception occurs during
      the evaluation. In that case the exception is caught and
      the patterns <c>ExceptionPattern</c> with the right exception
      class <c>Class</c> are sequentially matched against the caught
      exception. If a match succeeds and the optional guard sequence
      <c>ExceptionGuardSeq</c> is true, the corresponding
      <c>ExceptionBody</c> is evaluated to become the return value.</p>
    <p><c>Stacktrace</c>, if specified, must be the name of a variable
      (not a pattern). The stack trace is bound to the variable when
      the corresponding <c>ExceptionPattern</c> matches.</p>
    <p>If an exception occurs during evaluation of <c>Exprs</c> but
      there is no matching <c>ExceptionPattern</c> of the right
      <c>Class</c> with a true guard sequence, the exception is passed
      on as if <c>Exprs</c> had not been enclosed in a <c>try</c>
      expression.</p>
    <p>If an exception occurs during evaluation of <c>ExceptionBody</c>,
      it is not caught.</p>
    <p>It is allowed to omit <c>Class</c> and <c>Stacktrace</c>.
      An omitted <c>Class</c> is shorthand for <c>throw</c>:</p>

    <code type="none">
try Exprs
catch
    ExceptionPattern1 [when ExceptionGuardSeq1] ->
        ExceptionBody1;
    ExceptionPatternN [when ExceptionGuardSeqN] ->
        ExceptionBodyN
end</code>

    <p>The <c>try</c> expression can have an <c>of</c>
      section:
      </p>
    <code type="none">
try Exprs of
    Pattern1 [when GuardSeq1] ->
        Body1;
    ...;
    PatternN [when GuardSeqN] ->
        BodyN
catch
    Class1:ExceptionPattern1[:Stacktrace] [when ExceptionGuardSeq1] ->
        ExceptionBody1;
    ...;
    ClassN:ExceptionPatternN[:Stacktrace] [when ExceptionGuardSeqN] ->
        ExceptionBodyN
end</code>
    <p>If the evaluation of <c>Exprs</c> succeeds without an exception,
      the patterns <c>Pattern</c> are sequentially matched against
      the result in the same way as for a
      <seeguide marker="#case">case</seeguide> expression, except that if
      the matching fails, a <c>try_clause</c> run-time error occurs instead of a
      <c>case_clause</c>.</p>
    <p>Only exceptions occurring during the evaluation of <c>Exprs</c> can be
      caught by the <c>catch</c> section. Exceptions occurring in a <c>Body</c>
      or due to a failed match are not caught.</p>
    <p>The <c>try</c> expression can also be augmented with an
      <c>after</c> section, intended to be used for cleanup with side
      effects:</p>
    <code type="none">
try Exprs of
    Pattern1 [when GuardSeq1] ->
        Body1;
    ...;
    PatternN [when GuardSeqN] ->
        BodyN
catch
    Class1:ExceptionPattern1[:Stacktrace] [when ExceptionGuardSeq1] ->
        ExceptionBody1;
    ...;
    ClassN:ExceptionPatternN[:Stacktrace] [when ExceptionGuardSeqN] ->
        ExceptionBodyN
after
    AfterBody
end</code>
    <p><c>AfterBody</c> is evaluated after either <c>Body</c> or
      <c>ExceptionBody</c>, no matter which one. The evaluated value of
      <c>AfterBody</c> is lost; the return value of the <c>try</c>
      expression is the same with an <c>after</c> section as without.</p>
    <p>Even if an exception occurs during evaluation of <c>Body</c> or
      <c>ExceptionBody</c>, <c>AfterBody</c> is evaluated. In this case
      the exception is passed on after <c>AfterBody</c> has been
      evaluated, so the exception from the <c>try</c> expression is
      the same with an <c>after</c> section as without.</p>
    <p>If an exception occurs during evaluation of <c>AfterBody</c>
      itself, it is not caught. So if <c>AfterBody</c> is evaluated after
      an exception in <c>Exprs</c>, <c>Body</c>, or <c>ExceptionBody</c>,
      that exception is lost and masked by the exception in
      <c>AfterBody</c>.</p>
    <p>The <c>of</c>, <c>catch</c>, and <c>after</c> sections are all
      optional, as long as there is at least a <c>catch</c> or an
      <c>after</c> section. So the following are valid <c>try</c>
      expressions:</p>
    <code type="none">
try Exprs of 
    Pattern when GuardSeq -> 
        Body 
after 
    AfterBody 
end

try Exprs
catch 
    ExpressionPattern -> 
        ExpressionBody
after
    AfterBody
end

try Exprs after AfterBody end</code>
    <p>Next is an example of using <c>after</c>. This closes the file,
      even in the event of exceptions in <c>file:read/2</c> or in
      <c>binary_to_term/1</c>. The exceptions are the same as
      without the <c>try</c>...<c>after</c>...<c>end</c> expression:</p>
    <code type="none">
termize_file(Name) ->
    {ok,F} = file:open(Name, [read,binary]),
    try
        {ok,Bin} = file:read(F, 1024*1024),
        binary_to_term(Bin)
    after
        file:close(F)
    end.</code>
    <p>Next is an example of using <c>try</c> to emulate <c>catch Expr</c>:</p>
    <code type="none">
try Expr
catch
    throw:Term -> Term;
    exit:Reason -> {'EXIT',Reason}
    error:Reason:Stk -> {'EXIT',{Reason,Stk}}
end</code>
  </section>

  <section>
    <title>Parenthesized Expressions</title>
    <pre>
(Expr)</pre>
    <p>Parenthesized expressions are useful to override
      <seeguide marker="#prec">operator precedences</seeguide>,
      for example, in arithmetic expressions:</p>
    <pre>
1> <input>1 + 2 * 3.</input>
7
2> <input>(1 + 2) * 3.</input>
9</pre>
  </section>

  <section>
    <title>Block Expressions</title>
    <pre>
begin
   Expr1,
   ...,
   ExprN
end</pre>
    <p>Block expressions provide a way to group a sequence of
      expressions, similar to a clause body. The return value is
      the value of the last expression <c>ExprN</c>.</p>
  </section>

  <section>
    <marker id="lcs"></marker>
    <title>List Comprehensions</title>
    <p>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.</p>
    <p>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.</p>
    <p>List comprehensions are written with the following syntax:</p>
    <pre>
[Expr || Qualifier1,...,QualifierN]</pre>
    <p>Here, <c>Expr</c> is an arbitrary expression, and each
      <c>Qualifier</c> is either a generator or a filter.</p>
    <list type="bulleted">
      <item>A <em>generator</em> is written as:      <br></br>

       &nbsp;&nbsp;<c><![CDATA[Pattern <- ListExpr]]></c>.      <br></br>
<c>ListExpr</c> must be an expression, which evaluates to a
       list of terms.</item>
<item>A <em>bit string generator</em> is written as:      <br></br>

       &nbsp;&nbsp;<c><![CDATA[BitstringPattern <= BitStringExpr]]></c>.      <br></br>
<c>BitStringExpr</c> must be an expression, which evaluates to a
       bitstring.</item>
      <item>A <em>filter</em> is an expression, which evaluates to
      <c>true</c> or <c>false</c>.</item>
    </list>
    <p>The variables in the generator patterns, shadow variables in the function
    clause, surrounding the list comprehensions.</p> <p>A list comprehension
    returns a list, where the elements are the result of evaluating <c>Expr</c>
    for each combination of generator list elements and bit string generator
    elements, for which all filters are true.</p>
    <p><em>Example:</em></p>
    <pre>
1> <input>[X*2 || X &lt;- [1,2,3]].</input>
[2,4,6]</pre>
    <p>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.</p>
    <p><em>Example:</em></p>
    <pre>
1> <input>[2 || is_integer(2)].</input>
[2]
2> <input>[x || is_integer(x)].</input>
[]</pre>
    <p>More examples are provided in
    <seeguide marker="system/programming_examples:list_comprehensions">
    Programming Examples.</seeguide></p>

  </section>

<section>
    <title>Bit String Comprehensions</title> 
   
    <p>Bit string comprehensions are
    analogous to List Comprehensions. They are used to generate bit strings
    efficiently and succinctly.</p> 
    <p>Bit string comprehensions are written with
    the following syntax:</p>
    <pre>
&lt;&lt; BitStringExpr || Qualifier1,...,QualifierN &gt;&gt;</pre>
    <p><c>BitStringExpr</c> is an expression that evalutes 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.</p>
    <list type="bulleted">
 <item>A <em>generator</em> is written as:      <br></br>
      &nbsp;&nbsp;<c><![CDATA[Pattern <- ListExpr]]></c>.      <br></br>
       <c>ListExpr</c> must be an expression that evaluates to a
       list of terms.</item>
      <item>A <em>bit string generator</em> is written as:      <br></br>

       &nbsp;&nbsp;<c><![CDATA[BitstringPattern <= BitStringExpr]]></c>.      <br></br>
<c>BitStringExpr</c> must be an expression that evaluates to a
       bitstring.</item>
      <item>A <em>filter</em> is an expression that evaluates to
      <c>true</c> or <c>false</c>.</item>
    </list>
    <p>The variables in the generator patterns, shadow variables in
      the function clause, surrounding the bit string comprehensions.</p>
    <p>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.</p>
    <p><em>Example:</em></p>
    <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>
    <p>More examples are provided in
    <seeguide marker="system/programming_examples:bit_syntax">
    Programming Examples.</seeguide></p>
  </section>

  <section>
    <marker id="guards"></marker>
    <title>Guard Sequences</title>

    <p>A <em>guard sequence</em> is a sequence of guards, separated
      by semicolon (;). The guard sequence is true if at least one of
      the guards is true. (The remaining guards, if any, are not
      evaluated.)</p>
    <p><c>Guard1;...;GuardK</c></p>
    <p>A <em>guard</em> is a sequence of guard expressions, separated
      by comma (,). The guard is true if all guard expressions
      evaluate to <c>true</c>.</p>
    <p><c>GuardExpr1,...,GuardExprN</c></p>
  </section>

  <section>
    <marker id="guard_expressions"></marker>
    <title>Guard Expressions</title>
    <p>The set of valid <em>guard expressions</em> is a subset of the
    set of valid Erlang expressions.  The reason for restricting the
    set of valid expressions is that evaluation of a guard expression
    must be guaranteed to be free of side effects. Valid guard
    expressions are the following:</p>
    <list type="bulleted">
      <item>Variables</item>
      <item>Constants (atoms, integer, floats, lists, tuples, records,
      binaries, and maps)</item>
      <item>Expressions that construct atoms, integer, floats, lists,
      tuples, records, binaries, and maps</item>
      <item>Expressions that update a map</item>
      <item>The record epxressions <c>Expr#Name.Field</c> and <c>#Name.Field</c></item>
      <item>Calls to the BIFs specified in tables <em>Type Test BIFs</em> and
      <em>Other BIFs Allowed in Guard Expressions</em></item>
      <item>Term comparisons</item>
      <item>Arithmetic expressions</item>
      <item>Boolean expressions</item>
      <item>Short-circuit expressions (<c>andalso</c>/<c>orelse</c>)</item>
    </list>
    <table>
      <row>
        <cell align="left" valign="middle"><c>is_atom/1</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>is_binary/1</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>is_bitstring/1</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>is_boolean/1</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>is_float/1</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>is_function/1</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>is_function/2</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>is_integer/1</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>is_list/1</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>is_map/1</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>is_number/1</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>is_pid/1</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>is_port/1</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>is_record/2</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>is_record/3</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>is_reference/1</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>is_tuple/1</c></cell>
      </row>
      <tcaption>Type Test BIFs</tcaption>
    </table>
    <p>Notice that most type test BIFs have older equivalents, without
      the <c>is_</c> prefix. These old BIFs are retained for backwards
      compatibility only and are not to be used in new code. They are
      also only allowed at top level. For example, they are not allowed
      in Boolean expressions in guards.</p>
    <table>
      <row>
        <cell align="left" valign="middle"><c>abs(Number)</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>bit_size(Bitstring)</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>byte_size(Bitstring)</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>element(N, Tuple)</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>float(Term)</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>hd(List)</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>length(List)</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>map_get(Key, Map)</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>map_size(Map)</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>node()</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>node(Pid|Ref|Port)</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>round(Number)</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>self()</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>size(Tuple|Bitstring)</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>tl(List)</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>trunc(Number)</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>tuple_size(Tuple)</c></cell>
      </row>
      <tcaption>Other BIFs Allowed in Guard Expressions</tcaption>
    </table>

    <p>If an arithmetic expression, a Boolean expression, a
    short-circuit expression, or a call to a guard BIF fails (because
    of invalid arguments), the entire guard fails. If the guard was
    part of a guard sequence, the next guard in the sequence (that is,
    the guard following the next semicolon) is evaluated.</p>

  </section>

  <section>
    <marker id="prec"></marker>
    <title>Operator Precedence</title>
    <p>Operator precedence in falling priority:</p>
    <table>
      <row>
        <cell align="left" valign="middle">:</cell>
        <cell align="left" valign="middle">&nbsp;</cell>
      </row>
      <row>
        <cell align="left" valign="middle">#</cell>
        <cell align="left" valign="middle">&nbsp;</cell>
      </row>
      <row>
        <cell align="left" valign="middle">Unary + - bnot not</cell>
        <cell align="left" valign="middle">&nbsp;</cell>
      </row>
      <row>
        <cell align="left" valign="middle">/ * div rem band and</cell>
        <cell align="left" valign="middle">Left associative</cell>
      </row>
      <row>
        <cell align="left" valign="middle">+ - bor bxor bsl bsr or xor</cell>
        <cell align="left" valign="middle">Left associative</cell>
      </row>
      <row>
        <cell align="left" valign="middle">++ --</cell>
        <cell align="left" valign="middle">Right associative</cell>
      </row>
      <row>
        <cell align="left" valign="middle">== /= =&lt; &lt; >= > =:= =/=</cell>
        <cell align="left" valign="middle">&nbsp;</cell>
      </row>
      <row>
        <cell align="left" valign="middle">andalso</cell>
        <cell align="left" valign="middle">&nbsp;</cell>
      </row>
      <row>
        <cell align="left" valign="middle">orelse</cell>
        <cell align="left" valign="middle">&nbsp;</cell>
      </row>
      <row>
        <cell align="left" valign="middle">= !</cell>
        <cell align="left" valign="middle">Right associative</cell>
      </row>
      <row>
        <cell align="left" valign="middle">catch</cell>
        <cell align="left" valign="middle">&nbsp;</cell>
      </row>
      <tcaption>Operator Precedence</tcaption>
    </table>
    <p>When evaluating an expression, the operator with the highest
      priority is evaluated first. Operators with the same priority
      are evaluated according to their associativity.</p>
      <p><em>Example:</em></p>
      <p>The left associative arithmetic operators are evaluated left to
      right:</p>
    <pre>
<input>6 + 5 * 4 - 3 / 2</input> evaluates to
<input>6 + 20 - 1.5</input> evaluates to
<input>26 - 1.5</input> evaluates to
<input>24.5</input></pre>
  </section>
</chapter>

