path: root/lib/compiler/doc/src/compile.xml

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<!DOCTYPE erlref SYSTEM "erlref.dtd">

<erlref>
  <header>
    <copyright>
      <year>1996</year><year>2021</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>compile</title>
    <prepared>Robert Virding</prepared>
    <docno></docno>
    <date>1996-11-04</date>
    <rev>A</rev>
    <file>compile.sgml</file>
  </header>
  <module since="">compile</module>
  <modulesummary>Erlang Compiler</modulesummary>
  <description>
    <p>This module provides an interface to the standard Erlang
      compiler. It can generate either a new file, which contains
      the object code, or return a binary, which can be loaded directly.
    </p>
  </description>

  <datatypes>
    <datatype>
      <name>option() = term()</name>
      <desc><p>See <seemfa marker="#file/2">file/2</seemfa> for detailed description</p></desc>
    </datatype>
    <datatype>
      <name>forms() = term()</name>
      <desc><p>List of Erlang abstract or Core Erlang format representations, as used by <seemfa marker="#forms/2">forms/2</seemfa></p></desc>
    </datatype>
  </datatypes>

  <funcs>
    <func>
      <name since="OTP 19.0">env_compiler_options()</name>
      <fsummary>
	Compiler options defined via the environment variable
	<c>ERL_COMPILER_OPTIONS</c>
      </fsummary>
      <desc>
	<p>Return compiler options given via the environment variable
	  <c>ERL_COMPILER_OPTIONS</c>. If the value is a list, it is
	  returned as is. If it is not a list, it is put into a list.
	</p>
      </desc>
    </func>
    <func>
      <name since="">file(File)</name>
      <fsummary>Compiles a file.</fsummary>
      <desc>
        <p>Is the same as
	  <c>file(File, [verbose,report_errors,report_warnings])</c>.
	</p>
      </desc>
    </func>

    <func>
      <name since="">file(File, Options) -> CompRet</name>
      <fsummary>Compiles a file.</fsummary>
      <type>
        <v>CompRet = ModRet | BinRet | ErrRet</v>
        <v>ModRet = {ok,ModuleName} | {ok,ModuleName,Warnings}</v>
        <v>BinRet = {ok,ModuleName,Binary} | {ok,ModuleName,Binary,Warnings}</v>
        <v>ErrRet = error | {error,Errors,Warnings}</v>
      </type>
      <desc>
        <p>Compiles the code in the file <c>File</c>, which is an
	  Erlang source code file without the <c>.erl</c> extension.
	  <c>Options</c> determine the behavior of the compiler.</p>

        <p>Returns <c>{ok,ModuleName}</c> if successful, or <c>error</c>
	  if there are errors. An object code file is created if
	  the compilation succeeds without errors. It is considered
	  to be an error if the module name in the source code is
	  not the same as the basename of the output file.</p>

        <p>Available options:</p>
        <taglist>
	  <tag><c>brief</c></tag>
          <item>
            <p>Restricts error and warning messages to a single line of output.
            As of OTP 24, the compiler will by default also display the part
            of the source code that the message refers to.</p>
          </item>

          <tag><c>basic_validation</c></tag>
          <item>
            <p>This option is a fast way to test whether a module will
	      compile successfully. This is useful for code generators
              that want to verify the code that they emit. No code is
	      generated. If warnings are enabled, warnings generated by
	      the <c>erl_lint</c> module (such as warnings for unused
	      variables and functions) are also returned.</p>

            <p>Use option <c>strong_validation</c> to generate all
	      warnings that the compiler would generate.</p>
          </item>

          <tag><c>strong_validation</c></tag>
          <item>
            <p>Similar to option <c>basic_validation</c>. No code
	      is generated, but more compiler passes are run
	      to ensure that warnings generated by the optimization
	      passes are generated (such as clauses that will not match,
	      or expressions that are guaranteed to fail with an
	      exception at runtime).</p>
          </item>

          <tag><c>binary</c></tag>
          <item>
            <p>The compiler returns the object code in a
              binary instead of creating an object file. If successful,
              the compiler returns <c>{ok,ModuleName,Binary}</c>.</p>
          </item>

          <tag><c>bin_opt_info</c></tag>
          <item>
            <p>The compiler will emit informational warnings about binary
	    matching optimizations (both successful and unsuccessful).
	    For more information, see the section about
	    <seeguide marker="system/efficiency_guide:binaryhandling#bin_opt_info">bin_opt_info</seeguide>
	    in the Efficiency Guide.</p>
          </item>

          <tag><c>{compile_info, [{atom(), term()}]}</c></tag>
          <item>
            <p>Allows compilers built on top of <c>compile</c> to attach
            extra compilation metadata to the <c>compile_info</c> chunk
            in the generated beam file.</p>

            <p>It is advised for compilers to remove all non-deterministic
            information if the <c>deterministic</c> option is supported and
            it was supplied by the user.</p>
          </item>

          <tag><c>compressed</c></tag>
          <item>
            <p>The compiler will compress the generated object code,
	    which can be useful for embedded systems.</p>
          </item>

          <tag><c>debug_info</c></tag>
          <item>
            <marker id="debug_info"></marker>
            <p>Includes debug information in the form of <seeguide marker="erts:absform">
              Erlang Abstract Format</seeguide> in the <c>debug_info</c>
              chunk of the compiled beam module. Tools such as Debugger,
              Xref, and Cover require the debug information to be included.</p>

            <p><em>Warning</em>: Source code can be reconstructed from
              the debug information. Use encrypted debug information
              (<c>encrypt_debug_info</c>) to prevent this.</p>

            <p>For details, see
              <seeerl marker="stdlib:beam_lib#debug_info">beam_lib(3)</seeerl>.</p>
          </item>

          <tag><c>{debug_info, {Backend, Data}}</c></tag>
          <item>
            <marker id="debug_info"></marker>
            <p>Includes custom debug information in the form of a
              <c>Backend</c> module with custom <c>Data</c> in the compiled beam module.
              The given module must implement a <c>debug_info/4</c> function
              and is responsible for generating different code representations,
              as described in the <c>debug_info</c> under
              <seeerl marker="stdlib:beam_lib#debug_info">beam_lib(3)</seeerl>.</p>

            <p><em>Warning</em>: Source code can be reconstructed from
              the debug information. Use encrypted debug information
              (<c>encrypt_debug_info</c>) to prevent this.</p>
          </item>

          <tag><c>{debug_info_key,KeyString}</c></tag>
          <item></item>
          <tag><c>{debug_info_key,{Mode,KeyString}}</c></tag>
          <item>
            <marker id="debug_info_key"></marker>
            <p>Includes debug information, but encrypts it so that it
              cannot be accessed without supplying the key. (To give
              option <c>debug_info</c> as well is allowed, but
              not necessary.) Using this option is a good way to always
              have the debug information available during testing, yet
              protecting the source code.</p>
            <p><c>Mode</c> is the type of crypto algorithm to be used
              for encrypting the debug information. The default
              (and currently the only) type is <c>des3_cbc</c>.</p>
            <p>For details, see
              <seeerl marker="stdlib:beam_lib#debug_info">beam_lib(3)</seeerl>.</p>
          </item>

          <tag><c>encrypt_debug_info</c></tag>
          <item>
            <marker id="encrypt_debug_info"></marker>
            <p>Similar to the <c>debug_info_key</c> option, but
	      the key is read from an <c>.erlang.crypt</c> file.
	    </p>

	    <p>For details, see
              <seeerl marker="stdlib:beam_lib#debug_info">beam_lib(3)</seeerl>.</p>
          </item>

          <tag><c>deterministic</c></tag>
          <item>
            <p>Omit the <c>options</c> and <c>source</c> tuples in
            the list returned by <c>Module:module_info(compile)</c>, and
            reduce the paths in stack traces to the module name alone.
	    This option will make it easier to achieve reproducible builds.
	    </p>
          </item>

          <tag><c>makedep</c></tag>
          <item>
            <p>Produces a Makefile rule to track headers dependencies.
              No object file is produced.
            </p>
            <p>By default, this rule is written to
              <c><![CDATA[<File>.Pbeam]]></c>. However, if option
                <c>binary</c> is set, nothing is written and the rule is
                returned in <c>Binary</c>.
            </p>
            <p>The output will be encoded in UTF-8.</p>
            <p>For example, if you have the following module:
            </p>
            <code>
-module(module).

-include_lib("eunit/include/eunit.hrl").
-include("header.hrl").</code>
            <p>The Makefile rule generated by this option looks as follows:
            </p>
            <code>
module.beam: module.erl \
  /usr/local/lib/erlang/lib/eunit/include/eunit.hrl \
  header.hrl</code>
          </item>

          <tag><c>makedep_side_effect</c></tag>
          <item>
	    <p>The dependecies are created as a side effect to the
	    normal compilation process. This means that the object
	    file will also be produced. This option override the
	    <c>makedep</c> option.
	    </p>
	  </item>

          <tag><c>{makedep_output, Output}</c></tag>
          <item>
            <p>Writes generated rules to <c>Output</c> instead of the
              default <c><![CDATA[<File>.Pbeam]]></c>. <c>Output</c>
              can be a filename or an <c>io_device()</c>. To write to
              stdout, use <c>standard_io</c>. However, if <c>binary</c>
              is set, nothing is written to <c>Output</c> and the
              result is returned to the caller with
              <c>{ok, ModuleName, Binary}</c>.
            </p>
          </item>

          <tag><c>{makedep_target, Target}</c></tag>
          <item>
            <p>Changes the name of the rule emitted to <c>Target</c>.
            </p>
          </item>

          <tag><c>makedep_quote_target</c></tag>
          <item>
            <p>Characters in <c>Target</c> special to make(1) are quoted.
            </p>
          </item>

          <tag><c>makedep_add_missing</c></tag>
          <item>
            <p>Considers missing headers as generated files and adds them to the
              dependencies.
            </p>
          </item>

          <tag><c>makedep_phony</c></tag>
          <item>
            <p>Adds a phony target for each dependency.
            </p>
          </item>

          <tag><c>'P'</c></tag>
          <item>
            <p>Produces a listing of the parsed code, after preprocessing
	      and parse transforms, in the file
	      <c><![CDATA[<File>.P]]></c>. No object file is produced.
	    </p>
          </item>

          <tag><c>'E'</c></tag>
          <item>
            <p>Produces a listing of the code, after all source code
	      transformations have been performed, in the file
	      <c><![CDATA[<File>.E]]></c>. No object file is produced.
	    </p>
          </item>

          <tag><c>'S'</c></tag>
          <item>
            <p>Produces a listing of the assembler code in the file
              <c><![CDATA[<File>.S]]></c>. No object file is produced.
	    </p>
          </item>

          <tag><c>recv_opt_info</c></tag>
          <item>
            <p>The compiler will emit informational warnings about selective
              receive optimizations (both successful and unsuccessful). For
              more information, see the section about
              <seeguide marker="system/efficiency_guide:processes#receiving-messages">
              selective receive optimization</seeguide> in the Efficiency
              Guide.</p>
          </item>

          <tag><c>report_errors/report_warnings</c></tag>
          <item>
            <p>Causes errors/warnings to be printed as they occur.</p>
          </item>

          <tag><c>report</c></tag>
          <item>
            <p>A short form for both <c>report_errors</c> and
              <c>report_warnings</c>.</p>
          </item>

          <tag><c>return_errors</c></tag>
          <item>
            <p>If this flag is set,
	      <c>{error,ErrorList,WarningList}</c> is returned when
	      there are errors.</p>
          </item>

          <tag><c>return_warnings</c></tag>
          <item>
            <p>If this flag is set, an extra field, containing
              <c>WarningList</c>, is added to the tuples returned on
              success.</p>
          </item>

          <tag><c>warnings_as_errors</c></tag>
          <item>
            <p>Causes warnings to be treated as errors. This option is supported
            since R13B04.</p>
          </item>

	  <tag><c>{error_location,line | column}</c></tag>
	  <item>
            <p>If the value of this flag is <c>line</c>, the location
            <seeerl marker="#error_information"><c>ErrorLocation</c></seeerl>
            of warnings and errors is a line number. If
            the value is <c>column</c>, <c>ErrorLocation</c> includes
            both a line number and a column number.
	    Default is <c>column</c>. This option is supported
            since Erlang/OTP 24.0.</p>
            <p>If the value of this flag is <c>column</c>,
            <seeerl marker="#debug_info">debug information</seeerl>
	    includes column information.</p>
	  </item>

          <tag><c>return</c></tag>
          <item>
            <p>A short form for both <c>return_errors</c> and
              <c>return_warnings</c>.</p>
          </item>

	  <tag><c>verbose</c></tag>
          <item>
            <p>Causes more verbose information from the compiler,
              describing what it is doing.</p>
          </item>

	  <tag><c>{source,FileName}</c></tag>
          <item>
            <p>Overrides the source file name as presented in
              <c>module_info(compile)</c> and stack traces.</p>
          </item>

	  <tag><c>{outdir,Dir}</c></tag>
          <item>
            <p>Sets a new directory for the object code. The current
              directory is used for output, except when a directory
              has been specified with this option.</p>
          </item>

	  <tag><c>export_all</c></tag>
          <item>
            <p>Causes all functions in the module to be exported.</p>
          </item>

	  <tag><c>{i,Dir}</c></tag>
          <item>
            <p>Adds <c>Dir</c> to the list of directories to be searched
              when including a file. When encountering an
	      <c>-include</c> or <c>-include_lib</c> directive,
	      the compiler searches for header files in the following
	      directories:</p>
            <list type="ordered">
              <item>
                <p><c>"."</c>, the current working directory of
		  the file server</p>
              </item>
              <item>
                <p>The base name of the compiled file</p>
              </item>
              <item>
                <p>The directories specified using option <c>i</c>;
                  the directory specified last is searched first</p>
              </item>
            </list>
          </item>

          <tag><c>{d,Macro}</c></tag>
          <item></item>
          <tag><c>{d,Macro,Value}</c></tag>
          <item>
            <p>Defines a macro <c>Macro</c> to have the value
            <c>Value</c>. <c>Macro</c> is of type atom, and <c>Value</c> can be any term.
            The default <c>Value</c> is <c>true</c>.</p>
          </item>

	  <tag><c>{parse_transform,Module}</c></tag>
          <item>
            <p>Causes the parse transformation function
              <c>Module:parse_transform/2</c> to be applied to the
              parsed code before the code is checked for errors.</p>
          </item>

	  <tag><c>from_abstr</c></tag>
          <item>
            <p>The input file is expected to contain Erlang terms representing
            forms in abstract format (default file suffix ".abstr"). Note
            that the format of such terms can change between releases.</p>
            <p>See also the <c>no_lint</c> option.</p>
          </item>

	  <tag><c>from_asm</c></tag>
          <item>
            <p>The input file is expected to be assembler code (default
	      file suffix ".S"). Notice that the format of assembler files
              is not documented, and can change between releases.</p>
          </item>

	  <tag><c>from_core</c></tag>
          <item>
            <p>The input file is expected to be core code (default
	      file suffix ".core"). Notice that the format of core files
              is not documented, and can change between releases.</p>
          </item>

          <tag><c>no_spawn_compiler_process</c></tag>
          <item>
            <p>By default, all code is compiled in a separate process
              which is terminated at the end of compilation. However,
              some tools, like Dialyzer or compilers for other BEAM languages,
              may already manage their own worker processes and spawning
              an extra process may slow the compilation down.
              In such scenarios, you can pass this option to stop the
              compiler from spawning an additional process.</p>
          </item>

          <tag><c>no_strict_record_tests</c></tag>
          <item>
            <p>This option is not recommended.</p>

	    <p>By default, the generated code for
	      operation <c>Record#record_tag.field</c> verifies that
	      the tuple <c>Record</c> has the correct size for
	      the record, and that the first element is the tag
	      <c>record_tag</c>. Use this option to omit
	      the verification code.</p>
          </item>

          <tag><c>no_error_module_mismatch</c></tag>
          <item>
            <p>Normally the compiler verifies that the module name
	    given in the source code is the same as the base name
	    of the output file and refuses to generate an output file
	    if there is a mismatch. If you have a good reason (or
	    other reason) for having a module name unrelated to the
	    name of the output file, this option disables that verification
	    (there will not even be a warning if there is a mismatch).</p>
          </item>

          <tag><c>{no_auto_import,[{F,A}, ...]}</c></tag>
          <item>
	  <p>Makes the function <c>F/A</c> no longer being
          auto-imported from the <c>erlang</c> module, which resolves
          BIF name clashes. This option must be used to resolve name
          clashes with BIFs auto-imported before R14A, if it is needed to
          call the local function with the same name as an
          auto-imported BIF without module prefix.</p>
	  <note>
	  <p>As from R14A and forward, the compiler resolves calls
	  without module prefix to local or imported functions before
	  trying with auto-imported BIFs. If the BIF is to be
	  called, use the <c>erlang</c> module prefix in the call, not
	  <c>{no_auto_import,[{F,A}, ...]}</c>.</p>
	  </note>
	  <p>If this option is written in the source code, as a 
	  <c>-compile</c> directive, the syntax <c>F/A</c> can be used instead 
	  of <c>{F,A}</c>, for example:</p>
	  <code>-compile({no_auto_import,[error/1]}).</code>
	  </item>

          <tag><c>no_auto_import</c></tag>
          <item>
            <p>Do not auto-import any functions from <c>erlang</c> module.</p>
          </item>

          <tag><c>no_line_info</c></tag>
          <item>
            <p>Omits line number information to produce a slightly
	      smaller output file.
	    </p>
          </item>

          <tag><c>no_lint</c></tag>
          <item>
            <p>Skips the pass that checks for errors and warnings. Only
            applicable together with the <c>from_abstr</c> option. This is
            mainly for implementations of other languages on top of Erlang,
            which have already done their own checks to guarantee
            correctness of the code.</p>
            <p>Caveat: When this option is used, there are no guarantees
            that the code output by the compiler is correct and safe to
            use. The responsibility for correctness lies on the code or
            person generating the abstract format. If the code contains
            errors, the compiler may crash or produce unsafe code.</p>
          </item>

          <tag><c>{extra_chunks, [{binary(), binary()}]}</c></tag>
          <item>
            <p>Pass extra chunks to be stored in the <c>.beam</c> file.
              The extra chunks must be a list of tuples with a four byte
              binary as chunk name followed by a binary with the chunk contents.
              See <seeerl marker="stdlib:beam_lib">beam_lib</seeerl> for
              more information.
	    </p>
          </item>
        </taglist>

        <p>If warnings are turned on (option <c>report_warnings</c>
          described earlier), the following options control what type of
          warnings that are generated.
	  <marker id="erl_lint_options"></marker>
          Except from <c>{warn_format,Verbosity}</c>, the following options 
	have two forms:</p>
	  <list type="bulleted">
	    <item>A <c>warn_xxx</c> form, to turn on the warning.</item>
	    <item>A <c>nowarn_xxx</c> form, to turn off the warning.</item>
	  </list>
	  <p>In the descriptions that follow, the form that is used to change 
	  the default value are listed.</p>

        <taglist>
          <tag><c>{warn_format, Verbosity}</c></tag>
          <item>
            <p>Causes warnings to be emitted for malformed format
              strings as arguments to <c>io:format</c> and similar
              functions.</p>
	      <p><c>Verbosity</c> selects the number of warnings:</p>
	      <list type="bulleted">
		<item><c>0</c> = No warnings</item>
		<item><c>1</c> = Warnings for invalid format strings and incorrect 
		number of arguments</item>
		<item><c>2</c> = Warnings also when the validity cannot 
		be checked, for example, when the format string argument is a
		variable.</item>
	      </list>
	      <p>The default verbosity is <c>1</c>. Verbosity <c>0</c> can
              also be selected by option <c>nowarn_format</c>.</p>
          </item>

          <tag><c>nowarn_bif_clash</c></tag>
          <item>
            <p>This option is removed, it generates a fatal error if used.</p>

	    <warning>
	    <p>As from beginning with R14A, the compiler no longer calls the
	    auto-imported BIF if the name clashes with a local or
	    explicitly imported function, and a call without explicit
	    module name is issued. Instead, the local or imported
	    function is called. Still accepting <c>nowarn_bif_clash</c> would 
	    make a module calling functions clashing with auto-imported BIFs
	    compile with both the old and new compilers, but with
	    completely different semantics. This is why the option is removed.</p>

	    <p>The use of this option has always been discouraged.
	    As from R14A, it is an error to use it.</p>
	    <p>To resolve BIF clashes, use explicit module names or the
	    <c>{no_auto_import,[F/A]}</c> compiler directive.</p>
	    </warning>
          </item>

          <tag><c>{nowarn_bif_clash, FAs}</c></tag>
          <item>
            <p>This option is removed, it generates a fatal error if used.</p>

	    <warning>
	    <p>The use of this option has always been discouraged.
	    As from R14A, it is an error to use it.</p>
	    <p>To resolve BIF clashes, use explicit module names or the
	    <c>{no_auto_import,[F/A]}</c> compiler directive.</p>
	    </warning>
          </item>

	  <tag><c>nowarn_export_all</c></tag>
          <item>
	      <p>Turns off warnings for uses of the <c>export_all</c>
	      option. Default is to emit a warning if option
	      <c>export_all</c> is also given.</p>
          </item>

	  <tag><c>warn_export_vars</c></tag>
          <item>
	    <p>Emits warnings for all implicitly exported variables 
	      referred to after the primitives where they were first defined. 
	      By default, the compiler only emits warnings for exported 
	      variables referred to in a pattern.</p>
          </item>

	  <tag><c>nowarn_shadow_vars</c></tag>
          <item>
	      <p>Turns off warnings for "fresh" variables 
	      in functional objects or list comprehensions with the same 
	      name as some already defined variable. Default is to 
	      emit warnings for such variables.</p>   
          </item>

	  <tag><c>nowarn_unused_function</c></tag>
          <item>
            <p>Turns off warnings for unused local functions. Default 
	    is to emit warnings for all local functions that are not called
	      directly or indirectly by an exported function.
              The compiler does not include unused local functions in
	      the generated beam file, but the warning is still useful
	      to keep the source code cleaner.</p>
          </item>

	  <tag><c>{nowarn_unused_function, FAs}</c></tag>
          <item>
            <p>Turns off warnings for unused local functions like
              <c>nowarn_unused_function</c> does, but only for the mentioned
              local functions. <c>FAs</c> is a tuple <c>{Name,Arity}</c>
              or a list of such tuples.</p>
          </item>

	  <tag><c>nowarn_deprecated_function</c></tag>
          <item>
            <p>Turns off warnings for calls to deprecated functions. Default 
	    is to emit warnings for every call to a function known by the 
	    compiler to be deprecated. Notice that the compiler does not know
	      about attribute <c>-deprecated()</c>, but uses an
	      assembled list of deprecated functions in Erlang/OTP. To
	      do a more general check, the Xref tool can be used.
	      See also
	      <seeerl marker="tools:xref#deprecated_function">xref(3)</seeerl>
              and the function
	      <seemfa marker="tools:xref#m/1">xref:m/1</seemfa>, also
              accessible through the function 
	    <seemfa marker="stdlib:c#xm/1">c:xm/1</seemfa>.</p>
          </item>

	  <tag><c>{nowarn_deprecated_function, MFAs}</c></tag>
          <item>
            <p>Turns off warnings for calls to deprecated functions like
	      <c>nowarn_deprecated_function</c> does, but only for
	      the mentioned functions. <c>MFAs</c> is a tuple
              <c>{Module,Name,Arity}</c> or a list of such tuples.</p>
          </item>

	  <tag><c>nowarn_deprecated_type</c></tag>
          <item>
            <p>Turns off warnings for use of deprecated types. Default 
	    is to emit warnings for every use of a type known by the compiler
	    to be deprecated.</p>
          </item>

	  <tag><c>nowarn_removed</c></tag>
          <item>
            <p>Turns off warnings for calls to functions that have
            been removed. Default is to emit warnings for every call
            to a function known by the compiler to have been recently
            removed from Erlang/OTP.</p>
          </item>

	  <tag><c>{nowarn_removed, ModulesOrMFAs}</c></tag>
          <item>
            <p>Turns off warnings for calls to modules or functions
            that have been removed. Default is to emit warnings for
            every call to a function known by the compiler to have
            been recently removed from Erlang/OTP.</p>
          </item>

	  <tag><c>nowarn_obsolete_guard</c></tag>
          <item>
            <p>Turns off warnings for calls to old type testing BIFs, 
	    such as <c>pid/1</c> and <c>list/1</c>. See the
	    <seeguide marker="system/reference_manual:expressions#guards">Erlang Reference Manual</seeguide>
	      for a complete list of type testing BIFs and their old
	      equivalents. Default is to emit warnings for calls to
	      old type testing BIFs.</p>
          </item>

	  <tag><c>warn_unused_import</c></tag>
          <item>
            <p>Emits warnings for unused imported functions. 
	    Default is to emit no warnings for unused imported functions.</p>
          </item>

          <tag><c>nowarn_underscore_match</c></tag>
          <item>
            <p>By default, warnings are emitted when a variable that begins
              with an underscore is matched after being bound. Use this
              option to turn off this kind of warning.</p>
          </item>

          <tag><c>nowarn_unused_vars</c></tag>
          <item>
            <p>By default, warnings are emitted for unused variables,
	      except for variables beginning with an underscore 
	      ("Prolog style warnings").
              Use this option to turn off this kind of warning.</p>
          </item>

	  <tag><c>nowarn_unused_record</c></tag>
          <item>
            <p>Turns off warnings for unused record definitions. Default is to
	    emit warnings for unused locally defined records.</p>
          </item>

	  <tag><c>{nowarn_unused_record, RecordNames}</c></tag>
          <item>
            <p>Turns off warnings for unused record definitions. Default is to
	    emit warnings for unused locally defined records.</p>
          </item>

	  <tag><c>nowarn_unused_type</c></tag>
          <item>
            <p>Turns off warnings for unused type declarations. Default is to
	    emit warnings for unused local type declarations.</p>
          </item>

          <tag><c>nowarn_nif_inline</c></tag>
          <item>
              <p>By default, warnings are emitted when inlining is enabled in
                a module that may load NIFs, as the compiler may inline NIF
                fallbacks by accident. Use this option to turn off this kind of
                warnings.</p>
          </item>

          <tag><c>warn_missing_spec</c></tag>
          <item>
              <p>By default, warnings are not emitted when a specification
                (or contract) for an exported function is not given. Use this
                option to turn on this kind of warning.</p>
          </item>

          <tag><c>warn_missing_spec_all</c></tag>
          <item>
              <p>By default, warnings are not emitted when a specification
                (or contract) for an exported or unexported function is not
                given. Use this option to turn on this kind of warning.</p>
          </item>
        </taglist>

	<p>Other kinds of warnings are <em>opportunistic
	warnings</em>. They are generated when the compiler happens to
	notice potential issues during optimization and code
	generation.</p>

	<note>
	  <p>The compiler does not warn for expressions that it
          does not attempt to optimize. For example, the compiler will
          emit a warning for <c>1/0</c> but not for <c>X/0</c>,
          because <c>1/0</c> is a constant expression that the compiler
          will attempt to evaluate.</p>

          <p>The absence of warnings does not mean that there are no
          remaining errors in the code.</p>
        </note>

        <p>Opportunistic warnings can be disabled using the following
        options:</p>

        <taglist>
          <tag><c>nowarn_opportunistic</c></tag>
          <item><p>Disable all opportunistic warnings.</p></item>

          <tag><c>nowarn_failed</c></tag>
          <item><p>Disable warnings for expressions that will always
          fail (such as <c>atom+42</c>).</p></item>

          <tag><c>nowarn_ignored</c></tag>
          <item><p>Disable warnings for expressions whose
          values are ignored.</p></item>

          <tag><c>nowarn_nomatch</c></tag>
          <item><p>Disable warnings for patterns that will never match
          (such as <c>a=b</c>) and for guards that always evaluate to
          <c>false</c>.</p></item>
        </taglist>

	<note>
	<p>All options, except the include path
	  (<c>{i,Dir}</c>), can also be given in the file with attribute
	  <c>-compile([Option,...])</c>.
	  Attribute <c>-compile()</c> is allowed after the function
	  definitions.</p>
	</note>
	
	<note>
	<p>Before OTP 22, the option <c>{nowarn_deprecated_function,
	MFAs}</c> was only recognized when given in the file with
	attribute <c>-compile()</c>. (The option
	<c>{nowarn_unused_function,FAs}</c> was incorrectly documented
	to only work in a file, but it also worked when given in the
	option list.) Starting from OTP 22, all options that can be
	given in the file can also be given in the option list.</p>
	</note>

        <p>For debugging of the compiler, or for pure curiosity,
          the intermediate code generated by each compiler pass can be
	  inspected.
          To print a complete list of the options to produce list files,
	  type <c>compile:options()</c> at the Erlang shell prompt.
          The options are printed in the order that the passes are
	  executed. If more than one listing option is used, the one
	  representing the earliest pass takes effect.</p>

        <p>Unrecognized options are ignored.</p>

        <p>Both <c>WarningList</c> and <c>ErrorList</c> have
	  the following format:</p>
        <code>
[{FileName,[ErrorInfo]}].</code>

	<p><c>ErrorInfo</c> is described later in this section. 
	The filename is included here, as the compiler uses the 
	Erlang pre-processor <c>epp</c>, which allows the code to be 
	included in other files. It is therefore important to know to
	  <em>which</em> file the location of an error or a warning refers.
	</p>
      </desc>
    </func>

    <func>
      <name since="">forms(Forms)</name>
      <fsummary>Compiles a list of forms.</fsummary>
      <desc>
        <p>Is the same as
	  <c>forms(Forms, [verbose,report_errors,report_warnings])</c>.
	</p>
      </desc>
    </func>

    <func>
      <name since="">forms(Forms, Options) -> CompRet</name>
      <fsummary>Compiles a list of forms.</fsummary>
      <type>
        <v>Forms = <seetype marker="#forms">forms()</seetype></v>
        <v>forms() = [<seetype marker="stdlib:erl_parse#abstract_form">erl_parse:abstract_form</seetype>] | <seeerl marker="cerl#type-c_module">cerl:c_module()</seeerl></v>
        <v>Options = [<seetype marker="#option">option()</seetype>]</v>
        <v>CompRet = BinRet | ErrRet</v>
        <v>BinRet = {ok,ModuleName,BinaryOrCode} | {ok,ModuleName,BinaryOrCode,Warnings}</v>
        <v>ModuleName = module()</v>
        <v>BinaryOrCode = binary() | term()</v>
        <v>ErrRet = error | {error,Errors,Warnings}</v>
        <v>Warnings = Errors = [{<seetype marker="kernel:file#filename">file:filename()</seetype>, [{<seetype marker="stdlib:erl_anno#location">erl_anno:location()</seetype> | 'none', module(), term()}]}]</v>
      </type>
      <desc>
        <p>Analogous to <c>file/1</c>, but takes a list of forms (in
	  either Erlang abstract or Core Erlang format representation)
      as first argument.
          Option <c>binary</c> is implicit, that is, no object code
	  file is produced. For options that normally produce a listing 
	  file, such as 'E', the internal format for that compiler pass 
	  (an Erlang term, usually not a binary) is returned instead of 
	a binary.</p>
      </desc>
    </func>

    <func>
      <name since="">format_error(ErrorDescriptor) -> chars()</name>
      <fsummary>Formats an error descriptor.</fsummary>
      <type>
        <v>ErrorDescriptor = errordesc()</v>
      </type>
      <desc>
        <p>Uses an <c>ErrorDescriptor</c> and returns a deep list of
	  characters that describes the error. This function is
	  usually called implicitly when an <c>ErrorInfo</c> structure 
	(described in section 
	<seeerl marker="#error_information">Error Information</seeerl>) is processed.</p>
      </desc>
    </func>

    <func>
      <name since="">output_generated(Options) -> true | false</name>
      <fsummary>Determines whether the compiler generates an output file.</fsummary>
      <type>
        <v>Options = [term()]</v>
      </type>
      <desc>
        <p>Determines whether the compiler generates a <c>beam</c>
	file with the given options. <c>true</c> means that a <c>beam</c>
	file is generated. <c>false</c> means that the compiler
	generates some listing file, returns a binary, or merely
	checks the syntax of the source code.</p>
      </desc>
    </func>

    <func>
      <name since="">noenv_file(File, Options) -> CompRet</name>
      <fsummary>Compiles a file (ignoring <c>ERL_COMPILER_OPTIONS)</c>.</fsummary>
      <desc>
      <p>Works like <seemfa marker="#file/2">file/2</seemfa>,
	 except that the environment variable <c>ERL_COMPILER_OPTIONS</c>
	 is not consulted.</p>
      </desc>
    </func>

    <func>
      <name since="">noenv_forms(Forms, Options) -> CompRet</name>
      <fsummary>Compiles a list of forms (ignoring <c>ERL_COMPILER_OPTIONS)</c>.</fsummary>
      <desc>
        <p>Works like <seemfa marker="#forms/2">forms/2</seemfa>,
	 except that the environment variable <c>ERL_COMPILER_OPTIONS</c>
	 is not consulted.</p>
      </desc>
    </func>

    <func>
      <name since="">noenv_output_generated(Options) -> true | false</name>
      <fsummary>Determines whether the compiler generates an output file 
      (ignoring <c>ERL_COMPILER_OPTIONS)</c>.</fsummary>
      <type>
        <v>Options = [term()]</v>
      </type>
      <desc>
        <p>Works like
	<seemfa marker="#output_generated/1">output_generated/1</seemfa>,
	except that the environment variable <c>ERL_COMPILER_OPTIONS</c>
	is not consulted.</p>
      </desc>
    </func>

  </funcs>

  <section>
    <title>Default Compiler Options</title>
    <p>The (host operating system) environment variable
      <c>ERL_COMPILER_OPTIONS</c> can be used to give default compiler
      options. Its value must be a valid Erlang term. If the value is a
      list, it is used as is. If it is not a list, it is put
      into a list.</p>

      <p>The list is appended to any options given to
      <seemfa marker="#file/2">file/2</seemfa>,
      <seemfa marker="#forms/2">forms/2</seemfa>, and
      <seemfa marker="#output_generated/1">output_generated/2</seemfa>.
      Use the alternative functions
      <seemfa marker="#noenv_file/2">noenv_file/2</seemfa>,
      <seemfa marker="#noenv_forms/2">noenv_forms/2</seemfa>, or
      <seemfa marker="#noenv_output_generated/1">noenv_output_generated/2</seemfa>
      if you do not want the environment variable to be consulted,
      for example, if you are calling the compiler recursively from
      inside a parse transform.</p>

      <p>The list can be retrieved with
      <seemfa marker="#env_compiler_options/0">env_compiler_options/0</seemfa>.</p>
  </section>

  <section>
    <title>Inlining</title>
    <p>The compiler can do function inlining within an Erlang
      module. Inlining means that a call to a function is replaced with
      the function body with the arguments replaced with the actual
      values. The semantics are preserved, except if exceptions are
      generated in the inlined code. Exceptions are reported as
      occurring in the function the body was inlined into. Also,
      <c>function_clause</c> exceptions are converted to similar
      <c>case_clause</c> exceptions.</p>

    <p>When a function is inlined, the original function is
      kept if it is exported (either by an explicit export or if the
      option <c>export_all</c> was given) or if not all calls to the
      function are inlined.</p>
    
    <p>Inlining does not necessarily improve running time.
      For example, inlining can increase Beam stack use, which
      probably is detrimental to performance for recursive functions.
    </p>

    <p>Inlining is never default. It must be explicitly enabled with a
      compiler option or a <c>-compile()</c> attribute in the source
      module.</p>

    <p>To enable inlining, either use the option <c>inline</c> to
    let the compiler decide which functions to inline, or
    <c>{inline,[{Name,Arity},...]}</c> to have the compiler inline
    all calls to the given functions. If the option is given inside
    a <c>compile</c> directive in an Erlang module, <c>{Name,Arity}</c>
    can be written as <c>Name/Arity</c>.</p>

    <p>Example of explicit inlining:</p>

    <pre>
-compile({inline,[pi/0]}).

pi() -> 3.1416.
    </pre>

    <p>Example of implicit inlining:</p>
    <pre>
-compile(inline).</pre>

    <p>The option <c>{inline_size,Size}</c> controls how large functions
      that are allowed to be inlined. Default is <c>24</c>, which
      keeps the size of the inlined code roughly the same as
      the un-inlined version (only relatively small functions are
      inlined).</p>

    <p>Example:</p>
    <pre>
%% Aggressive inlining - will increase code size.
-compile(inline).
-compile({inline_size,100}).</pre>
  </section>

  <section>
    <title>Inlining of List Functions</title>
    <p>The compiler can also inline various list manipulation functions
      from the module <c>list</c> in STDLIB.</p>

    <p>This feature must be explicitly enabled with a compiler option or a
      <c>-compile()</c> attribute in the source module.</p>

    <p>To enable inlining of list functions, use option <c>inline_list_funcs</c>.</p>

    <p>The following functions are inlined:</p>
    <list type="bulleted">
      <item><seemfa marker="stdlib:lists#all/2">lists:all/2</seemfa></item>
      <item><seemfa marker="stdlib:lists#any/2">lists:any/2</seemfa></item>
      <item><seemfa marker="stdlib:lists#foreach/2">lists:foreach/2</seemfa></item>
      <item><seemfa marker="stdlib:lists#map/2">lists:map/2</seemfa></item>
      <item><seemfa marker="stdlib:lists#flatmap/2">lists:flatmap/2</seemfa></item>
      <item><seemfa marker="stdlib:lists#filter/2">lists:filter/2</seemfa></item>
      <item><seemfa marker="stdlib:lists#foldl/3">lists:foldl/3</seemfa></item>
      <item><seemfa marker="stdlib:lists#foldr/3">lists:foldr/3</seemfa></item>
      <item><seemfa marker="stdlib:lists#mapfoldl/3">lists:mapfoldl/3</seemfa></item>
      <item><seemfa marker="stdlib:lists#mapfoldr/3">lists:mapfoldr/3</seemfa></item>
    </list>
  </section>

  <section>
    <title>Parse Transformations</title>

    <p>Parse transformations are used when a programmer wants to use
      Erlang syntax but with different semantics. The original Erlang
      code is then transformed into other Erlang code.</p>

    <p>See <seeerl marker="stdlib:erl_id_trans">erl_id_trans(3)</seeerl>
      for an example and an explanation of the function
      <c>parse_transform_info/0</c>.</p>
  </section>

  <section>
    <marker id="error_information"></marker>
    <title>Error Information</title>

    <p>The <c>ErrorInfo</c> mentioned earlier is the standard
      <c>ErrorInfo</c> structure, which is returned from all I/O modules.
      It has the following format:</p>
    <code>
{ErrorLocation, Module, ErrorDescriptor}</code>

    <p><c>ErrorLocation</c> is the atom <c>none</c> if the error does
    not correspond to a specific location, for example, if the source file does
    not exist.</p>

    <p>A string describing the error is obtained with the following
      call:</p>
    <code>
Module:format_error(ErrorDescriptor)</code>
  </section>

  <section>
    <title>See Also</title>
    <p>
      <seeerl marker="stdlib:epp">epp(3)</seeerl>,
      <seeerl marker="stdlib:erl_id_trans">erl_id_trans(3)</seeerl>,
      <seeerl marker="stdlib:erl_lint">erl_lint(3)</seeerl>,
      <seeerl marker="stdlib:beam_lib">beam_lib(3)</seeerl>
    </p>
  </section>
</erlref>