man compile () - Erlang Compiler


compile - Erlang Compiler


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.



Is the same as file(File, [verbose, report_errors, report_warnings]).

file(File, Options) -> CompRet

CompRet = ModRet | BinRet | ErrRet

ModRet = {ok, ModuleName} | {ok, ModuleName, Warnings}

BinRet = {ok, ModuleName, Binary} | {ok, ModuleName, Binary, Warnings}

ErrRet = error | {error, Errors, Warnings}

Compiles the code in the file File, which is an Erlang source code file without the .erl extension. Options determine the behavior of the compiler.

Returns {ok, ModuleName} if successful, or error if there are errors. An object code file is created if the compilation succeeds with no errors.

Here follows first all elements of Options that in some way control the behavior of the compiler.

basic_validation: This option is fast way to test whether a module will compile successfully (mainly useful for code generators that want to verify the code they emit). No code will generated. If warnings are enabled, warnings generated by the erl_lint module (such as warnings for unused variables and functions) will be returned too.

Use the strong_validation option to generate all warnings that the compiler would generate.

strong_validation: Similar to the basic_validation option, no code will be generated, but more compiler passes will be run to ensure also 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 run-time).
binary: Causes the compiler to return the object code in a binary instead of creating an object file. If successful, the compiler returns {ok, ModuleName, Binary}
debug_info: Include debug information in the compiled beam module. Examle of Erlang/OTP applications that can use the debug information are Debugger, Xref, and Cover.

Warning: Note that the source code can be reconstructed from the abstract code. If it is important to keep the source code secret, use the debug_info_key option (described next) to encrypt the debug information, or strip the debug information using the beam_lib module before shipping your code.

{debug_info_key, KeyString}:
{debug_info_key, {Mode, KeyString}}: Include debug information, but encrypt it, so that it cannot be accessed without supplying the key. (To give the debug_info option as well is allowed, but is not necessary.) Using this option is a good way to always have the debug information available during testing, yet protect the source code.

Mode is the type of crypto algorithm to be used for encrypting the debug information. The default mode, and currently the only, is des3_cbc (three rounds of DES). The KeyString will be scrambled (using erlang:md5) to generate the actual keys used for des3_cbc. It is recommended that the key string contains at least 32 characters, and that both upper and lower case letters as well as digits and special characters are used.

Note: As far as we know by the time of writing, it is infeasible to break des3_cbc encryption without any knowledge of the key. Therefore, as long as the key is kept safe and is unguessable, the encrypted debug information should be safe from intruders.

See the beam_lib module on how to register a key so that utilities such as Xref or the Debugger can access the debug information.

encrypt_debug_info: Like the debug_info_key option above, except that the key will be read from an .erlang.crypt file. See the beam_lib module for syntax of the .erlang.crypt file.
'P': Produces a listing of the parsed code after preprocessing and parse transforms, in the file <File>.P. No object file is produced.
'E': Produces a listing of the code after all source code transformations have been performed, in the file <File>.E. No object file is produced.
'S': Produces a listing of the assembler code in the file <File>.S. No object file is produced.
report_errors/report_warnings: Causes errors/warnings to be printed as they occur.
report: This is a short form for both report_errors and report_warnings.
return_errors: If this flag is set, then {error, ErrorList, WarningList} is returned when there are errors.
return_warnings: If this flag is set, then an extra field containing WarningList is added to the tuples returned on success.
return: This is a short form for both return_errors and return_warnings.
verbose: Causes more verbose information from the compiler describing what it is doing.
{outdir, Dir}: 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.
export_all: Causes all functions in the module to be exported.
{i, Dir}: Add Dir to the list of directories to be searched when including a file. When encountering an -include or -include_dir directive, the compiler searches for header files in the following directories:

".", the current working directory of the file server;
the base name of the compiled file;
the directories specified using the i option. The directory specified last is searched first.
{d, Macro}:
{d, Macro, Value}: Defines a macro Macro to have the value Value. The default is true).
{parse_transform, Module}: Causes the parse transformation function Module:parse_transform/2 to be applied to the parsed code before the code is checked for errors.
asm: The input file is expected to be assembler code (default file suffix ".S"). Note that the format of assembler files is not documented, and may change between releases - this option is primarily for internal debugging use.
ignore_try: try is a reserved keyword from the R9 release and may not be used as atom names or field names in records (unless single-quoted). To compile old code where try is used, the ignore_try option can be given.
ignore_cond: cond is a reserved keyword starting with the R9 release and may not be used as atom names or field names in records (unless single-quoted). To compile old code where cond is used, the ignore_cond option can be given.

If warnings are turned on (the report_warnings option described above), the following options control what type of warnings that will be generated. With the exception of {warn_format, Verbosity} all options below have two forms; one warn_xxx form to turn on the warning and one nowarn_xxx form to turn off the warning. In the description that follows, the form that is used to change the default value is listed.

{warn_format, Verbosity}: Causes warnings to be emitted for malformed format strings as arguments to io:format and similar functions. Verbosity selects the amount of warnings: 0 = no warnings; 1 = warnings for invalid format strings and incorrect number of arguments; 2 = warnings also when the validity could not be checked (for example, when the format string argument is a variable). The default verbosity is 1. Verbosity 0 can also be selected by the option nowarn_format.
nowarn_bif_clash: By default, a warning will be emitted when a module contains an exported function with the same name as an auto-imported BIF (such as size/1) AND there is a call to it without a qualifying module name. The reason is that the BIF will be called, not the function in the same module. The recommended way to eliminate that warning is to use a call with a module name - either erlang to call the BIF or ?MODULE to call the function in the same module. The warning can also be turned off using nowarn_bif_clash, but that is not recommended.
warn_export_vars: Causes warnings to be emitted for all implicitly exported variables referred to after the primitives where they were first defined. No warnings for exported variables unless they are referred to in some pattern, which is the default, can be selected by the option nowarn_export_vars.
warn_shadow_vars: Causes warnings to be emitted for "fresh" variables in functional objects or list comprehensions with the same name as some already defined variable. The default is to warn for such variables. No warnings for shadowed variables can be selected by the option nowarn_shadow_vars.
nowarn_unused_function: Turns off warnings for unused local functions. By default (warn_unused_function), warnings are emitted for all local functions that are not called directly or indirectly by an exported function. The compiler does include unused local functions in the generated beam file, but the warning is still useful to keep the source code cleaner.
warn_unused_import: Causes warnings to be emitted for unused imported functions. No warnings for imported functions, which is the default, can be selected by the option nowarn_unused_import.
nowarn_unused_vars: By default, warnings are emitted for variables which are not used, with the exception of variables beginning with an underscore ("Prolog style warnings"). Use this option to turn off this kind of warnings.

Another class of warnings (introduced in the R10B release) are generated by the compiler during optimization and code generation. They warn about patterns that will never match (such as a=b), guards that will always evaluate to false, and expressions that will always fail (such as atom+42). Currently, those warnings cannot be disabled (except by disabling all warnings).


Obviously, the absence of warnings does not mean that there are no remaining errors in the code.

Note that all the options except the include path ({i, Dir}) can also be given in the file with a -compile([Option, ...]). attribute.

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

Unrecognized options are ignored.

Both WarningList and ErrorList have the following format:


ErrorInfo is described below. The file name has been included here as the compiler uses the Erlang pre-processor epp, which allows the code to be included in other files. For this reason, it is important to know to which file an error or warning line number refers.


Is the same as forms(File, [verbose, report_errors, report_warnings]).

forms(Forms, Options) -> CompRet

Forms = [Form]

CompRet = BinRet | ErrRet

BinRet = {ok, ModuleName, BinaryOrCode} | {ok, ModuleName, BinaryOrCode, Warnings}

BinaryOrCode = binary() | term() <V>ErrRet = error | {error, Errors, Warnings}

Analogous to file/1, but takes a list of forms (in the Erlang abstract format representation) as first argument. The option binary is implicit; i.e., no object code file is produced. Options that would ordinarily produce a listing file, such as 'E', will instead cause the internal format for that compiler pass (an Erlang term; usually not a binary) to be returned instead of a binary.

format_error(ErrorDescriptor) -> string()

ErrorDescriptor = errordesc()

Uses an ErrorDescriptor and returns a string which describes the error. This function is usually called implicitly when an ErrorInfo structure is processed. See below.

Default compiler options

The (host operating system) environment variable ERL_COMPILER_OPTIONS can be used to give default compiler options. Its value must be a valid Erlang term. If the value is a list, it will be used as is. If it is not a list, it will be put into a list. The list will be appended to any options given to file/2 or forms/2.


The compiler can now 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 will be reported as occurring in the function the body was inlined into. Also, function_clause exceptions will be converted to similar case_clause exceptions.

When a function is inlined, the original function may be kept as a separate function as well, because there might still be calls to it. Therefore, inlining almost always increases code size.

Inlining does not necessarily improve running time. For instance, inlining may increase Beam stack usage which will probably be detrimental to performance for recursive functions.

Inlining is never default; it must be explicitly enabled with a compiler option or a '-compile()' attribute in the source module.

To enable inlining, use the 'inline' option.



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


%% Aggressive inlining - will increase code size.

Parse Transformations

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.

Error Information

The ErrorInfo mentioned above is the standard ErrorInfo structure which is returned from all IO modules. It has the following format

    {ErrorLine, Module, ErrorDescriptor}

A string describing the error is obtained with the following call:

apply(Module, format_error, ErrorDescriptor)

See Also


Bjorn Gustavsson -