Erlang -- The Preprocessor

User's Guide
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Erlang Reference Manual
User's Guide
Version 5.8.3

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Chapters

- Introduction
  - Top of chapter
  - Purpose
  - Prerequisites
  - Document Conventions
  - Complete List of BIFs
  - Reserved Words
  - Character Set
- Data Types
  - Top of chapter
  - Terms
  - Number
  - Atom
  - Bit Strings and Binaries
  - Reference
  - Fun
  - Port Identifier
  - Pid
  - Tuple
  - List
  - String
  - Record
  - Boolean
  - Escape Sequences
  - Type Conversions
- Pattern Matching
  - Top of chapter
  - Pattern Matching
- Modules
  - Top of chapter
  - Module Syntax
  - Module Attributes
  - Comments
  - The module_info/0 and module_info/1 functions
- Functions
  - Top of chapter
  - Function Declaration Syntax
  - Function Evaluation
  - Tail recursion
  - Built-In Functions, BIFs
- Types and Function Specifications
  - Top of chapter
  - Introduction of Types
  - Types and their Syntax
  - Type declarations of user-defined types
  - Type information in record declarations
  - Specifications for functions
- Expressions
  - Top of chapter
  - Expression Evaluation
  - Terms
  - Variables
  - Patterns
  - Match
  - Function Calls
  - If
  - Case
  - Send
  - Receive
  - Term Comparisons
  - Arithmetic Expressions
  - Boolean Expressions
  - Short-Circuit Expressions
  - List Operations
  - Bit Syntax Expressions
  - Fun Expressions
  - Catch and Throw
  - Try
  - Parenthesized Expressions
  - Block Expressions
  - List Comprehensions
  - Bit String Comprehensions
  - Guard Sequences
  - Operator Precedence
- The Preprocessor
  - Top of chapter
  - File Inclusion
  - Defining and Using Macros
  - Predefined Macros
  - Macros Overloading
  - Flow Control in Macros
  - Stringifying Macro Arguments
- Records
  - Top of chapter
  - Defining Records
  - Creating Records
  - Accessing Record Fields
  - Updating Records
  - Records in Guards
  - Records in Patterns
  - Nested records
  - Internal Representation of Records
- Errors and Error Handling
  - Top of chapter
  - Terminology
  - Exceptions
  - Handling of Run-Time Errors in Erlang
  - Exit Reasons
- Processes
  - Top of chapter
  - Processes
  - Process Creation
  - Registered Processes
  - Process Termination
  - Message Sending
  - Links
  - Error Handling
  - Monitors
  - Process Dictionary
- Distributed Erlang
  - Top of chapter
  - Distributed Erlang System
  - Nodes
  - Node Connections
  - epmd
  - Hidden Nodes
  - C Nodes
  - Security
  - Distribution BIFs
  - Distribution Command Line Flags
  - Distribution Modules
- Compilation and Code Loading
  - Top of chapter
  - Compilation
  - Code Loading
  - Code Replacement
  - Running a function when a module is loaded
- Ports and Port Drivers
  - Top of chapter
  - Ports
  - Port Drivers
  - Port BIFs

# 8 The Preprocessor

### 8.1 File Inclusion

A file can be included in the following way:

    -include(File).
    -include_lib(File).

File, a string, should point out a file. The contents of this file are included as-is, at the position of the directive.

Include files are typically used for record and macro definitions that are shared by several modules. It is recommended that the file name extension .hrl be used for include files.

File may start with a path component $VAR, for some string VAR. If that is the case, the value of the environment variable VAR as returned by os:getenv(VAR) is substituted for $VAR. If os:getenv(VAR) returns false, $VAR is left as is.

If the filename File is absolute (possibly after variable substitution), the include file with that name is included. Otherwise, the specified file is searched for in the current working directory, in the same directory as the module being compiled, and in the directories given by the include option, in that order. See erlc(1) and compile(3) for details.

Examples:

    -include("my_records.hrl").
    -include("incdir/my_records.hrl").
    -include("/home/user/proj/my_records.hrl").
    -include("$PROJ_ROOT/my_records.hrl").

include_lib is similar to include, but should not point out an absolute file. Instead, the first path component (possibly after variable substitution) is assumed to be the name of an application. Example:

    -include_lib("kernel/include/file.hrl").

The code server uses code:lib_dir(kernel) to find the directory of the current (latest) version of Kernel, and then the subdirectory include is searched for the file file.hrl.

### 8.2 Defining and Using Macros

A macro is defined the following way:

    -define(Const, Replacement).
    -define(Func(Var1,...,VarN), Replacement).

A macro definition can be placed anywhere among the attributes and function declarations of a module, but the definition must come before any usage of the macro.

If a macro is used in several modules, it is recommended that the macro definition is placed in an include file.

A macro is used the following way:

    ?Const
    ?Func(Arg1,...,ArgN)

Macros are expanded during compilation. A simple macro ?Const will be replaced with Replacement. Example:

    -define(TIMEOUT, 200).
    ...
    call(Request) ->
        server:call(refserver, Request, ?TIMEOUT).

This will be expanded to:

    call(Request) ->
        server:call(refserver, Request, 200).

A macro ?Func(Arg1,...,ArgN) will be replaced with Replacement, where all occurrences of a variable Var from the macro definition are replaced with the corresponding argument Arg. Example:

    -define(MACRO1(X, Y), {a, X, b, Y}).
    ...
    bar(X) ->
        ?MACRO1(a, b),
        ?MACRO1(X, 123)

This will be expanded to:

    bar(X) ->
        {a,a,b,b},
        {a,X,b,123}.

It is good programming practice, but not mandatory, to ensure that a macro definition is a valid Erlang syntactic form.

To view the result of macro expansion, a module can be compiled with the 'P' option. compile:file(File, ['P']). This produces a listing of the parsed code after preprocessing and parse transforms, in the file File.P.

### 8.3 Predefined Macros

The following macros are predefined:

?MODULE
  The name of the current module.
?MODULE_STRING.
  The name of the current module, as a string.
?FILE.
  The file name of the current module.
?LINE.
  The current line number.
?MACHINE.
  The machine name, 'BEAM'.

### 8.4 Macros Overloading

It is possible to overload macros, except for predefined macros. An overloaded macro has more than one definition, each with a different number of arguments.

The feature was added in Erlang 5.7.5/OTP R13B04.

A macro ?Func(Arg1,...,ArgN) with a (possibly empty) list of arguments results in an error message if there is at least one definition of Func with arguments, but none with N arguments.

Assuming these definitions:

    -define(F0(), c).
    -define(F1(A), A).
    -define(C, m:f).

the following will not work:

    f0() ->
        ?F0. % No, an empty list of arguments expected.

    f1(A) ->
        ?F1(A, A). % No, exactly one argument expected.

On the other hand,

    f() ->
        ?C().

will expand to

    f() ->
        m:f().

### 8.5 Flow Control in Macros

The following macro directives are supplied:

-undef(Macro).
  Causes the macro to behave as if it had never been defined.
-ifdef(Macro).
  Evaluate the following lines only if Macro is defined.
-ifndef(Macro).
  Evaluate the following lines only if Macro is not defined.
-else.
  Only allowed after an ifdef or ifndef directive. If that condition was false, the lines following else are evaluated instead.
-endif.
  Specifies the end of an ifdef or ifndef directive.

Note

The macro directives cannot be used inside functions.

Example:

    -module(m).
    ...

    -ifdef(debug).
    -define(LOG(X), io:format("{~p,~p}: ~p~n", [?MODULE,?LINE,X])).
    -else.
    -define(LOG(X), true).
    -endif.

    ...

When trace output is desired, debug should be defined when the module m is compiled:

    % erlc -Ddebug m.erl

    or

    1> c(m, {d, debug}).
    {ok,m}

?LOG(Arg) will then expand to a call to io:format/2 and provide the user with some simple trace output.

### 8.6 Stringifying Macro Arguments

The construction ??Arg, where Arg is a macro argument, will be expanded to a string containing the tokens of the argument. This is similar to the #arg stringifying construction in C.

The feature was added in Erlang 5.0/OTP R7.

Example:

    -define(TESTCALL(Call), io:format("Call ~s: ~w~n", [??Call, Call])).

    ?TESTCALL(myfunction(1,2)),
    ?TESTCALL(you:function(2,1)).

results in

    io:format("Call ~s: ~w~n",["myfunction ( 1 , 2 )",m:myfunction(1,2)]),
    io:format("Call ~s: ~w~n",["you : function ( 2 , 1 )",you:function(2,1)]).

That is, a trace output with both the function called and the resulting value.