CODEWITHSUNDEEP
erlang
Kevin Le - Khnle
Kevin Le - Khnle

Reputation: 10887

What does in ? operator/prefix in Erlang mean?

What does the question mark in ?MODULE (which can be seen in all generated code by Mochiweb make command) mean? 

-export([start/1, stop/0, loop/2]).

start(Options) ->
    {DocRoot, Options1} = get_option(docroot, Options),
       Loop = fun (Req) ->
               ?MODULE:loop(Req, DocRoot)
       end,
    mochiweb_http:start([{name, ?MODULE}, {loop, Loop} | Options1]).

stop() ->
    mochiweb_http:stop(?MODULE).

loop(Req, DocRoot) ->
    ...

Upvotes: 12

Views: 1249

Answers (3)

Bhuvan
Bhuvan

Reputation: 429

-define(Macro,Replacement). is used by the preprocessor to supports macros to have more readable programs. It can be used to have a conditional compilation. It is recommended that If a macro is used in several modules, it's definition is placed in an include file.

A macro definition example:

-define(TIMEOUT, 200).

For using macro:

?TIMEOUT.

List of predefined macros:

?MODULE: The name of the current module.
?FILE: The file name of the current module.
?LINE: The current line number.
?MACHINE: The machine name.

Source: https://www.dcs.gla.ac.uk/~amirg/tutorial/erlang/

Upvotes: 1

Muzaaya Joshua
Muzaaya Joshua

Reputation: 7836

Well this is the way we represent MACROS in Erlang. At compile time, these macros are replaced with the actual meanings. They save on re-writing pieces of code or on abstracting out a parameter you may change in future without changing your code (would only require a re-compilation of the source that depends on the MACRO).

Forexample:

-module(square_plus).
-compile(export_all).
-define(SQUARE(X),X * X).

add_to_square(This,Number)-> ?SQUARE(This) + Number.

Is the same as:

-module(square_plus).
-compile(export_all).

add_to_square(This,Number)-> (This * This) + Number.

Upvotes: 10

Cat Plus Plus
Cat Plus Plus

Reputation: 130014

It denotes a preprocessor macro. ?MODULE is one of the predefined macro constants that expand to current module's name.

Upvotes: 20

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