Erlang: "extending" an existing module with new functions

Question

I'm currently writing some functions that are related to lists that I could possibly be reused.

My question is:

Are there any conventions or best practices for organizing such functions?

To frame this question, I would ideally like to "extend" the existing lists module such that I'm calling my new function the following way: lists:my_funcion(). At the moment I have lists_extensions:my_function(). Is there anyway to do this?

I read about erlang packages and that they are essentially namespaces in Erlang. Is it possible to define a new namespace for Lists with new Lists functions?

Note that I'm not looking to fork and change the standard lists module, but to find a way to define new functions in a new module also called Lists, but avoid the consequent naming collisions by using some kind namespacing scheme.

Any advice or references would be appreciated.

Cheers.

Answer 1

To frame this question, I would ideally like to "extend" the existing lists module such that I'm calling my new function the following way: lists:my_funcion(). At the moment I have lists_extensions:my_function(). Is there anyway to do this?

No, so far as I know.

I read about erlang packages and that they are essentially namespaces in Erlang. Is it possible to define a new namespace for Lists with new Lists functions?

They are experimental and not generally used. You could have a module called lists in a different namespace, but you would have trouble calling functions from the standard module in this namespace.

Answer 2

I give you reasons why not to use lists:your_function() and instead use lists_extension:your_function():

• Generally, the Erlang/OTP Design Guidelines state that each "Application" -- libraries are also an application -- contains modules. Now you can ask the system what application did introduce a specific module? This system would break when modules are fragmented.

However, I do understand why you would want a lists:your_function/N:

• It's easier to use for the author of your_function, because he needs the your_function(...) a lot when working with []. When another Erlang programmer -- who knows the stdlb -- reads this code, he will not know what it does. This is confusing.
• It looks more concise than lists_extension:your_function/N. That's a matter of taste.

Answer 3

I think this method would work on any distro:

You can make an application that automatically rewrites the core erlang modules of whichever distribution is running. Append your custom functions to the core modules and recompile them before compiling and running your own application that calls the custom functions. This doesn't require a custom distribution. Just some careful planning and use of the file tools and BIFs for compiling and loading. * You want to make sure you don't append your functions every time. Once you rewrite the file, it will be permanent unless the user replaces the file later. Could use a check with module_info to confirm of your custom functions exist to decide if you need to run the extension writer.

Pseudo Example:

lists_funs() -> ["myFun() -> <<"things to do">>."].
extend_lists() -> 
   {ok, Io} = file:open(?LISTS_MODULE_PATH, [append]),
   lists:foreach(fun(Fun) -> io:format(Io,"~s~n",[Fun]) end, lists_funs()),
   file:close(Io),
   c(?LISTS_MODULE_PATH).

* You may want to keep copies of the original modules to restore if the compiler fails that way you don't have to do anything heavy if you make a mistake in your list of functions and also use as source anytime you want to rewrite the module to extend it with more functions. * You could use a list_extension module to keep all of the logic for your functions and just pass the functions to list in this function using funName(Args) -> lists_extension:funName(Args). * You could also make an override system that searches for existing functions and rewrites them in a similar way but it is more complicated. I'm sure there are plenty of ways to improve and optimize this method. I use something similar to update some of my own modules at runtime, so I don't see any reason it wouldn't work on core modules also.

Answer 4

i guess what you want to do is to have some of your functions accessible from the lists module. It is good that you would want to convert commonly used code into a library.

one way to do this is to test your functions well, and if their are fine, you copy the functions, paste them in the lists.erl module (WARNING: Ensure you do not overwrite existing functions, just paste at the end of the file). this file can be found in the path $ERLANG_INSTALLATION_FOLDER/lib/stdlib-{$VERSION}/src/lists.erl. Make sure that you add your functions among those exported in the lists module (in the -export([your_function/1,.....])), to make them accessible from other modules. Save the file.

Once you have done this, we need to recompile the lists module. You could use an EmakeFile. The contents of this file would be as follows:

{"src/*", [verbose,report,strict_record_tests,warn_obsolete_guard,{outdir, "ebin"}]}.

Copy that text into a file called EmakeFile. Put this file in the path: $ERLANG_INSTALLATION_FOLDER/lib/stdlib-{$VERSION}/EmakeFile.

Once this is done, go and open an erlang shell and let its pwd(), the current working directory be the path in which the EmakeFile is, i.e. $ERLANG_INSTALLATION_FOLDER/lib/stdlib-{$VERSION}/.

Call the function: make:all() in the shell and you will see that the module lists is recompiled. Close the shell.

Once you open a new erlang shell, and assuming you exported you functions in the lists module, they will be running the way you want, right in the lists module. Erlang being open source allows us to add functionality, recompile and reload the libraries. This should do what you want, success.