[erlang-questions] Re: how to patch a standard module

Tomas Abrahamsson <>
Fri Jan 14 17:42:54 CET 2011

>> > > I'd like to extend httpd_request.erl to handle the OPTIONS method.
>> > > I've managed to do this by copying httpd_request.erl into my project
>> > > and making sure it overrides the standard version.
>> > > My question: is it possible to redefine a function in a standard
>> > > module at runtime? Then I could define my own validate(Method, Uri,
>> > > Version) function (just a few lines) and keep using the standard
>> > > httdp_request.erl.

> It is actually possible to to what you want. I have used a piece of
> code that could replace functions in existing modules by manipulating
> the beam code. In that project we only used it in eunit testing to stub
> functions in order to isolate the tested code.
> I'm not sure about using that method in a production system though.
> I don't have the code available any more so I can't provide any details
> about it but I think that Tomas Abrahamsson who sometimes contributes to
> this list could give some pointers on how to do it if he reads this.

The stubber that Ola refers to is a bit similar to meck
and some other mocking libraries. It provides the
possibility to override all or only some functions of a
module, while still making the rest of the functions
available unchanged.

The stubber has evolved a bit since: Klas Johansson
has done a fair bit of useful work on it, to say the
least, and I guess he might even make his work publicly
available eventually(?)

It is mainly intended for (e)unit testing, though.

The way the existing modules were extended with new or
changed functions, was by simply renaming the beam file,
then provide a new beam module which either calls the
renamed module for all non-changed functions, or uses the
-extends(RenamedModule) feature (undocumented, I think).
The new module was compiled to a binary and loaded with

See https://github.com/klajo/hacks/tree/master/beam
at Klas' repository for some code to rename a beam

See http://www.trapexit.org/Extend_Module for more info
on the -extends attribute.


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