[erlang-questions] Mocking in Erlang

Jeremy Wall <>
Tue Jun 3 14:58:06 CEST 2008

An Interesting approach. I'm going to have to try this later.

On Tue, Jun 3, 2008 at 7:14 AM, Dominic Williams <
> wrote:

> Hi Jeremy,
> > what is the typical way to handle the case where your module
> > uses ibrowse say.
> >
> > is the best practice in testing just to not mock ibrowse?  or
> > do you typically just isolate the ibrowse calls in a small
> > untested function and test everything else as normal?  or is
> > there some other trick I dont know about yet?
> This is one of the reasons that I do not subscribe to the
> "traditional" Erlang practise of writing interface modules that
> hide message passing behind function calls. I like to expose the
> message passing and have the message protocol /be/ the official
> interface. Testing and mocking become very much easier.
> However, when faced with existing applications that use the
> interface module approach (such as, in your case, ibrowse) , I
> resort to one of the following to perform mocking:
> - define my own set of messages (usually a small subset of the
>  applications full functionality) and introduce a bridge process
> - invert the dependency: avoid side-effects and don't call out to
>  ibrowse directly from the module that needs to be tested: have
>  it return lists of "ibrowse command" terms, and let the caller
>  pass these on to a dumb (untested) module that makes the actual
>  ibrowse calls given these terms
> - create higher-order functions: always pass into the functions
>  of the module you want to test (that makes the ibrowse calls)
>  the functions it should use to make the calls. When there are
>  too many functions to make this practical, either pass in a
>  module, or pass in a single function whose first argument could
>  be the ibrowse function name...
> - use parameterised modules, with the name of the module to be
>  used to make ibrowse calls being a parameter.
> I am driven to adopt such approaches in order to achieve good
> testability, but in my experience they turn out to be good
> designs in themselves, with more flexibility and modularity.
> Regards,
> Dominic Williams
> http://dominicwilliams.net
> ----

Jeremy Wall

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