[erlang-questions] Extracting pure functions

Judson Lester nyarly@REDACTED
Fri Feb 20 20:46:49 CET 2015

I'm thinking about how to separate pure functions from those that cause
side effects, and one of the places I'm trying to find a decent pattern for
is message passing.

I'm primarily concerned with being able to easily test the functional
components as units, and then use integration testing for the parts that
involve a side effect.

For instance, I have a gen_server that handles a message and emits an event

State=#state{collected_args=Args,event_server=Ev}) ->
  gen_event:notify(Ev, {some_event, Arg}),
  State#state{collected_args[Arg |Args]}.

(obviously, I'm simplifying a lot here)

What I'm wondering about is extracting something like:

State=#state{collected_args=Args,event_server=Ev}) ->
  {Effects, State} = pure_handle(Arg, State),
  [ apply(Mod, Fun, Args) || {Mod, Fun, Args} <- Effects],

pure_handle(Arg, State) ->
  { [{gen_event, notify, [Ev, {some_event, Arg}]}],
    State#state{collected_args[Arg |Args]} }.

Which would mean I could test the effects that pure_handle/2 intends in a
unit test.

Is this a pattern others use? A terrible delusion? Not interesting?

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