[erlang-questions] Extracting pure functions

[Gordon Guthrie]

Judson

That mechanism is perfect for building highly testable services.

Effectively you are transforming the testing domain from:
* if I receive these inputs then I emit these side effects

to:
* if I were to receive these inputs then I would have emitted these side effects

To test the former you have two choices:
* build a full system test
* meck the living bejesus out of your system

This mechanism is particularly useful if the serve contains a large data structure that needs to be manipulated in memory.

Gordon

> Le 20 févr. 2015 à 19:46, Judson Lester <nyarly@REDACTED> a écrit :
> 
> 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 sometimes.
> 
> handle_event_message(Arg, 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:
> 
> handle_event_message(Arg, State=#state{collected_args=Args,event_server=Ev}) ->
>   {Effects, State} = pure_handle(Arg, State),
>   [ apply(Mod, Fun, Args) || {Mod, Fun, Args} <- Effects],
>   State.
> 
> 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?
> 
> Judson
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