[erlang-questions] Avoiding boilerplate code when using gen_server

Torben Hoffmann torben.lehoff@REDACTED
Thu Mar 22 09:09:03 CET 2012

As OP I take the liberty to do a top post and try to summarise a few 
things at this point.

I am not alone when it comes to having an urge to remove some of the 
boilerplate code. I guess we all want value for our money and when we 
have to do something that does not provide that we want to improve. If 
we did not have that craving we might as well be coding enterprise Java 
applications using dozens of frameworks ;-)

I had a look at the e2project and it actually does remove some of the 
boilerplate that I find the most iritating, namely the tagging of 
messages from the API function to the handle_* function. It is a lot 
clearer in e2project where you reference a function in the 
e2_service:call - I like that a lot.

The approach with parse transforms has its merits, but I think that the 
e2project can do away with most of the pains I have been feeling, so I 
think I will leave out the semi-black magic of parse transforms for 
now... it is a tool that should be used with caution as other posters 
have mentioned.

The IDL approach I had in mind was nothing about CORBA - I should never 
have mentioned CORBA in my original mail, there are too many negative 
feelings associated to CORBA.
What I like about IDLs is that it allows you to spec your API in a nice 
way and in many cases it will be possible to avoid writing boilerplate 
code (either through code generation or using a parse transform).

Given the good point made about separating the client and the server 
side of things I think that what is needed is simply to spec up the API 
function (who doesn't do that already?!?!) and then try out e2project. 
The spec will give me a slap over my fingers if I do not provide the 
function (I like that) and e2project seems to minimise the tedious 
boilerplate stuff from standard gen_server without limiting my ability 
to express what I need.

A big thanks to all posters - I am glad I asked the question!


On 21/3/12 21:10 , Tim Watson wrote:
> On 21 Mar 2012, at 19:22, Garrett Smith wrote:
>> This may come down to personal taste. I don't like having to dig
>> around in parse transform code to understand what my module looks
>> like.
> Neither do I - that's why I simply run 'escript `evm 
> site`/parse_trans/ebin/parse_trans_pp.beam ebin/<target>.beam' over 
> the generated beam to see what it looks like at source level. ;)
>> eunit is a great example. If you really love eunit, then I'm not going
>> to convince you :)
> Common test and PropEr do exactly the same thing. I'm not sure what 
> you're expected testing frameworks to do instead!?
>>>> - Hiding the client-server distinction in Erlang is a terrible 
>>>> disservice
>>> I agree with this wholeheartedly, but don't see what it has to do 
>>> with code generation.
>> If you consolidate both the client and server functions into one
>> function, you've obfuscated an important distinction.
> I never suggested doing that at all. Not that I mean to be prickly, as 
> we're all part of the same community and whatnot, but nobody suggested 
> separating both functions. What the OP suggested was that you could 
> define the interface declaratively - e.g., without writing the code 
> directly - and that was what I was responding to. As I mentioned later 
> on, in this particular gen_server case I actually think your approach 
> is probably cleaner and more appropriate, but it's good to separate 
> these points and refine the discussion I think.
>>>> IMO, e2 solves the problem of "too much boiler plate". It's really
>>>> easy and requires zero magic.
>>>> Here's a "gen_server" equivalent in e2:
>>>> https://github.com/gar1t/e2v2/blob/master/examples/ping/src/ping_server.erl
>>> This is pretty cool though. :)
>>>> You might object to the separation of "ping" and "handle_msg" -- it
>>>> appears to be just one function, so why break it up into two pieces?
>>>> The problem is that it's not one function -- it's definitely *two*
>>>> very separate pieces of code.
>>> Absolutely and a good point!
>>>> When you write a gen_server style module, you're writing code that
>>>> support client-server interactions. You have, in the same module, code
>>>> that is called by the "client" and code that is called by the
>>>> "server". If you don't grok this, you're missing the entire point of
>>>> this type of module.
>>>> Code that hides this difference -- e.g. a parse transform that gloms
>>>> the client and server functions into one -- is IMO a Really Bad Idea.
>>> I don't necessarily agree with this, as the parse transform can be 
>>> applied to a -spec which is potentially as intention revealing as a 
>>> hand written function. I do take your point about them being two 
>>> separate functions and perhaps in this particular case (for 
>>> gen_server) you are actually correct and having two hand coded parts 
>>> is actually better. I don't buy 'code-gen is bad' as a general 
>>> argument (which perhaps you weren't going so far as to make anyway), 
>>> and I do see your point in this instance.
>> I'm certainly not arguing against code generation. I think Erlang's
>> parse transform scheme is *very* good -- flexible enough to enable
>> elegant solutions but with enough complexity to scare off casual "meta
>> programmers".
>> But -- I think there should be a very clear payoff for using a parse
>> transform. I don't think the discussion here, which is "boilerplate"
>> is a good application for that.
> I think it depends on how much boilerplate you're looking at. As an 
> OCaml programmer I have seen a *lot* of work going into removing 
> boilerplate code, and the same in Haskell (although there I am less 
> experienced) and overall I think the payoff you're describing is a 
> value judgement based on the negative impact of using code 
> generation. Generated code is brittle - there's no two ways about it. 
> But it doesn't prevent you from testing or documenting, nor does it 
> have to make your code opaque and difficult to understand, so long as 
> it is not overused - a sprinkle here and there rather than a bucket load.
>> A good example of a value-add parse transform IMO is modlib, which
>> lets you create mods for inets httpd withou causing your brain to
>> explode [1].
>>>> As an example of how this client/server difference is important,
>>>> consider form validation in a web app. There are two places you can
>>>> run code to validate form input -- you can validate it in the browser
>>>> using JavaScript, or you can send the form data to the server. Web
>>>> developers may opt for browser validation to avoid the expense of
>>>> sending data to the server -- or they might prefer it on the server.
>>>> Either way, it's an important consideration.
>>>> This same dynamic applies to gen_server style modules. If you stick
>>>> with it, I think you'll appreciate the separateness of client and
>>>> server facing code.
>>>> As for the boilerplate, I couldn't agree with you more!
>>>> Garrett
>>>> P.S. I'm giving a talk on e2 at the SF Factory in a couple weeks,
>>>> where I'll get into this in more details. Also, the e2 docs and github
>>>> project are in slight disarray at the moment, but will be put in order
>>>> this weekend!
>> [1] https://github.com/gar1t/modlib


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