[erlang-questions] Avoiding boilerplate code when using gen_server

Tim Watson <>
Wed Mar 21 21:10:27 CET 2012


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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