03 March 2014

I got an programming assignment in a course I was taking. The task was to create an overlay network topology and implement a routing protocol for it with some given constraints — where I quickly realized a hypercube mesh would meet the rating criteria. (This shows my age — hypercube networks were a hot topic in the 90’s. They were used in supercomputers such as the CM-5.)

Unfortunately a binary N-cube routing algorithm is pretty much trivial. Here’s the whole routing algorithm written in Erlang:

find_route(_,_,[],_) ->
{error,noroute};

find_route(Id,Dst,[Route|Routes]) ->
if Id band 1 /= Dst band 1 ->
{ok,Route};
true ->
find_route(Id bsr 1, Dst bsr 1, Routes)
end.


That’s it. 8 lines of code. The first function could be omitted (deducting two lines) as it guaranteed to be never called. (Id is this node’s address and Dst is destination address, each element in the Routes list is a neighbor in the matching dimension.)

Since the actual network problem became trivial you can see why I picked up Erlang. It was for the sole purpose of making the assignment more interesting. I had not previously used Erlang — I was familiar with the syntax and could read Erlang programs — but all the libraries, conventions etc. were new to me. I knew of Erlang’s approach to distributed computing and parallelism and wanted to give it a spin.

So, what I learned? I’ll first summarize its pros and cons from my viewpoint and later elaborate on these:

ProsCons
• Language and core libraries are compact, consistent and mature
• Built-in concurrency and messaging
• Pattern matching
• Symbols are always sexy
• Registered processes
• Cryptic compile-time and runtime errors
• Package management
• Structured data ~ painful syntax
• No hierarchical namespaces
• Registered processes

The overall result for me is:

• Erlang is a very nice language, it has great features and I’d love to use it again.
• … but it won’t become my default go-to language.

Please note that I’m basing this post on my experiences. I might have missed or misinterpreted things that are obvious to other people, so don’t take this post as any kind of gospel truth of Erlang.

Now the long version. Erlang is really nice in several aspects:

• The language is compact and consistent and the standard libraries are mature (e.g. well documented and debugged). There’s also a good variety of non-core libraries available which I didn’t have any trouble of using.

• Its built-in support for massive concurrency and distributed messaging are just manna from heavens.

Erlang’s lightweight process model just kicks ass. I’ve spawned 15k Erlang processes (e.g. threads) without problems whereas in Python 1000 threads? Forget it (you’ll hit the maxproc limit). 99.99% of the time parallelism is a tool to achieve asynchronous behavior so that case should be as least limiting as possible. Like Erlang does (it runs green threads on multiple native threads, getting the best of both worlds).

Also the shared-nothing process model removes most problems with shared resources. It does make some things more cumbersome and less efficient, but hey, I’m quite willing to trade a little inefficiency to programming with a massively less error-prone concurrency model.

• Pattern matching in functions, assignment and conditionals is sinfully easy. Don’t care to handle errors, but still want the process to fail when they occur? {ok,Result} = maybe_failing_function() — if the function does not return {ok,_} the runtime will signal that as an error. And of course guards.

• Symbols are always a good thing. Scheme, Ruby and Erlang (among others) do this right. Oh Python, when will you realize symbols are a very useful first-class citizen?

• Registered processes. They are very useful when they fit the need, but see below when they don’t.

On the minus side there are a few things that will mean that Erlang won’t be my choice as a default go-to language in the future:

• Compiler and runtime errors. So I forgot to make the variable uppercase and now it thinks I want to do pattern matching with a symbol? My bad. But you just should give a less cryptic error message about it.

• Package management. Rebar can pull dependencies automatically, but there’s still a world of difference between writing PyYAML into setup.py vs. writing {jiffy, "0\.8\.5", {git, "https://github.com/davisp/jiffy", {tag, "0.8.5"}}} into rebar.config over and over again. This is not a problem for large projects where dependency setup is one-time-only affair, but when doing smaller or one-off programming jobs it would add up quickly to the workload.

• Horrendous syntax for structured data. Changing a field? NewStruct = Struct#struct{value=Struct#struct.value + 1} A little syntatic sugar here would do miracles. Yes, you can use parse transformations to help. But that then gets you into another problem of having to first get those parse transforms (see previous) and second to apply them.

• Registered processes. These are nice, I would very much want to use them but can’t. The idea is that you can register processes by name, say identify the router process as router and use it directly in messaging like router ! {route,Packet}. Add a supervisor which will re-spawn a failed router thread you’ve got a model where you always know how to reach a working “router” process.

Except registered processes are global. I needed to run multiple hypercube nodes in a single server process, with each having a separate router and separate local message handler and separate remote connectors and separate state manager.

I think this is due to the history of Erlang. It was designed to run in a loosely coupled but purpose-designed system (telco exchanges). In that context it made perfect sense to have a globally identified process. After all, there was only a single system, so why there would be any need for multiples of any single process?

Which does not work in a multitenancy scenario where you have multiple “domains” of processes, where intra-domain visibility is a good thing but inter-domain visibility is verboten.

So I was stuck to using the process dictionary and lugging process identifiers around in function arguments.

• Lack of hierarchical namespaces. It is two levels and only two levels. Module and function name and that’s it. So when your frobnizer app needs to have internal module, it is frobnizer_internal_module and not frobnizer.internal.module. Module hierarchy and scoping isn’t only syntatic sugar regardless what hard-core erlangistas say. I personally have found module hierarchy and its close ally, scoping rules a useful feature in other languages. So why not here? I don’t understand the opposition for such a simple and non-intrusive change.

I love Erlang’s pragmatic approach to functional programming, contrasted with Haskell for example, which as a language I simply admire but Always. Find. It. Painful. To. Do. Anything. Useful. Using. It. We don’t write programs (at least mostly) for the pleasure of seeing beautiful and pure programs. We write programs to get things done in a real-world environment where interactions with that non-functional world is the primus motor. So why make that painful?

Erlang is a functional language which understands its purpose of interacting with the non-functional real world. Functional but does not try to whack you with the +4 Mace of Lambda the Pure every time you interact with the world.

Interestingly I see a pattern in my choice of programming languages. The languages I use the most have the following traits:

• Good package management with a centralized package directory.
• Ability to write quick one-off programs easily (scripting).

I could add “nice syntax” etc., but that’s beside the point. I don’t do non-nice languages. I want to retain my sanity. (So goodbye the lucrative MUMPS jobs there.)

In a world where you do not write your own JSON parser, networking library, UI framework, HTTP request processor etc. etc. the ability to easily discover, pull and manage external dependencies is important. My life is too short to waste on libraries and languages which start with “to install, first … then … then” instead of pip install thispackage or even “download, unpack, ./configure && make install”.

Somehow Erlang falls short of my definition of “good package management” and “good scripting”. Not by much, but still.