Category Archives: Computing

XML: Xtensively Mucked-up Lists (or “How A Committee Screwed Up Sexps”)

Some folks are puzzled at why I avoid XML. They ask why I “try so hard to avoid XML” and do crazy things like write ASN.1 specs, use native language terms when possible (like Python config files in Python dicts, Erlang configs in Erlang terms, etc.), consider YAML/JSON a decent last resort, and regard XML to a non-option.

I maintain that XML sucks. I believe that it is, to date, the most perfectly subtle (or not-so-subtle) way to screw up sexps. Consider that: “screw up sexps”. What a thing to say! How in the world would one even propose to screw up such a simple idea? Let’s consider an example…

Can you identify the semantic difference among the following examples?
(Inspired by the sample XML in the Python xml.etree docs)

Verson 1

<country name="Liechtenstein">
  <rank>1</rank>
  <year>2008</year>
  <gdppc>141100</gdppc>
  <neighbor name="Austria" direction="E"/>
  <neighbor name="Switzerland" direction="W"/>
</country>

Version 2

<country>
  <name>Liechtenstein</name>
  <rank>1</rank>
  <year>2008</year>
  <gdppc>141100</gdppc>
  <neighbor>
    <name>Austria</name>
    <direction>E</direction>
  </neighbor>
  <neighbor>
    <name>Switzerland</name>
    <direction>W</direction>
  <neighbor>
</country>

Version 3

<country name="Liechtenstein" rank="1" year="2008" gdppc="141100">
  <neighbor name="Austria" direction="E"/>
  <neighbor name="Switzerland" direction="W"/>
</country>

Version 4

And here there is a deliberate semantic difference, meant to be illustrative of a certain property of trees… which is supposedly the whole point.

<entries>
  <country rank="1" year="2008" gdppc="141100">
    <name>Liechtenstein</name>
    <neighbors>
      <name direction="E">Austria</name>
      <name direction="W">Switzerland</name>
    </neighbors>
  </country>
</entries>

Which one should you choose for your application? Which one is obvious to a parser? From which could you more than likely write a general parsing routine that could pull out data that meant something? Which one could you turn into a program by defining the identifier tags as functions somewhere?

Consider that last two questions carefully. The so-called “big data” people are hilarious, especially when they are XML people. There is a difference between “not a large enough sample to predict anything specific” and “a statistically significant sample from which generalities can be derived”, certainly, but that has a lot more to do with representative sample data than the sheer number of petabytes you have sitting on disk somewhere. “Big Data” should really be about “Big Meaning”, but we seem to have all missed the boat on that. The reason I so hate XML is because the complexity and ambiguity introduced in an effort to make the X in XML mean something has made meanings a lot less clear to the people who have to parse (or design, document, discuss, edit, etc.) XML schemas.

Erlang: Writing Terms to a File for file:consult/1

I notice that there are a few little helper functions I seem to always wind up writing given different contexts. In Erlang one of these is an inverse function for file:consult/1, which I have to write any time I use a text file to store config data*.

Very simply:

write_terms(Filename, List) ->
    Format = fun(Term) -> io_lib:format("~tp.~n", [Term]) end,
    Text = lists:map(Format, List),
    file:write_file(Filename, Text).

[Note that this *should* return the atom 'ok' — and if you want to check and assertion or crash on failure, you want to do ok = write_terms(Filename, List) in your code.]

This separates each term in a list by a period in the text file, which causes file:consult/1 to return the same list back (in order — though this detail usually does not matter because most conf files are used as proplists and are keysearched anyway).

An annoyance with most APIs is a lack of inverse functions where they could easily be written. Even if the original authors of the library don’t conceive of a use for an inverse of some particular function, whenever there is an opportunity for this leaving it out just makes an API feel incomplete (and don’t get me started on “web APIs”… ugh). This is just one case of that. Why does Erlang have a file:consult/1 but not a file:write_terms/2 (or “file:deconsult/2” or whatever)? I don’t know. But this bugs me in most libs in most languages — this is the way I usually deal with this particular situation in Erlang.

[* term_to_binary/1 ←→ binary_to_term/1 is not an acceptable solution for config data!]

Erlang: Maps, Comprehensions and Side-effecty Iteration

In Erlang it is fairly common to want to perform a side-effecty operation over a list of values, not because you want to collect an aggregate (fold), actually map the input list to the output list (map), or build a new list in some way (list comprehension) but because you just want the side-effects of the operation.

The typical idiom (especially for broadcasting messages to a list of processes) is to use a list comprehension for this, or sometimes lists:map/2:

%% Some procedural side-effect, like spawning list of processes or grabbing
%% a list of resources:
[side_effecty_procedure(X) || X <- ListOfThings],

%% Or, for broadcasting:
[Pid ! SomeMessage || Pid <- ListOfPids],

%% And some old farts still do this:
lists:map(fun side_effecty_procedure/1, ListOfThings),

%% Or even this (gasp!) which is actually made for this sort of thing:
lists:foreach(fun side_effecty_procedure/1, ListOfThings),
%% but lacks half of the semantics I describe below, so this part
%% of the function namespace is already taken... (q.q)

That works just fine, and is so common that list comprehensions have been optimized to handle this specific situation in a way that avoids creating a return list value if it is clearly not going to be assigned to anything. I remember thinking this was sort of ugly, or at least sort of hackish before I got accustomed to the idiom, though. “Hackish” in the sense that this is actually a syntax intended for the construction of lists and only incidentally a useful way to write a fast side-effect operation over a list of values, and “ugly” in the sense that it is one of the few places in Erlang you can’t force an assertion to check the outcome of a side-effecty operation.

For example, there is no equivalent to the assertive ok = some_procedure() idiom, or even the slightly more complex variation used in some other situations:

case foo(Bar) of
    ok    -> Continue();
    Other -> Other
end,

a compromise could be to write an smap/2 function defined something like

smap(F, List) ->
    smap(F, 1, List).

smap(F, N, []) -> ok;
smap(F, N, [H|T]) ->
    case F(H) of
        ok              -> smap(F, N + 1, T);
        {error, Reason} -> {error, {N, Reason}}
    end.

But this now has the problem of requiring that whatever is passed as F/1 have a return type of ok | {error, Reason} which is unrealistic without forcing a lot of folks to wrap existing side-effecty functions in something that coerces the return type to match this. Though that might not be bad practice ultimately, its still perhaps more trouble than its worth.

It isn’t like most Erlang code is written in a way where side-effecty iteration over a list is likely to fail, and if it actually does fail the error data from a crash will contain whatever was passed to the side-effecty function that failed. But this still just doesn’t quite sit right with me — that leaves the prospect of list iteration in the interest of achieving a series of side effects as at least a little bit risky to do in the error kernel (or “crash kernel”) of an application.

On the other hand, the specific case of broadcasting to a list of processes would certainly be nice to handle exactly the same way sending to a single process works:

%% To one message
Pid ! SomeMessage,
%% To a list of messages
Pids ! SomeMessage,

Which seems particularly obvious syntax, considering that the return of the ! form of send/2 is the message itself, meaning that the following two would be equivalent if the input to the ! form of send/2 also accepted lists:

%% The way it works now
Pid1 ! Pid2 ! SomeMessage,
%% Another way I wish it worked
[Pid1, Pid2] ! SomeMessage,

In any case, this is clearly not the sort of language wart that gets much attention, and its never been a handicap for me. It just seems a bit hackish and ugly to essentially overload the semantics of list comprehensions or (ab)use existing list operations like maps and folds to achieve the side effect of iterating over a list instead of having a function like smap/2 which is explicitly designed to achieve side-effecty iteration.

OpenSSL RSA DER public key PKCS#1 OID header madness

(Wow, what an utterly unappealing post title…)

I have run into an annoying issue with the DER output of RSA public keys in OpenSSL. The basic problem is that OpenSSL adds an OID header to its ASN.1 DER output, but other tools are not expecting this (be it iOS, a keystore in Java, the iPhone keystore, or Erlang’s public_key module).

I first noticed this when experiencing decode failures in Erlang trying to use the keys output by an openssl command sequence like:

openssl genpkey \
    -algorithm rsa \
    -out $keyfile \
    -outform DER \
    -pkeyopt rsa_keygen_bits:8192

openssl rsa \
    -inform DER \
    -in $keyfile \
    -outform DER \
    -pubout \
    -out $pubfile

Erlang’s public_key:der_decode(‘RSAPrivateKey’, KeyBin) would give me the correct #’RSAPrivateKey’ record but public_key:der_decode(‘RSAPublicKey’, PubBin) would choke and give me an asn1 parse error (ML thread I posted about this). Of course, OpenSSL is expecting the OID header, so it works fine there, just not anywhere else.

Most folks probably don’t notice this, though, because the primary use case for most folks is either to use OpenSSL directly to generate and then use keys, use a tool that calls OpenSSL through a shell to do the same, or use a low-level tool to generate the keys and then use the same library to use the keys. Heaven forbid you try to use an OpenSSL-generated public key in DER format somewhere other than OpenSSL, though! (Another reason this sort of thing usually doesn’t cause problems is that almost nobody fools around with crypto stuff in their tools to begin with, leaving this sort of thing as an issue far to the outside of mainstream hackerdom…)

Of course, the solution could be to chop off the header, which happens to be 24-bits long:

1> {ok, OpenSSLBin} = file:read_file("rsa3.pub.der.openssl").
{ok,<<48,130,4,34,48,13,6,9,42,134,72,134,247,13,1,1,1,5,
      0,3,130,4,15,0,48,130,4,...>>}
2> {ok, ErlangBin} = file:read_file("rsa3.pub.der.erlang").
{ok,<<48,130,4,10,2,130,4,1,0,202,167,130,153,242,77,196,
      252,167,142,159,17,13,69,148,41,161,50,...>>}
3> <<_:24/binary, ChoppedBin/binary>> = OpenSSLBin.
<<48,130,4,34,48,13,6,9,42,134,72,134,247,13,1,1,1,5,0,3,
  130,4,15,0,48,130,4,10,2,...>>
4> ChoppedBin = ErlangBin.
<<48,130,4,10,2,130,4,1,0,202,167,130,153,242,77,196,252,
  167,142,159,17,13,69,148,41,161,50,44,138,...>

But… that’s pretty darn arbitrary. It turns out the header is always:

<<48,130,4,34,48,13,6,9,42,134,72,134,247,13,1,1,1,5,0,3,130,4,15,0>>

I could match on that, but it still feels weird because that particular binary string just doesn’t mean anything to me, so matching on it is still the same hack. I could, of course, go around that by writing just the public key as PEM, loading it, encoding it to DER, and saving that as the DER file from within Erlang (thus creating a same-tool-to-same-tool situation). But that’s goofy too: PEM is just a base64 encoded DER binary wrapped in a text header/footer! Its completely arbitrary that PEM should work but DER shouldn’t!

-module(keygen).
-export([start/1]).

start([Prefix]) ->
    PemFile = string:concat(Prefix, ".pub.pem"),
    KeyFile = string:concat(Prefix, ".key.der"),
    PubFile = string:concat(Prefix, ".pub.der"),
    {ok, PemBin} = file:read_file(PemFile),
    [PemData] = public_key:pem_decode(PemBin),
    Pub = public_key:pem_entry_decode(PemData),
    PubDer = public_key:der_encode('RSAPublicKey', Pub),
    ok = file:write_file(PubFile, PubDer),
    io:format("Wrote private key to: ~ts.~n", [KeyFile]),
    io:format("Wrote public key to:  ~ts.~n", [PubFile]),
    case check(KeyFile, PubFile) of
        true  ->
            ok = file:delete(PemFile),
            io:format("~ts and ~ts agree~n", [KeyFile, PubFile]),
            init:stop();
        false ->
            io:format("Something has gone wrong.~n"),
            init:stop(1)
    end.

check(KeyFile, PubFile) ->
    {ok, KeyBin} = file:read_file(KeyFile),
    {ok, PubBin} = file:read_file(PubFile),
    Key = public_key:der_decode('RSAPrivateKey', KeyBin),
    Pub = public_key:der_decode('RSAPublicKey', PubBin),
    TestMessage = <<"Some test data to sign.">>,
    Signature = public_key:sign(TestMessage, sha512, Key),
    public_key:verify(TestMessage, sha512, Signature, Pub).

This is so silly its maddening — and apparently folks from the iOS, PHP and Java worlds have essentially built themselves hacks to handle DER keys that deal directly with this.

I finally found a way that is both semantically meaningful (well, somewhat) and is sure to either generate a proper PKCS#1 DER public RSA key or fail telling me that its looking at badly-formed ASN.1 (or binary trash): the magical command “openssl asn1parse -strparse [offset]”.

A key generation script therefore looks something like this:

#! /bin/bash

prefix=${1:?"Cannot proceed without a file prefix."}

keyfile="$prefix"".key.der"
pubfile="$prefix"".pub.der"

# Generate 8192 RSA key
openssl genpkey \
    -algorithm rsa \
    -out $keyfile \
    -outform DER \
    -pkeyopt rsa_keygen_bits:8192

# OpenSSL's PKCS#1 ASN.1 output adds a 24-byte header that
# other tools (Erlang, iOS, Java, etc.) choke on, so we clip
# the OID header off with asn1parse.
openssl rsa \
    -inform DER \
    -in $keyfile \
    -outform DER \
    -pubout \
| openssl asn1parse \
    -strparse 24 \
    -inform DER \
    -out $pubfile

The output on stdout will look something like:

$ openssl asn1parse -strparse 24 -inform DER -in rsa3.pub.der.openssl 
    0:d=0  hl=4 l=1034 cons: SEQUENCE          
    4:d=1  hl=4 l=1025 prim: INTEGER          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
 1033:d=1  hl=2 l=   3 prim: INTEGER           :010001

And if we screw it up and don’t actually align with the ASN.1 header properly things explode:

$ openssl asn1parse -strparse 32 -inform DER -in rsa3.pub.der.openssl 
Error parsing structure
140488074528416:error:0D07207B:asn1 encoding routines:ASN1_get_object:header too long:asn1_lib.c:150:
140488074528416:error:0D068066:asn1 encoding routines:ASN1_CHECK_TLEN:bad object header:tasn_dec.c:1306:
140488074528416:error:0D06C03A:asn1 encoding routines:ASN1_D2I_EX_PRIMITIVE:nested asn1 error:tasn_dec.c:814:

Now my real question is… why isn’t any of this documented? This was a particularly annoying issue to work my way around, has obviously affected others, and yet is obscure enough that almost no mention of it can be found in documentation anywhere. GHAH!

Why OTP? Why “pure” and not “raw” Erlang?

I’ve been working on my little instructional project for the last few days and today finally got around to putting a very minimal, but working, chat system into the ErlMUD “scaffolding” code. (The commit with original comment is here. Note the date. By the time this post makes its way into Google things will probably be a lot different.)

I commented the commit on GitHub, but felt it was significant enough to reproduce here (lightly edited and linked). The state of the “raw Erlang” ErlMUD codebase as of this commit is significant because it clearly demonstrates the need for many Erlang community conventions, and even more significantly why OTP was written in the first place. Not only does it demonstrate the need for them, the non-trivial nature of the problem being handled has incidentally given rise to some very clear patterns which are already recognizable as proto-OTP usage patterns (without the important detail of having written any behaviors just yet). Here is the commit comment:

Originally chanman had been written to monitor, but not link or trap exits of channel processes [example]. At first glance this appears acceptable, after all the chanman doesn’t have any need to restart channels since they are supposed to die when they hit zero participants, and upon death the participant count winds up being zero.

But this assumes that the chanman itself will never die. This is always a faulty assumption. As a user it might be mildly inconvenient to suddenly be kicked from all channels, but it isn’t unusual for chat services to hiccup and it is easy to re-join whatever died. Resource exhaustion and an inconsistent channel registry is worse. If orphaned channels are left lying about the output of \list can never match reality, and identically named ones can be created in ways that don’t make sense. Even a trivial chat service with a tiny codebase like this can wind up with system partitions and inconsistent states (oh no!).

All channels crashing with the chanman might suck a little, but letting the server get to a corrupted state is unrecoverable without a restart. That requires taking the game and everything else down with it just because the chat service had a hiccup. This is totally unacceptable. Here we have one of the most important examples of why supervision trees matter: they create a direct chain of command, and enforce a no-orphan policy by annihilation. Notice that I have been writing “managers” not “supervisors” so far. This is to force me to (re)discover the utility of separating the concepts of process supervision and resource management (they are not the same thing, as we will see later).

Now that most of the “scaffolding” bits have been written in raw Erlang it is a good time to sit back and check out just how much repetitive code has been popping up all over the place. The repetitions aren’t resulting from some mandatory framework or environment boilerplate — I’m deliberately making an effort to write really “low level” Erlang, so low that there are no system or framework imposed patterns — they are resulting from the basic, natural fact that service workers form constellations of similarly defined processes and supervision trees provide one of the only known ways to guarantee fallback to a known state throughout the entire system without resorting to global restarts.

Another very important thing to notice is how inconsistent my off-the-cuff implementation of several of these patterns has been. Sometimes a loop has a single State variable that wraps the state of a service, sometimes bits are split out, sometimes it was one way to begin with and switched a few commits ago (especially once the argument list grew long enough to annoy me when typing). Some code_change/N functions have flipped back and forth along with this, and that required hand tweaking code that really could have been easier had every loop accepted a single wrapped State (or at least some standard structure that didn’t change every time I added something to the main loop without messing with code_change). Some places I start with a monitor and wind up with a link or vice versa, etc.

While the proper selection of OTP elements is more an art than a science in many cases, having commonly used components of a known utility already grouped together avoids the need for all this dancing about in code to figure out just what I want to do. I suppose the most damning point about all this is that none of the code I’ve been flip-flopping on has been essential to the actual problem I’m trying to solve. I didn’t set out to write a bunch of monitor or link or registry management code. The only message handler I care about is the one that sends a chat message to the right people. Very little of my code has been about solving that particular problem, and instead I consumed a few hours thinking through how I want the system to support itself, and spent very little time actually dealing with the problem I wanted to treat. Of course, writing this sort of thing without the help of any external libraries in any other language or environment I can think of would have been much more difficult, but the commit history today is a very strong case for making an effort to extract the common patterns used and isolate them from the actual problem solving bits.

The final thing to note is something I commented on a few commits ago, which is just how confusing tracing message passage can be when not using module interface functions. The send and receive locations are distant in the code, so checking for where things are sent from and where they are going to is a bit of a trick in the more complex cases (and fortunately none of this has been particularly complex, or I probably would have needed to write interface functions just to get anything done). One of the best things about using interface functions is the ability to glance at them for type information while working on other modules, use tools like Dialyzer (which we won’t get into we get into “pure Erlang” in v0.2), and easily grep or let Emacs or an IDE find calling sites for you. This is nearly impossible with pure ad hoc messaging. Ad hoc messaging is fine when writing a stub or two to test a concept, but anything beyond that starts getting very hard to keep track of, because the locations significant to the message protocol are both scattered about the code (seemingly at random) and can’t be defined by any typing tools.

I think this code proves three things:

  • Raw Erlang is amazingly quick for hacking things together that are more difficult to get right in other languages, even when writing the “robust” bits and scaffolding without the assistance of external libraries or applications. I wrote a robust chat system this afternoon that can be “hot” updated, from scratch, all by hand, with no framework code — that’s sort of amazing. But doing it sucked more than it needed to since I deliberately avoided adhering to most coding standards, but it was still possible and relatively quick. I wouldn’t want to have to maintain this two months from now, though — and that’s the real sticking point if you want to write production code.
  • Code convention recommendations from folks like Joe Armstrong (who actually does a good bit of by-hand, pure Erlang server writing — but is usually rather specific about how he does it), and standard set utilities like OTP exists for an obvious reason. Just look at the mess I’ve created!
  • Deployment clearly requires a better solution than this. We won’t touch on this issue for a while yet, but seriously, how in the hell would you automate deployment of a scattering of files like this?

Finally working on an intermediate Erlang instructional

After contemplating the state of the tiny constellation of Erlang resources available, both in-depth instructionals and books, I’ve finally decided that what is most lacking is an intermediate resource. So I’ve started working on one. The goal is to demonstrate a non-trivial system (a MUD, in this case) as it would be written in raw, beginner-style Erlang and show how a project can evolve to a for-real OTP system by providing complete project examples and code commentary throughout the intermediate stages of evolution.

Hopefully this will force me to cover many of the giant, undocumented gaps Erlangers tend to encounter when trying to make the transition from Erlang beginner to OTP master, and I’ll undoubtedly learn a lot in the process myself. That last bit about self education is, of course, my own selfish motive for doing this. I have yet to find an aggregate body of Erlang best practices or collection of “common things people do by hand in Erlang and the idiomatic way to handle them in OTP”, and I really wish there was such a thing. I especially wish there were such a thing based on a single, non-trivial project instead of toy examples so that learners could get a grasp on why OTP matters by reading a real project instead of trying to assemble a mental model of the grander sculpture by examining bits of dust cast off by the chisel.

I chose a MUD as an example for ease of understanding, non-triviality (too many example systems are unrealistically small), freedom from graphical distractions, and the striking number of parallels between many MUD subsystems and real-world business, game, and social software. Hopefully work and real life don’t interfere to the point that I have to stop this mid-way through!

Speaking of police states, whatever happened to StackExchange?

StackOverflow, StackExchange, etc. and its whole community rather saddens me these days. It seems the assburgers who dump their angst on Wikipedia have found a new home making StackExchange sites as unfun as possible. The situation isn’t helped at all by the recent rabbit-like proliferation of SE sites which exhibit so much subject overlap as to make determining the “proper” place of most posts nearly impossible — all that has done is given the Post Police reason, in every case, to force re-posting, closing, pausing, deletion, or otherwise official vandalism of valid posts and answers.

Let’s say I have a question about travelling to Austria to see historical sites. Let’s put it in travel. But if the question is phrased in such a way as to reference a specific historical event, like say a little battle in which Polish king demonstrated how much he appreciated the Ottoman visitors, then it is an almost certain thing that some twerp who knows nothing about history or travelling in Austria will promptly vote to close or move the question on the grounds that it belongs on the SE site for history and not travel. The post will face the same fate once it arrives there, of course, and the asker will be helpless.

This sort of thing goes on all the time between StackOverflow, “Programmers”, and “Workplace” as many questions professional programmers have cut sharply across all three lines.

Another annoyance is unnecessary Post Police comments like the one left on this post. Sure, I was being silly in the way I did it, but my response represents the general consensus of the Erlang community on this particular subject and is, in essence, a fairly standard response. Obviously not good enough for 19-year-old Alexy Schmalko, Protector of the Realm.

Whatever happened to the sense of community I used to get from, you know, the community? Did that all die when usenet got retarded in the early 90’s? Did it evaporate with the coming of the cool web kids? I’ve probably got kids somewhere near his age…

The Lightweight Nature of Erlang Processes

Understanding the difference between Erlang processes and OS processes can be a bit confusing at first, partly because the term “process” means something different in each case, and partly because the semantics of programming terms have become polluted by marketing, political and religious wars. A post to the Erlang questions mailing list asking why Erlang processes are so fast and OS processes are so slow reminded me of this today.

Erlang processes are more similar to the “objects” found in most OOP languages than the “processes” managed by an OS kernel, but have a proper message passing semantics added on in a way that abstracts the OS network, pipe and socket mechanisms. We wouldn’t be surprised if the Python runtime handled its objects with less overhead than the OS kernel handles a process, of course, and it should come as no surprise that the Erlang runtime handles its processes with less overhead than the OS kernel. After all, a Python “object” and an Erlang “process” are very nearly the same thing underneath.

Most OOP runtimes implement “objects” as a special syntactical form of a higher order function, one that forms a closure around its state, includes pointers to methods as a part of that state (usually with their own special syntax that abstracts the difference between a label, a pointer and a variable) and returns a dispatch function which manages access to its internal methods. Once you get down to assembly, this is the only way things work anyhow (and on von Neuman architectures there is exactly zero difference between pointers to data, pointers to data, instructions and pointers to a next instruction). If you strip that special syntax away there is no practical difference between directly writing a higher order function that does this and using the special class definition syntax.

Even in a higher language the higher-order functional nature of an “object”s class definition can be illustrated. For example, the following Python class and function definitions are equivalent.

class Point():
    def __init__(self, x=0, y=0):
        self.x = x
        self.y = y

    def set_x(self, x):
        self.x = x

    def set_y(self, y):
        self.y = y

    def get_x(self):
        return self.x

    def get_y(self):
        return self.y


def gen_point(x=0, y=0):
    coords = {"x": x, "y": y}

    def set_x(x):
        coords["x"] = x

    def set_y(y):
        coords["y"] = y

    def get_x():
        return coords["x"]

    def get_y():
        return coords["y"]

    def dispatch(message, value=0):
        if message == "set x":
            set_x(value)
        elif message == "set y":
            set_y(value)
        elif message == "get x":
            return get_x()
        elif message == "get y":
            return get_y()
        else:
            return "Bad message"

    return dispatch

We would be utterly unsurprised that both the class definition and the function definition return entities that are lighter weight than OS processes. This is not so far from being the difference between Erlang processes and OS processes.

Of course, the above code is ridiculous to do in Python either way. The whole point of the language is to let you avoid dealing with this exact sort of code. Also, Python has certain scoping rules which are designed to minimize the confusion surrounding variable masking in dynamic languages — and the use of a dictionary to hold the (X, Y) state is a hack to get around this. (A more complete example that uses explicit returns and reassignment is available here.)

For a more direct example, consider how this can be done in Guile/Scheme:

(define (point x y)
  (define (setter coord value)
    (cond ((eq? coord 'x) (set! x value))
          ((eq? coord 'y) (set! y value))))
  (define (getter coord)
    (cond ((eq? coord 'x) x)
          ((eq? coord 'y) y)))
  (define (dispatch m)
    (cond ((eq? m 'set) setter)
          ((eq? m 'get) getter)
          (else (error "point: Unknown request"))))
  dispatch)

OOP packages for Lisps wrap this technique in a way that abstracts away the boilerplate and makes it less messy, but its the same idea. This can be done in assembler or C directly as well. Equivalent examples are a bit longer, so you’ll have to take my word for it. (A commented version of the Guile example above can be found here.)

While OOP languages typically focus on access to state and access to methods as state, Erlang focuses like a laser on the idea of message passing. Easy, universal access to state in OOP languages makes it natural to do things like share state, usually by doing something innocent like declaring a name in an internal scope that points to an independent object from somewhere outside.

Erlang forbids this, and forces all data to either be a part of a the definitions that describe the process (things declared in functions or their arguments), or go through messages. Combined with recursive returns and assignment in a fresh scope (akin to the last Python example in the extra code file) this means state is effectively mutable and side effects can occur without violating single assignment, but that everything that changes must change in an explicit way.

This restriction comes at the cost of requiring a sophisticated routing and filtering system. Erlang has an unusually complete message concept, going far beyond the “signals and slots” style found in some of the more interesting OOP systems. In fact, Erlang goes so far with the idea that it abstracts message, filters, a process scheduler and the entire network layer with it. And hence we have a very safe environment for concurrent processing — using “processes” that certainly feel like OS type processes, but are actually named locations Erlang’s runtime keeps track of in the same way an OOP runtime does objects, functions and other declared thingies. They feel like OS processes because of the way Erlang handles access to them in the same way that Java objects feel like my mother-in-law’s purse because of the way the JVM handles access to them — but underneath they are much more alike each other than either are to OS processes.

In the end, all this stuff is just long lines of bits standing in memory. The special thing is the rules we invent for ourselves that tell us how to interpret those bits. Within those rules we have various ways of declaring our semantics, but in the end the lines of bits don’t care if you think of them as “objects”, as “processes”, as “closures”, as “structs with pointers to code and data” or as “lists of lists with their own embedded processing rules”. OSes have particularly heavy sets of rules regarding how bits are accessed and moved around. Runtimes tend not to. Erlang “processes” are of a kind with Python “objects”, so we shouldn’t be surprised that they are significantly lighter weight than the “processes” found in the OS.

Republication of GNU’s Guile 2.0 Manual

I’ve republished the “one page per node” html version of the GNU Guile 2.0 Manual here. I’ve been referencing the manual quite a bit lately and noticed that, at least from my location, the site responds quite slowly at times. So I’m mirroring the manual where I know its fast and I can always find it.

I hope that the reason the GNU servers are responding slowly is legitimate load and not something more sinister. Attacks on community sites are one of the more stupid forms of tragedy that these commons experience.

Using the alternatives command

A quick demonstration of using alternatives:

[root@taco ~]# cat /opt/foo/this
#! /bin/bash
echo "I am this."
[root@taco ~]# cat /opt/foo/that
#! /bin/bash
echo "I am that."
[root@taco ~]# cat /opt/foo/somethingelse 
#! /bin/bash
echo "I am somethingelse."
[root@taco ~]# which thisorthat
/usr/bin/which: no thisorthat in (/usr/lib64/qt-3.3/bin:/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin)
[root@taco ~]# alternatives --install /usr/bin/thisorthat thisorthat /opt/foo/this 1
[root@taco ~]# alternatives --install /usr/bin/thisorthat thisorthat /opt/foo/that 2
[root@taco ~]# which thisorthat
/usr/bin/thisorthat
[root@taco ~]# ls -l /usr/bin/thisorthat 
lrwxrwxrwx. 1 root root 28 Mar 23 10:38 /usr/bin/thisorthat -> /etc/alternatives/thisorthat
[root@taco ~]# ls -l /etc/alternatives/thisorthat 
lrwxrwxrwx. 1 root root 13 Mar 23 10:38 /etc/alternatives/thisorthat -> /opt/foo/that
[root@taco ~]# thisorthat
I am that.
[root@taco ~]# alternatives --set thisorthat /opt/foo/that 
[root@taco ~]# thisorthat
I am that.
[root@taco ~]# alternatives --set thisorthat /opt/foo/somethingelse 
/opt/foo/somethingelse has not been configured as an alternative for thisorthat
[root@taco ~]# alternatives --display thisorthat
thisorthat - status is manual.
 link currently points to /opt/foo/that
/opt/foo/this - priority 1
/opt/foo/that - priority 2
Current `best' version is /opt/foo/that.

Once upon a time, in a time long forgotten, the alternatives command on Fedora-descended systems (RHEL, Scientific Linux, CentOS, etc.) seemed a magical thing. It permitted two versions of the same utility to exist on a system at the same time by automatically switching links from a canonical path to the actual versioned path when necessary. Very cool. This was so cool, in fact, that other system utilities came to rely on it, namely, the post-install section on quite a few RPMs calls alternatives to set up parallel versions of things like language runtimes.

The downside to this automation is use of alternatives itself seems to have become a lost art (or possibly its that the popularity of this distro family has simply diluted the knowledge pool as the user ranks have swelled with newcomers who simply take the automation for granted).

It should be noted that alternatives is a system-wide command, so when root sets an alternative, it affects everyone’s view of the system. It should be further noted that this problem has other, but very similar, solutions on other systems like Gentoo and Debian. Gentoo’s eselect system is a bit more sophisticated and can manage families of alternatives at once (links to a number of disparate language utilities which have to change in arbitrary ways based on which underlying runtime is selected, for example). Fortunately its not very difficult to write a wrapper for alternatives that can provide a similar experience, but unfortunately I don’t have the time this morning to get into that here.