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119        <TD class=headline align=left>Regular Expressions: Erlang Is Worth a Look</TD>
120    <TD class=headdate align=right>March 2002</TD>
121      </TR>
122  <TR vAlign=top align=left> 
123        <TD colSpan=2> 
124      <P> by Cameron Laird and Kathryn Soraiz
125      <P>
126Let's write highly-reliable, high-performance distributed applications.
127While we're at it, let's choose a development language with a "track
128record" of high productivity.
129<P>
130We'll use the Erlang programming language.
131<P>
132<H2>Erlang for Real</H2>
133      <P> The name "Erlang" honors a Danish mathematician by that name,
134        whose work in stochastic theories flourished at the beginning of the twentieth
135        century. Although Erlang's name-recognition is low among U.S. programmers
136        ("Erlang? Isn't that one of the new Winter Olypmics events?"), its accomplishments
137        are plenty serious. First implemented in 1986 by an engineer at Ericsson
138        Enterprise, Erlang is the language in which many production telecommunications
139        and other mission-critical applications are written. Its use on general-purpose
140        "personal computers" and servers is also growing.
141      <P> One of the reasons for the interest in Erlang is that the software world
142        is finally acknowledging the importance of distributed computation. This
143        has been a good season for distributed computing -- <I>Scientific American</I> 
144        has popularized the topic; the new Usenet newsgroup comp.distributed has
145        hosted a handful of worthwhile discussions since its launch in mid-February;
146        and the Global Grid Forum (<A href="http://www.gridforum.org/" target="_blank">http://www.gridforum.org/</A>)
147        has become one of the hottest events in computing. Ideas about large-scale
148        operating systems (OSes) are moving out of research laboratories; the
149        .NET and Web Services marketing machines are starting to propagate technical
150        content; and peer-to-peer and CORBA partisans are fighting back with significant
151        engineering advances. This is territory that Erlang experts have been
152        exploring for more than a decade.
153      <P>Erlang was originally designed as a "language for programming <I>distributed
154        fault-tolerant soft real-time non-stop applications</I>." That formulation
155        correctly suggests there's more to Erlang than just its capacity for distributed
156        calculations. Erlang is a functional programming language, with deep roots
157        in logic and functional semantics that contribute to program correctness.
158        Like such wildly different languages as Ada and Eiffel, Erlang's correctness
159        characteristics make it easier for programmers to compose applications
160        that behave predictably. Erlang generalizes correctness to "fault-tolerance"
161        with its powerful "exception" mechanisms. Ericsson telephone switches
162        need very high uptimes -- 99.999% and up. Erlang's fault-tolerance, related
163        to the exceptions in Java and Python, helps achieve this level of reliability.
164      <P>
165<H2>See for Yourself</H2>
166<P>
167What's it like to work in a language shaped for fault-tolerance,
168high productivity, and distribution?  As with most of the subjects
169we choose for "Regular Expressions," the best answer is likely to
170be, "See for yourself."  An open-source version of Erlang has been
171available since 1998, and extensive resources are available on the
172Web. 
173      <P> Source or binary downloads for the recent versions of the standard Erlang
174        distribution are just over 10 megabytes in size. Although this is toward
175        the top of the range for the languages "Regular Expressions" most often
176        covers, it's significantly smaller than typical Java installations. In
177        our experience, Unix generations of Erlang from source always go smoothly;
178        whatever errors the latest releases have, they don't show up during installation
179        or simple tests.
180      <P> Once that's done, Erlang's "Hello, World" preliminaries go quickly enough.
181        Erlang's home site even has a "Getting Started" page at <A href="http://www.erlang.org/starting.html" target="_blank">http://www.erlang.org/starting.html</A>.
182        Like other typical modern interpreters, the Erlang language processor,
183        <CODE>erl</CODE>, supports both interactive and "batch" operation. You
184        can bring it up interactively as a convenient calculator:
185      <PRE>     #> erl
186     Erlang (BEAM) emulator version 5.1 [source]
187
188     Eshell V5.1  (abort with ^G)   
189     1> pwd().
190     /usr/local/src/otp_src_R8B-0
191     ok   
192     2> ls().
193     AUTHORS           EPLICENCE         Makefile          Makefile.in
194     README            bin               bootstrap         config.cache
195     config.log        config.status     configure         configure.in
196     erts              lib               make
197     ok
198     3> 123 * 456 - 999.
199     55089 </PRE> 
200   
201Notice that a period terminates every complete line.
202
203      <P>
204<H2>Safe Power</H2>
205<P>
206The Erlang downloads are so large because the
207standard distribution is intended to "include batteries."
208Every Erlang installation builds in functions
209that mediate ASN encoding, manage CORBA transactions,
210perform cryptographic transformations, implement such
211standard networking protocols as FTP, HTTP, SNMP, and SSL,
212and even connect to Java classes.
213      <P> This powerful library combines with Erlang's succinct expressivity to
214        make remarkable results possible in just a few lines. It's hard to show
215        Erlang at its best by simply looking at common programming tasks; it doesn't
216        do the things that Perl or C do any better than those languages. Erlang
217        comes into its own on applications that must run continuously (think of
218        air-traffic control), cooperatively (across many machines), and without
219        surprises. Erlang is largely immune to the buffer overflows, memory leaks,
220        off-by-one, and other mundane errors that affect many applications.
221      <P>
222A favorite example of Erlang's brevity is this generic
223client-server:
224   <PRE>     -module(cs).
225     -export([start/3, rpc/2, loop/2]).
226
227     start(Name, Data, Fun) ->
228         register(Name,
229             spawn(fun() ->
230                 loop(Data, Fun)
231             end)).
232
233     rpc(Name, Q) ->         
234         Tag = ref(),
235         Name ! -query, self(), Tag, Q),
236         receive
237             (Tag, Reply) -> Reply
238         end.
239
240     loop(Data, Fun) ->
241         receive
242             (query, Pid, Tag, Q) ->
243                 (Reply, Data1) = Fun(Q, Data),
244                 Pid ! (Tag, Reply),
245                 loop(Data1, Fun)
246     end.
247   </PRE>
248   <P>
249Invocation of <CODE>start()</CODE> launches a complete
250network endpoint. The code above handles communications
251and concurrency, while all that's left for an application
252developer is to bind <CODE>Fun</CODE> to a definition
253that executes the application-specific operation.
254<P>
255If your programs really, REALLY have to work right every
256time, consider using Erlang.
257<P>
258<H2>Administrative Wrap-Up</H2>
259      <P> We're collecting notes and hyperlinks on Erlang at <A href="http://starbase.neosoft.com/~claird/comp.lang.functional/Erlang.html" target="_blank">http://starbase.neosoft.com/~claird/comp.lang.functional/Erlang.html</A>.
260        One ambition for this page is to introduce a functional language for the
261        "working programmer" in a way that makes the academic abstractions found
262        in this specialty more appealing. As always, e-mail to us @regularexpressions.com
263        is welcome. Incidentally, if you want to try out parallel programming
264        in a more mainstream language than Erlang, consider PyPAR (<A href="http://datamining.anu.edu.au/pypar" target="_blank">http://datamining.anu.edu.au/pypar</A>).
265        This well-behaved Python extension was just released in its 1.0 version
266        last month.
267      <P>
268Next time, we'll look at a completely
269different approach to construction of correct programs:
270use of static syntax analyzers.  Until then, good luck
271in solving your problems at a high level.
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