Industrial Use of a Functional Language

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Industrial Use of a Functional Language

• Thomas Arts
• Ericsson
• Computer Science Laboratory
• Stockholm, Sweden

thomas_at_cslab.ericsson.se http//www.ericsson.se/cs
lab/thomas
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Telecom industry

• Switches, routers,
• base-stations
• Networks
• Mobile telephones

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Computer ScienceLaboratory

• Founded 1983
• Research on implementation tools, methods and
  techniques for telecommunication applications
• Intermediate between universities and Ericsson
  product units

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Telecom requirements

• Requirements of telecom software
• Concurrency
• Distribution
• Soft real-time
• Robust
• Non-stop system
• External interfaces

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A good language?

• Experiments in the 80s with Ada, C, ML, CML,
  Prolog...
• large programs (million lines)
• change code in running system
• fast message passing
• low memory overhead
• no memory fragmentation/leaks
• recover from errors

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Erlang/OTP

• A functional language successfully used for
  programming large real-time control systems.
• OTP is the set of libraries that is used with
  Erlang for product development

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Erlang/OTP

• Erlang/OTP develop/maintenance
• Erlang consultancy courses
• Erlang used many systems, e.g. ATM switch and
  new GSM network (GPRS)
• Erlang Open Source

www.erlang.org
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Erlangsequential program

• -module(math).
• -export(fac/1).
• fac(N) when Ngt0 -gt
• Nfac(N-1)
• fac(N)-gt
• 1.

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Erlangdatatypes

• atoms (true,foo,Hello)
• numbers (1212864187154)
• floats (3.141592)
• tuples (a,123)
• lists (1,123,2,56)
• process identifiers
• ...

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Erlangdatatypes

• dynamically typed language
• poor mechanism to build your own datatypes

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Erlangcontrol structures

• Matching
• case X of
• ok,List -gt hd(List)
• resend,Data -gt submit
• error -gt exit(error)
• _ -gt retry(X)
• end

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Erlangcontrol structures

• Guards
• f(....) when guard -gt ...
• If
• f(X) -gt
• if guard1 -gt ...
• guard2 -gt ...
• end

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Erlangcontrol structures

• Higher order functions
• f(F,X) -gt F(X)
• map(F,1,2,3,4).
• List comprehensions
• X X,Ylt-Set, guard(X)

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Erlangcontrol structures

• Naming of objects/data
• f(X) -gt
• Dev update_device(X),
• Date,Time now(),
• h(Dev,Date).

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Erlangcontrol structures

• Sequence
• f(X) -gt
• action1(X),
• action2(X)
• update(X) -gt
• log(X,myfile), new(X).

side-effects
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Erlangcontrol - concurrency/distribution

• Creating a process
• Pid spawn(F,Arg1,...,ArgN)

B spawn(F,Args)
P2
P1
P2
F(Arg1,...,ArgN)
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Erlangcontrol - concurrency/distribution

• Sending messages
• Pid ! Message

B!self(),hej
P1
P2
P1,hej
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Erlangcontrol - concurrency/distribution

• Receiving messages
• receive
• Pattern -gt ...
• end

P1
P2
P1,hej

receive From,Msg -gt From !
ok,Msg end
P1 ! ok,hej
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Erlangchanging code in running system
P0
loop(F) -gt receive change,G -gt
loop(G) exec,Pid,Arg -gt
Pid!F(Arg), loop(F) end
loop(F) -gt receive exec,Pid,Arg -gt
Pid!F(Arg), loop(F) end
P1
P0 ! exec,self(),15, N receive Answer
-gt Answer end,
N F(15),
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Erlangfault tolerance
Processes can be linked to each other
PidA
PidB

• Links are created by using either
• link(Pid), or
• spawn_link(Module, Function, Args)
• Links are bi-directional.
• They can be removed using unlink(Pid).

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Erlangfault tolerance
When a process terminates, an exit signal is sent
to all processes the process is linked to
PidA
PidB
PidA
A process can terminate normally, due to a
run-time error, or when explicitly ordered to do
so.
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Erlangfault tolerance
If a process terminates abnormally, the emitted
exit signal will (by default) cause the recipient
to terminate
The termination reason in the transmitted exit
signals will be the same as in the received one
(the exit signal is propagated).
PidC
PidD
PidE
PidA
PidB
PidB
PidD
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Erlangfault tolerance
A process can terminate itself using
exit(Reason). This will cause exit signals with
termination reason Reason to be emitted.
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Erlangfault tolerance
A process can explicitly send an exit signal to
another process using exit(Pid, Reason)

• The calling process is not affected.
• The processes do not need to be linked.

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Erlangfault tolerance
A process can trap exit signals
using process_flag(trap_exit, true). Incoming
exit signals will be transformed into messages of
the form 'EXIT', Pid, Reason These exit
messages are delivered to the process mailbox in
the normal way.
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Erlangfault tolerance
PidC terminates with reason error, PidD is
trapping exits
PidB terminates, propagating the exit signal.
PidD will receive an exit message
'EXIT', PidC, error.
error
error
PidC
PidD
PidE
PidA
PidB
PidB
PidA
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Erlangfault tolerance
Robust systems can be made by layering.
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Erlangfault tolerance
supervision trees and restart strategies
P4
P3
P1
P2
P3
P1
P6
P5
P1
P2
P3
P4
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Erlangcomponent based development

• recognize frequently occurring
• patterns and transfer them into
• standard components.
• faster development
• uniform code (maintenance)
• less errors

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Erlang

• Developed with the application of the language
  constantly in mind
• Practical usability more priority than purity

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Erlang is used

• AXD 301, scalable ATM switch (up to 160 GB/sec)

• four years work
• more than 500,000 lines of Erlang
• several hundreds of programmers

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Erlang is used
GPRS next generation GSM network.

• Eigth times faster internet access in mobile
  phones
• always connected
• development in three countries, hundreds of
  people

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Erlang is used

• The success of the language in industry is due to
• Language features
• Support and libraries
• Design patterns / architectures

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Erlang is used
over 300 downloads per month

• Become a user yourself!
• passive make a phonecall
• active download Erlang for free

www.erlang.org
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Erlang

• The functional language
• for industry