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<!DOCTYPE html>
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<div id = "showleftcol" > </div>
<H1>More Stuff</H1>
<p style="margin-top:-8px; margin-bottom:-18px">
Leslie Lamport<p>
<font size=-1><I> Last modified on 23 August 2019</I></font>
</td>
<!-- td style="vertical-alight:top;width:auto" -->
<!-- img src="tla-logo.png" style="width:170px;margin-top:14px"> </img -->
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<hr style="margin-bottom:-5px;margin-top:-11px">
<P style="margin-top:0px"> </P>
<DIV class="hidden-div" style="color:red;margin-bottom:-22px"><b>
You'll miss a lot on this web site unless you enable Javascript
in your browser. </b></DIV>
<!--
<H2 id ="h2intro"
class="show-hide" onclick="showHide('hide-intro','intro')">
Introduction Section
<font
id="hide-intro" >
[show]</font>
</H2>
<DIV id="intro" class = "hidden-div">
This is the section that is displayed when the page is opened
normally.
</DIV>
-->
<H2 id="h2videos" class="show-hide"
onclick="showHide('hide-videos','videos')">Videos of My Talks
about Specification
<font id="hide-videos">
[show]</font>
</H2>
<DIV id="videos" class = "hidden-div">
<h4> If You're Not Writing a Program, Don't Use a Programming Language</h4>
<!-- lamport/talks/state-machines/heidelberg18.pdf -->
Given in Heidelberg on 24 September 2018 at the <i>2018 Heidelberg
Laureate Forum</i>.
<!-- Discusses specifying systems mathematically as state machines in TLA+,
and how it works in practice.
-->
<div class="abstract">
<b>Abstract</b>
Algorithms are not programs. They can and should be written with math
rather than programming languages or pseudo-languages. This applies
to many more algorithms than the ones taught in algorithm courses.
</div>
<p class="link">
<a href="https://www.heidelberg-laureate-forum.org/blog/video/lecture-monday-september-24-2018-leslie-lamport/">
[video]</a>
<h4>How to Write a 21st Century Proof</h4>
Given in Heidelberg on 26 September 2017 at the <i>2017 Heidelberg
Laureate Forum</i>.
<!-- talks/proof/palais11/heidelberg.pdf -->
<div class="abstract">
<b>Abstract</b>
Mathematicians have made a lot of progress in the last 350 years, but
not in writing proofs. The proofs they write today are just like the
ones written by Newton. This makes it all too easy to prove things
that aren’t true. I’ll describe a better way that I’ve been using for
more than 25 years.
</div>
<p class="link">
<a href="https://www.heidelberg-laureate-forum.org/blog/video/lecture-tuesday-september-26-2017-leslie-lamport/">
[video]</a>
<h4>The PlusCal Algorithm Language</h4>
Given in Heidelberg on 22 September 2016 at the <i>2016 Heidelberg
Laureate Forum</i>.
<!-- talks/pluscal/heidelberg16.pdf -->
<div class="abstract">
<b>Abstract</b>
An algorithm is not a program, so why describe it with a programming
language? PlusCal is a tiny toy-like language that is infinitely more
expressive than any programming language because an expression can be
any mathematical formula.
</div>
<p class="link">
<a href="https://www.heidelberg-laureate-forum.org/blog/video/lecture-thursday-september-22-2016-leslie-lamport/">
[video]</a>
<h4>A Mathematical View of Computer Systems</h4>
Given in Heidelberg on 24 August 2015 at the <i>2015 Heidelberg
Laureate Forum</i> and in Beijing on 28 October 2015 at the
<i>Computing in the 21st Century Conference</i>.
<!-- talks/oslo15/heidelberg15.pdf -->
<div class="abstract">
<b>Abstract</b>
Mathematics provides what I believe to be the simplest and most
powerful way to describe computer systems.
</div>
<p class="link">
<a href="https://www.heidelberg-laureate-forum.org/blog/video/lecture-monday-august-24-2015-leslie-lamport/">
[Heidelberg video]</a> <a
href="https://www.youtube.com/watch?v=oIwdN-irLqs">[Beijing video]</a>
<h4 name="raynal"
style="margin-top:0px;margin-bottom:0px">The Bakery Algorithm in 2015</h4>
<!-- talks/pluscal/raynal.pdf -->
Given in Rennes, France on 5 May 2017 at the
<i>International Workshop on Distributed Computing in the honor of
Michel Raynal</i>.
<div class="abstract">
<b>Abstract</b>
An algorithm is not a program, so why describe it with a programming
language? PlusCal is a tiny toy-like language that is infinitely more
expressive than any programming language because an expression can be
any mathematical formula.
</div>
<!-- It describes PlusCal by showing how the
bakery algorithm is written in it. -->
<p class="link">
<a href="https://www.canal-u.tv/video/inria/the_bakery_algorithm_in_2015.40143"
target="_blank">[video]</a> <a
href="http://videos.rennes.inria.fr/workshop-MichelRaynal/raynal_leslie_Lamport.pdf"
target="_blank">[slides]</a>
<h4 name="more-than-coding">Programming Should be More than Coding</h4>
<!-- talks/india13/stanford15.pdf -->
Given at Stanford University on 8 April 2015.
<div class="abstract">
<b>Abstract</b>
Writing a program involves three tasks:
<ol style="margin-top:2px;margin-bottom:2px">
<li> Deciding what the program should do. </li>
<li> Deciding how the program should do it. </li>
<li>Coding: Implementing these decisions in code. </li>
</ol>
Too often, all three are combined into the process of coding. This
talk explains why they should be separated, and discusses how to
perform the first two.
</div>
<!-- Yet another of my talks about thinking mathematically above the
code level, using TLA+ to show that it can be done.-->
<p class="link">
<a href="https://www.youtube.com/watch?v=6QsTfL-uXd8" target="_blank">
[video]</a>
<h4>Thinking for Programmers</h4>
<!-- talks/india13/build.pdf -->
Given in San Francisco on 3 April 2014 at
the <i>Build</i> conference.
<div class="abstract">
<b>Abstract</b>
[I]ntroduces techniques and tools that help programmers think above the
code level to determine what applications and services should do and
ensure that they do it. Depending on the task, the appropriate tools
can range from simple prose to formal, tool-checked models written in
TLA+ or PlusCal.
</div>
<!-- The same general message as <i>Programming Should be More than Coding</i>
<a href="#more-than-coding">above</a>. -->
<p class="link">
<a href="https://www.youtube.com/watch?v=4nhFqf_46ZQ" target="_blank">
[video]</a>
<h4>Who Builds a Skyscraper without Drawing Blueprints?</h4>
<!-- talks/india13/mit14.pdf -->
Given in San Francisco on 19 Novembr 2014 at
the <i>React</i> conference.
<div class = "abstract"> <!-- style="font-size:16px;line-height:18px"-->
<b>Abstract</b> Architects draw detailed plans before
construction begins. Software engineers don't. Can this be why
buildings seldom collapse and programs often crash? A blueprint for
software is called a specification. TLA+ specifications have been
described as exhaustively testable pseudo-code. High-level TLA+
specifications can catch design errors that are now found only by
examining the rubble after a system has collapsed.
</div>
<!-- The same general message as <i>Programming Should be More than Coding</i>
<a href="#more-than-coding">above</a>. -->
<P class="link">
<a href="https://www.youtube.com/watch?v=-4Yp3j_jk8Q"
target="_blank">[video]</a> <a
href="#"
target="_blank">[slides to appear]</a>
<h4>Thinking Above the Code</h4>
<!-- lamport/talks/india13/faculty-summit14.pdf -->
Given in Redmond, Washington on 14 July 2014 at the
<i>Microsoft Faculty Summit</i>.
<div class="abstract">
<b>Abstract</b>
Architects draw detailed blueprints before a brick is laid or a nail
is hammered. Programmers and software engineers seldom do. A
blueprint for software is called a specification. The need for
extremely rigorous specifications before coding complex or critical
systems should be obvious--especially for concurrent and distributed
systems. This talk explains why some sort of specification should be
written for any software.
</div>
<!-- Discusses both formal TLA+ specs and writing informal specs
above the code level. -->
<P class="link"> <!-- style="margin-top:7px"-->
<a href="https://www.youtube.com/watch?v=iCRqE59VXT0" target="_blank">
[video]</a>
<h4>What is Computation?</h4>
<!-- talks/state-machines/computation-paris12.pptx or maybe
talks/state-machines/computation-paris12-short.pptx -->
Given in Haifa, Israel on 2 June 2011 at the Technion — Israel Institute
of Technology.
<div class="abstract">
A simple introduction to thinking of computations as sequences
of states and thinking of computing devices as state machines
that can be described with elementary math.
</div>
<P class="link">
<a href="https://www.youtube.com/watch?v=BDPHfRuAFnU"
target="_blank">[video]</a>
</DIV>
<H2 id="h2papers" class="show-hide"
onclick="showHide('hide-papers','papers')">My Papers About TLA and TLA+
<font id="hide-papers" >
[show]</font>
</H2>
<DIV id="papers" class = "hidden-div">
A page listing approximately all the papers I've written
about TLA and TLA+.
<p class="link">
<A href =
"papers4a79.html?back-link=more-stuff.html#papers?unhideBut@EQhide-papers@AMPunhideDiv@EQpapers"
>[click here]</A>
</DIV>
<H2 id="h2examples" class="show-hide"
onclick="showHide('hide-examples','examples')">Examples
<font id="hide-examples" >
[show]</font>
</H2>
<DIV id="examples" class = "hidden-div">
The TLA+ GitHub repository contains a few dozen examples of TLA+
specifications
<a href="https://github.com/tlaplus/Examples">here</a>.
They have not been edited and are of varying quality and
degree of complexity. Here are a few simple ones suitable
for newcomers to TLA+.
<h4><a href="https://github.com/tlaplus/Examples/tree/master/specifications/MissionariesAndCannibals">Missionaries and Cannibals</a></h4>
A well-known problem is specified in TLA+ and solved with
the TLC model checker. The spec assumes knowledge of TLA+;
everything you need to know is explained in comments.
<h4><a href="https://github.com/tlaplus/Examples/tree/master/specifications/N-Queens">
The <em> N </em> queens problem</a></h4>
This example generalizes the
<a href="https://en.wikipedia.org/wiki/Eight_queens_puzzle">
<em>eight queens puzzle</em></a> from 8 to  N 
and specifies an algorithm to solve it.
The file <code>Queens.tla</code> contains a TLA+ spec and
<code>QueensPluscal.tla</code> contains a PlusCal spec.
<h4><a href="https://github.com/tlaplus/Examples/tree/master/specifications/Prisoners">The
Prisoners Problem</a></h4>
A cute problem and its solution, described in the file
<code>Prisoners.tla</code>.
<h4><a href="https://github.com/tlaplus/Examples/tree/master/specifications/ewd840">
Termination detection in a ring</a></h4>
This is a TLA+ specification of an algorithm of Dijkstra, Fiejen, and
van Gasteren. There is a link to Dijkstra's tech report EWD840. Read
the first page of that report to understand the problem solved by the
algorithm, which is described by the spec in the file
<code>EWD840.tla</code>. Most people will not want to read the rest
of the tech report, which provides an informal explanation of the
algorithm and why it is correct, or the file
<code>EWD840_proof.tla</code> that contains a machine-checked
TLA+ correctness proof.
</DIV>
<H2 id="h2paxos" class="show-hide"
onclick="showHide('hide-paxos','paxos')">Videos of Paxos Lectures
<font id="hide-paxos" >
[show]</font>
</H2>
<DIV id="paxos" class = "hidden-div">
Videos of two lectures describing the Paxos consensus algorithm
with TLA+. In addition to explaining the algorithm, they provide a crash
course on TLA+.
<p class="link">
<A href =
"paxos-algorithm4a79.html?back-link=more-stuff.html#paxos?unhideBut@EQhide-paxos@AMPunhideDiv@EQpaxos"
>[click here]</A>
</DIV>
<H2 id="h2relevant" class="show-hide"
onclick="showHide('hide-relevant','relevant')">Other Relevant Links
<font id="hide-relevant" >
[show]</font>
</H2>
<DIV id="relevant" class = "hidden-div">
Links to pages related to TLA+ outside the TLA+ and TLAPS web
sites. If you'd like links to your work added to this page, <a
href="https://lamport.azurewebsites.net/">contact me</a>.
<DL>
<DT><A href="https://vimeo.com/264959035">Large Scale Model Checking 101</A>
<DD> This is a video by Markus Kuppe giving practical advice on
using the TLC model checker for checking large models.
Dated April 11, 2018.
<p>
<DT><A href="http://tla2012.loria.fr/program.html">2012 TLA+ Workshop</A>
<DD> These are the slides of talks and the submitted papers from
a TLA+ workshop that was held in Paris on 27 August 2012 in conjunction
with the <A href="http://fm2012.cnam.fr/">FM 2012 conference</a>.
<p>
<DT> <A HREF = "http://www.loria.fr/~merz/">
Stephan Merz</A></DT>
<DD> A page containing a link to Stephan Merz's publication list, which
includes several papers on TLA.
<!--
<p>
a link to
<A HREF =
"http://isabelle.in.tum.de/library/HOL/HOL-TLA/index.html">Isabelle/TLA</A>,
Merz's encoding of TLA in the higher-order logic of the generic interactive
theorem prover Isabelle.
Here's
<a href = "http://isabelle.in.tum.de/library/HOL/HOL-TLA-Memory/index.html">
a link
</a>
to a proof of
<A HREF = "http://lamport.azurewebsites.net/pubs/pubs.html#broy-specification-problem">
The Dagstuhl RPC-Memory Example</A>.)
-->
<p>
</DD>
<DT> <A HREF = "http://www.rvs.uni-bielefeld.de/publications/abstracts.html#TLA"> Work With and On Lamport's TLA </A>.</DT>
<DD> Publications by
the Networks and Distributed Systems group at the University of
Bielefeld, headed by
Peter Ladkin. They are all dated 1996 and 1997, but at least
some of them are still worth reading.
</DD>
<!-- Heiko Krumm's stuff, commented out because no longer of interest.
<P>
<DT> <A HREF =
"http://ls4-www.informatik.uni-dortmund.de/RVS/P-TLA/welcome.html">
Tools for TLA based specifications</A></DT>
<DD> This page describes work done in the early 1990s at the
University
of Dortmund, Department of Computer Science by the
Computer Networks and Distributed Systems group
headed by Dr. Heiko Krumm.
<DD>
-->
<!-- COMMENTED OUT. LINK NO LONGER WORKS.
<P>
<DT> <A HREF = "http://www.brics.dk/~urban/tlp/tlp.html">
TLP</A></DT>
<DD> <i><A HREF="http://tla.msr-inria.inria.fr/tlaps">
TLAPS</A>, the TLA+ Proof System, makes this page
obsolete.
The link is kept for historical interest only.</i>
TLP was a system for mechanically checking TLA proofs.
It was developed primarily by Urban Engberg
when he was a
doctoral student at Aarhus University, with help from
the late Peter Grønning.
The page has not been updated
since 1995, but the program should still work.
(It uses an obsolete version of the LP theorem prover,
which is available from the site. It also has Emacs
macros that were written for an old version of Emacs
and may or may not work with newer versions.)
</DD>
-->
</DL>
</DIV>
<H2 id="h2unclassed" class="show-hide"
onclick="showHide('hide-unclassed','unclassed')">Unclassifiable
<font id="hide-unclassed" >
[show]</font>
</H2>
<DIV id="unclassed" class = "hidden-div">
<h4 style="margin-top:5px">Why Don't Computer Scientists Learn Math?</h4>
<font class = "popup-blue" color="blue" id="math-knowledge"
onclick="popupw('math-knowledge.html',770,630)"><b>
Click here</b></font>
for an anecdote demonstrating the sad state of computer science education.
</DIV>
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