theory Intro

imports Base

begin

chapter \<open>Introduction\<close>

text \<open>

   \begin{flushright}

  {\em

  ``My thesis is that programming is not at the bottom of the intellectual \\

  pyramid, but at the top. It's creative design of the highest order. It \\

  isn't monkey or donkey work; rather, as Edsger Dijkstra famously \\

  claimed, it's amongst the hardest intellectual tasks ever attempted.''} \\[1ex]

  Richard Bornat, In {\em Defence of Programming}. \cite{Bornat-lecture}

  \end{flushright}

  \medskip

  then this tutorial is for you. It will guide you through the first steps of

  Isabelle programming, and also explain ``tricks of the trade''. We also hope

  the tutorial will encourage students and researchers to play with Isabelle

  and implement new ideas. The source code of Isabelle can look intimidating,

  but beginners can get by with knowledge of only a handful of concepts, 

  a small number of functions and a few basic coding conventions.

  There is also a considerable amount of code written in Scala that allows

  Isabelle interface with the Jedit GUI. Explanation of this part is beyond

  this tutorial.

  with the many code examples included in the tutorial. The code is as far as

  possible checked against the Isabelle

  distribution.%%\footnote{\input{version.tex}}

  If something does not work, then

  please let us know. It is impossible for us to know every environment,

  operating system or editor in which Isabelle is used. If you have comments,

  criticism or like to add to the tutorial, please feel free---you are most

  welcome!! The tutorial is meant to be gentle and comprehensive. To achieve

  this we need your help and feedback.

\<close>

section \<open>Intended Audience and Prior Knowledge\<close>

text \<open>

  This tutorial targets readers who already know how to use Isabelle for

  writing theories and proofs. We also assume that readers are familiar with

  the functional programming language ML, the language in which most of

  Isabelle is implemented. If you are unfamiliar with either of these two

  subjects, then you should first work through the Isabelle/HOL tutorial

  Isabelle has adopted a sizable amount of Scala code for a slick GUI

  based on jEdit. This part of the code is beyond the interest of this

  tutorial, since it mostly does not concern the regular Isabelle 

  developer.

\<close>

section \<open>Existing Documentation\<close>

text \<open>

  The following documentation about Isabelle programming already exists (and is

  part of the distribution of Isabelle):

  \begin{description}

  to be the main reference of Isabelle at a time when all proof scripts 

  now, but some  parts, particularly the chapters on tactics, are still 

  useful.

  \end{description}

  Then of course there are:

  \begin{description}

  \item[The Isabelle sources.] They are the ultimate reference for how

  things really work. Therefore you should not hesitate to look at the

  way things are actually implemented. While much of the Isabelle

  code is uncommented, some parts have very helpful comments---particularly

  the code about theorems and terms. Despite the lack of comments in most

  parts, it is often good to look at code that does similar things as you 

  want to do and learn from it. 

  This tutorial contains frequently pointers to the

  it is often also necessary to track the change history of a file or

  files. The Mercurial repository\footnote{\url{http://isabelle.in.tum.de/repos/isabelle/}} 

  for Isabelle  provides convenient interfaces to query the history of

  files and ``change sets''.

  \end{description}

\<close>

section \<open>Typographic Conventions\<close>

text \<open>

  All ML-code in this tutorial is typeset in shaded boxes, like the following 

  simple ML-expression:

  \begin{isabelle}

  \begin{graybox}

  \isacommand{ML}~\<open>\<verbopen>\<close>\isanewline

  \hspace{5mm}@{ML "3 + 4"}\isanewline

  \<open>\<verbclose>\<close>

  \end{graybox}

  \end{isabelle}

  These boxes correspond to how code can be processed inside the interactive

  environment of Isabelle. It is therefore easy to experiment with the code

  that is shown in this tutorial. However, for better readability we will drop

  the enclosing \isacommand{ML}~\<open>\<verbopen> \<dots> \<verbclose>\<close> and just

  write:

  @{ML [display,gray] "3 + 4"}

  Whenever appropriate we also show the response the code 

  generates when evaluated. This response is prefixed with a 

  @{text [quotes] ">"}, like:

  The user-level commands of Isabelle (i.e., the non-ML code) are written

  in \isacommand{bold face} (e.g., \isacommand{lemma}, \isacommand{apply},

  \isacommand{foobar} and so on).  We use \<open>$ \<dots>\<close> to indicate that a

  command needs to be run in a UNIX-shell, for example:

  @{text [display] "$ grep -R Thy_Output *"}

  Pointers to further information and Isabelle files are typeset in 

  \textit{italic} and highlighted as follows:

  \begin{readmore}

  Further information or pointers to files.

  \end{readmore}

  Note that pointers to Isabelle files are hyperlinked to the tip of the Mercurial

  repository at \href{http://isabelle.in.tum.de/repos/isabelle/}

  {http://isabelle.in.tum.de/repos/isabelle/}, not the latest stable release

  of Isabelle.

  A few exercises are scattered around the text. Their solutions are given 

  in Appendix~\ref{ch:solutions}. Of course, you learn most, if you first try

  to solve the exercises on your own, and then look at the solutions.

\<close>

section \<open>How To Understand Isabelle Code\<close>

text \<open>

  One of the more difficult aspects of any kind of programming is to

  understand code written by somebody else. This is aggravated in Isabelle by

  the fact that many parts of the code contain none or only few

  comments. There is one strategy that might be helpful to navigate your way:

  code on the fly (for example inside an \isacommand{ML}~\<open>\<verbopen>\<dots>\<verbclose>\<close> section). So you can copy (self-contained)

  chunks of existing code into a separate theory file and then study it

  alongside with examples. You can also install ``probes'' inside the copied

  code without having to recompile the whole Isabelle distribution. Such

  probes might be messages or printouts of variables (see chapter

  \ref{chp:firststeps}). Although PolyML also contains a debugger, it seems

  probing the code with explicit print statements is the most effective method

  for understanding what some piece of code is doing. However do not expect

  quick results with this! It is painful. Depending on the size of the code

  you are looking at, you will spend the better part of a quiet afternoon with

  it. And there seems to be no better way for understanding code in Isabelle.

\<close>

section \<open>Aaaaargh! My Code Does not Work Anymore\<close>

text \<open>

  One unpleasant aspect of any code development inside a larger system is that

  one has to aim at a ``moving target''. Isabelle is no exception of this. Every 

  update lets potentially all hell break loose, because other developers have

  changed code you are relying on. Cursing is somewhat helpful in such situations, 

  but taking the view that incompatible code changes are a fact of life 

  might be more gratifying. Isabelle is a research project. In most circumstances 

  it is just impossible to make research backward compatible (imagine Darwin 

  attempting to make the Theory of Evolution backward compatible).

  However, there are a few steps you can take to mitigate unwanted

  interferences with code changes from other developers. First, you can base

  your code on the latest stable release of Isabelle (it is aimed to have one

  such release at least once every year). This might cut you off from the

  latest feature implemented in Isabelle, but at least you do not have to

  track side-steps or dead-ends in the Isabelle development. Of course this

  means also you have to synchronise your code at the next stable release. If

  you do not synchronise, be warned that code seems to ``rot'' very

  quickly. Another possibility is to get your code into the Isabelle

  distribution. For this you have to convince other developers that your code

  or project is of general interest. If you managed to do this, then the

  problem of the moving target goes away, because when checking in new code,

  developers are strongly urged to test it against Isabelle's code base. If

  your project is part of that code base, then maintenance is done by

  others. Unfortunately, this might not be a helpful advice for all types of

  projects. A lower threshold for inclusion has the Archive of Formal

  Proofs, short AFP.\footnote{\url{http://afp.sourceforge.net/}} This archive

  has been created mainly for formalisations that are interesting but not

  necessarily of general interest. If you have ML-code as part of a

  formalisation, then this might be the right place for you. There is no

  problem with updating your code after submission. At the moment developers

  are not as diligent with checking their code against the AFP than with

  checking agains the distribution, but generally problems will be caught and

  the developer, who caused them, is expected to fix them. So also in this

  case code maintenance is done for you.

\<close>

section \<open>Serious Isabelle ML-Programming\<close>

text \<open>

  As already pointed out in the previous section, Isabelle is a joint effort 

  of many developers. Therefore, disruptions that break the work of others

  are generally frowned upon. ``Accidents'' however do happen and everybody knows

  this. Still to keep them to a minimum, you can submit your changes first to a rather 

  sophisticated \emph{testboard}, which will perform checks of your changes against the

  Isabelle repository and against the AFP. The advantage of the testboard is

  that the testing is performed by rather powerful machines saving you lengthy

  tests on, for example, your own laptop. You can see the results of the testboard 

  at 

  \begin{center}

  \url{http://isabelle.in.tum.de/testboard/Isabelle/}

  \end{center}

  which is organised like a Mercurial repository. A green point next to a change

  indicates that the change passes the corresponding tests (for this of course you

  have to allow some time). You can summit any changes to the testboard using the 

  command

  @{text [display] "$ hg push -f ssh://...@hgbroy.informatik.tu-muenchen.de\\

   //home/isabelle-repository/repos/testboard"}

  where the dots need to be replaced by your login name.  Note that for

  pushing changes to the testboard you need to add the option \<open>-f\<close>,

  which should \emph{never} be used with the main Isabelle

  repository. While the testboard is a great system for supporting Isabelle

  developers, its disadvantage is that it needs login permissions for the

  computers in Munich. So in order to use it, you might have to ask other

  developers to obtain one.

\<close>

section \<open>Some Naming Conventions in the Isabelle Sources\<close>

text \<open>

  There are a few naming conventions in the Isabelle code that might aid reading 

  and writing code. (Remember that code is written once, but read many 

  times.) The most important conventions are:

  \begin{itemize}

  \item \<open>t\<close>, \<open>u\<close>, \<open>trm\<close> for (raw) terms; ML-type: @{ML_type term}

  \item \<open>ct\<close>, \<open>cu\<close> for certified terms; ML-type: @{ML_type cterm}

  \item \<open>ty\<close>, \<open>T\<close>, \<open>U\<close> for (raw) types; ML-type: @{ML_type typ}

  \item \<open>S\<close> for sorts; ML-type: @{ML_type sort}

  \item \<open>th\<close>, \<open>thm\<close> for theorems; ML-type: @{ML_type thm}

  \item \<open>foo_tac\<close> for tactics; ML-type: @{ML_type tactic}

  \item \<open>thy\<close> for theories; ML-type: @{ML_type theory}

  \item \<open>ctxt\<close> for proof contexts; ML-type: @{ML_type Proof.context}

  \item \<open>lthy\<close> for local theories; ML-type: @{ML_type local_theory}

  \item \<open>context\<close> for generic contexts; ML-type @{ML_type Context.generic}

  \item \<open>mx\<close> for mixfix syntax annotations; ML-type @{ML_type mixfix}

  \item \<open>prt\<close> for pretty printing; ML-type @{ML_type Pretty.T}

  \item \<open>phi\<close> for morphisms; ML-type @{ML_type morphism}

  \end{itemize}

\<close>

section \<open>Acknowledgements\<close>

text \<open>

  Financial support for this tutorial was provided by the German 

  Research Council (DFG) under grant number URB 165/5-1. The following

  people contributed to the text:

  \begin{itemize}

  \item {\bf Stefan Berghofer} wrote nearly all of the ML-code of the

  \simpleinductive-package and the code for the \<open>chunk\<close>-antiquotation. He also wrote the first version of chapter

  \ref{chp:package} describing this package and has been helpful \emph{beyond

  measure} with answering questions about Isabelle.

  \item {\bf Jasmin Blanchette} helped greatly with section \ref{sec:pretty}

  and exercise \ref{fun:killqnt}.

  \ref{rec:external} and \ref{rec:oracle}. He also wrote section \ref{sec:conversion} 

  and helped with recipe \ref{rec:timing}. Parts of section \ref{sec:storing}

  are by him.

  \item {\bf Lukas Bulwahn} made me aware of a problem with recursive

  parsers, contributed exercise \ref{ex:contextfree} and contributed 

  to the ``introspection'' of theorems in section \ref{sec:theorems}.

  \item {\bf Jeremy Dawson} wrote the first version of chapter \ref{chp:parsing}

  about parsing.

  %%\item {\bf Florian Haftmann} helped with maintaining recipe \ref{rec:callml}.

  \item {\bf Armin Heller} helped with recipe \ref{rec:sat}.

  \item {\bf Rafal Kolanski} contributed to the ``introspection'' of theorems 

  in section \ref{sec:theorems}.

  \item {\bf Alexander Krauss} wrote a very early version of the ``first-steps'' 

  %%\item {\bf Tobias Nipkow} contributed recipe \ref{rec:callml}.

  \item {\bf Michael Norrish} proofread parts of the text.

  \item {\bf Andreas Schropp} improved and corrected section \ref{sec:univ} and

   contributed towards section \ref{sec:sorts}.

  \item {\bf Christian Sternagel} proofread the tutorial and made 

  many improvemets to the text. 

  \item {\bf Dmitriy Traytel} suggested to use the ML-antiquotation 

  \<open>command_spec\<close> in section~\ref{sec:newcommand}, which simplified the code. 

  \item {\bf Piotr Trojanek} proofread the text.

  \end{itemize}

  Please let me know of any omissions. Responsibility for any remaining

  errors lies with me.\bigskip

  \newpage

  \mbox{}\\[5cm]

  {\Large\bf

  This tutorial is still in the process of being written! All of the

  text is still under construction. Sections and 

  chapters that are under \underline{heavy} construction are marked 

  with TBD.}