theory ToyList

imports Datatype

begin

text{*\noindent

HOL already has a predefined theory of lists called @{text List} ---

@{text ToyList} is merely a small fragment of it chosen as an example. In

contrast to what is recommended in \S\ref{sec:Basic:Theories},

@{text ToyList} is not based on @{text Main} but on @{text Datatype}, a

theory that contains pretty much everything but lists, thus avoiding

ambiguities caused by defining lists twice.

*}

datatype 'a list = Nil                          ("[]")

                 | Cons 'a "'a list"            (infixr "#" 65);

text{*\noindent

The datatype\index{datatype@\isacommand {datatype} (command)}

\tydx{list} introduces two

constructors \cdx{Nil} and \cdx{Cons}, the

empty~list and the operator that adds an element to the front of a list. For

example, the term \isa{Cons True (Cons False Nil)} is a value of

type @{typ"bool list"}, namely the list with the elements @{term"True"} and

@{term"False"}. Because this notation quickly becomes unwieldy, the

datatype declaration is annotated with an alternative syntax: instead of

@{term[source]Nil} and \isa{Cons x xs} we can write

@{term"[]"}\index{$HOL2list@\isa{[]}|bold} and

@{term"x # xs"}\index{$HOL2list@\isa{\#}|bold}. In fact, this

alternative syntax is the familiar one.  Thus the list \isa{Cons True

(Cons False Nil)} becomes @{term"True # False # []"}. The annotation

\isacommand{infixr}\index{infixr@\isacommand{infixr} (annotation)} 

means that @{text"#"} associates to

the right: the term @{term"x # y # z"} is read as @{text"x # (y # z)"}

and not as @{text"(x # y) # z"}.

The @{text 65} is the priority of the infix @{text"#"}.

\begin{warn}

  Syntax annotations can be powerful, but they are difficult to master and 

  are never necessary.  You

  could drop them from theory @{text"ToyList"} and go back to the identifiers

  @{term[source]Nil} and @{term[source]Cons}.  Novices should avoid using

  syntax annotations in their own theories.

\end{warn}

Next, two functions @{text"app"} and \cdx{rev} are defined recursively,

in this order, because Isabelle insists on definition before use:

*}

primrec app :: "'a list \<Rightarrow> 'a list \<Rightarrow> 'a list" (infixr "@" 65) where

"[] @ ys       = ys" |

"(x # xs) @ ys = x # (xs @ ys)"

primrec rev :: "'a list \<Rightarrow> 'a list" where

"rev []        = []" |

"rev (x # xs)  = (rev xs) @ (x # [])"

text{*\noindent

Each function definition is of the form

\begin{center}

\isacommand{primrec} \textit{name} @{text"::"} \textit{type} \textit{(optional syntax)} \isakeyword{where} \textit{equations}

\end{center}

The equations must be separated by @{text"|"}.

%

Function @{text"app"} is annotated with concrete syntax. Instead of the

prefix syntax @{text"app xs ys"} the infix

@{term"xs @ ys"}\index{$HOL2list@\isa{\at}|bold} becomes the preferred

form.

\index{*rev (constant)|(}\index{append function|(}

The equations for @{text"app"} and @{term"rev"} hardly need comments:

@{text"app"} appends two lists and @{term"rev"} reverses a list.  The

keyword \commdx{primrec} indicates that the recursion is

of a particularly primitive kind where each recursive call peels off a datatype

constructor from one of the arguments.  Thus the

recursion always terminates, i.e.\ the function is \textbf{total}.

\index{functions!total}

The termination requirement is absolutely essential in HOL, a logic of total

functions. If we were to drop it, inconsistencies would quickly arise: the

``definition'' $f(n) = f(n)+1$ immediately leads to $0 = 1$ by subtracting

$f(n)$ on both sides.

% However, this is a subtle issue that we cannot discuss here further.

\begin{warn}

  As we have indicated, the requirement for total functions is an essential characteristic of HOL\@. It is only

  because of totality that reasoning in HOL is comparatively easy.  More

  generally, the philosophy in HOL is to refrain from asserting arbitrary axioms (such as

  function definitions whose totality has not been proved) because they

  quickly lead to inconsistencies. Instead, fixed constructs for introducing

  types and functions are offered (such as \isacommand{datatype} and

  \isacommand{primrec}) which are guaranteed to preserve consistency.

\end{warn}

\index{syntax}%

A remark about syntax.  The textual definition of a theory follows a fixed

syntax with keywords like \isacommand{datatype} and \isacommand{end}.

% (see Fig.~\ref{fig:keywords} in Appendix~\ref{sec:Appendix} for a full list).

Embedded in this syntax are the types and formulae of HOL, whose syntax is

extensible (see \S\ref{sec:concrete-syntax}), e.g.\ by new user-defined infix operators.

To distinguish the two levels, everything

HOL-specific (terms and types) should be enclosed in

\texttt{"}\dots\texttt{"}. 

To lessen this burden, quotation marks around a single identifier can be

dropped, unless the identifier happens to be a keyword, for example

\isa{"end"}.

When Isabelle prints a syntax error message, it refers to the HOL syntax as

the \textbf{inner syntax} and the enclosing theory language as the \textbf{outer syntax}.

\section{Evaluation}

\index{evaluation}

Assuming you have processed the declarations and definitions of

\texttt{ToyList} presented so far, you may want to test your

functions by running them. For example, what is the value of

@{term"rev(True#False#[])"}? Command

*}

value "rev (True # False # [])"

text{* \noindent yields the correct result @{term"False # True # []"}.

But we can go beyond mere functional programming and evaluate terms with

variables in them, executing functions symbolically: *}

normal_form "rev (a # b # c # [])"

text{*\noindent yields @{term"c # b # a # []"}.

Command \commdx{normal\protect\_form} works for arbitrary terms

but can be slower than command \commdx{value},

which is restricted to variable-free terms and executable functions.

To appreciate the subtleties of evaluating terms with variables,

try this one:

*}

normal_form "rev (a # b # c # xs)"

text{*

\section{An Introductory Proof}

\label{sec:intro-proof}

Having convinced ourselves (as well as one can by testing) that our

definitions capture our intentions, we are ready to prove a few simple

theorems. This will illustrate not just the basic proof commands but

also the typical proof process.

\subsubsection*{Main Goal.}

Our goal is to show that reversing a list twice produces the original

list.

*}

theorem rev_rev [simp]: "rev(rev xs) = xs";

txt{*\index{theorem@\isacommand {theorem} (command)|bold}%

\noindent

This \isacommand{theorem} command does several things:

\begin{itemize}

\item

It establishes a new theorem to be proved, namely @{prop"rev(rev xs) = xs"}.

\item

It gives that theorem the name @{text"rev_rev"}, for later reference.

\item

It tells Isabelle (via the bracketed attribute \attrdx{simp}) to take the eventual theorem as a simplification rule: future proofs involving

simplification will replace occurrences of @{term"rev(rev xs)"} by

@{term"xs"}.

\end{itemize}

The name and the simplification attribute are optional.

Isabelle's response is to print the initial proof state consisting

of some header information (like how many subgoals there are) followed by

@{subgoals[display,indent=0]}

For compactness reasons we omit the header in this tutorial.

Until we have finished a proof, the \rmindex{proof state} proper

always looks like this:

\begin{isabelle}

~1.~$G\sb{1}$\isanewline

~~\vdots~~\isanewline

~$n$.~$G\sb{n}$

\end{isabelle}

The numbered lines contain the subgoals $G\sb{1}$, \dots, $G\sb{n}$

that we need to prove to establish the main goal.\index{subgoals}

Initially there is only one subgoal, which is identical with the

main goal. (If you always want to see the main goal as well,

set the flag \isa{Proof.show_main_goal}\index{*show_main_goal (flag)}

--- this flag used to be set by default.)

Let us now get back to @{prop"rev(rev xs) = xs"}. Properties of recursively

defined functions are best established by induction. In this case there is

nothing obvious except induction on @{term"xs"}:

*}

apply(induct_tac xs);

txt{*\noindent\index{*induct_tac (method)}%

This tells Isabelle to perform induction on variable @{term"xs"}. The suffix

@{term"tac"} stands for \textbf{tactic},\index{tactics}

a synonym for ``theorem proving function''.

By default, induction acts on the first subgoal. The new proof state contains

two subgoals, namely the base case (@{term[source]Nil}) and the induction step

(@{term[source]Cons}):

@{subgoals[display,indent=0,margin=65]}

The induction step is an example of the general format of a subgoal:\index{subgoals}

\begin{isabelle}

~$i$.~{\isasymAnd}$x\sb{1}$~\dots$x\sb{n}$.~{\it assumptions}~{\isasymLongrightarrow}~{\it conclusion}

\end{isabelle}\index{$IsaAnd@\isasymAnd|bold}

The prefix of bound variables \isasymAnd$x\sb{1}$~\dots~$x\sb{n}$ can be

ignored most of the time, or simply treated as a list of variables local to

this subgoal. Their deeper significance is explained in Chapter~\ref{chap:rules}.

The {\it assumptions}\index{assumptions!of subgoal}

are the local assumptions for this subgoal and {\it

  conclusion}\index{conclusion!of subgoal} is the actual proposition to be proved. 

Typical proof steps

that add new assumptions are induction and case distinction. In our example

the only assumption is the induction hypothesis @{term"rev (rev list) =

  list"}, where @{term"list"} is a variable name chosen by Isabelle. If there

are multiple assumptions, they are enclosed in the bracket pair

\indexboldpos{\isasymlbrakk}{$Isabrl} and

\indexboldpos{\isasymrbrakk}{$Isabrr} and separated by semicolons.

Let us try to solve both goals automatically:

*}

apply(auto);

txt{*\noindent

This command tells Isabelle to apply a proof strategy called

@{text"auto"} to all subgoals. Essentially, @{text"auto"} tries to

simplify the subgoals.  In our case, subgoal~1 is solved completely (thanks

to the equation @{prop"rev [] = []"}) and disappears; the simplified version

of subgoal~2 becomes the new subgoal~1:

@{subgoals[display,indent=0,margin=70]}

In order to simplify this subgoal further, a lemma suggests itself.

*}

(*<*)

oops

(*>*)

subsubsection{*First Lemma*}

text{*

\indexbold{abandoning a proof}\indexbold{proofs!abandoning}

After abandoning the above proof attempt (at the shell level type

\commdx{oops}) we start a new proof:

*}

lemma rev_app [simp]: "rev(xs @ ys) = (rev ys) @ (rev xs)";

txt{*\noindent The keywords \commdx{theorem} and

\commdx{lemma} are interchangeable and merely indicate

the importance we attach to a proposition.  Therefore we use the words

\emph{theorem} and \emph{lemma} pretty much interchangeably, too.

There are two variables that we could induct on: @{term"xs"} and

@{term"ys"}. Because @{text"@"} is defined by recursion on

the first argument, @{term"xs"} is the correct one:

*}

apply(induct_tac xs);

txt{*\noindent

This time not even the base case is solved automatically:

*}

apply(auto);

txt{*

@{subgoals[display,indent=0,goals_limit=1]}

Again, we need to abandon this proof attempt and prove another simple lemma

first. In the future the step of abandoning an incomplete proof before

embarking on the proof of a lemma usually remains implicit.

*}

(*<*)

oops

(*>*)

subsubsection{*Second Lemma*}

text{*

We again try the canonical proof procedure:

*}

lemma app_Nil2 [simp]: "xs @ [] = xs";

apply(induct_tac xs);

apply(auto);

txt{*

\noindent

It works, yielding the desired message @{text"No subgoals!"}:

@{goals[display,indent=0]}

We still need to confirm that the proof is now finished:

*}

done

text{*\noindent

As a result of that final \commdx{done}, Isabelle associates the lemma just proved

with its name. In this tutorial, we sometimes omit to show that final \isacommand{done}

if it is obvious from the context that the proof is finished.

% Instead of \isacommand{apply} followed by a dot, you can simply write

% \isacommand{by}\indexbold{by}, which we do most of the time.

Notice that in lemma @{thm[source]app_Nil2},

as printed out after the final \isacommand{done}, the free variable @{term"xs"} has been

replaced by the unknown @{text"?xs"}, just as explained in

\S\ref{sec:variables}.

Going back to the proof of the first lemma

*}

lemma rev_app [simp]: "rev(xs @ ys) = (rev ys) @ (rev xs)";

apply(induct_tac xs);

apply(auto);

txt{*

\noindent

we find that this time @{text"auto"} solves the base case, but the

induction step merely simplifies to

@{subgoals[display,indent=0,goals_limit=1]}

Now we need to remember that @{text"@"} associates to the right, and that

@{text"#"} and @{text"@"} have the same priority (namely the @{text"65"}

in their \isacommand{infixr} annotation). Thus the conclusion really is

\begin{isabelle}

~~~~~(rev~ys~@~rev~list)~@~(a~\#~[])~=~rev~ys~@~(rev~list~@~(a~\#~[]))

\end{isabelle}

and the missing lemma is associativity of @{text"@"}.

*}

(*<*)oops(*>*)

subsubsection{*Third Lemma*}

text{*

Abandoning the previous attempt, the canonical proof procedure

succeeds without further ado.

*}

lemma app_assoc [simp]: "(xs @ ys) @ zs = xs @ (ys @ zs)";

apply(induct_tac xs);

apply(auto);

done

text{*

\noindent

Now we can prove the first lemma:

*}

lemma rev_app [simp]: "rev(xs @ ys) = (rev ys) @ (rev xs)";

apply(induct_tac xs);

apply(auto);

done

text{*\noindent

Finally, we prove our main theorem:

*}

theorem rev_rev [simp]: "rev(rev xs) = xs";

apply(induct_tac xs);

apply(auto);

done

text{*\noindent

The final \commdx{end} tells Isabelle to close the current theory because

we are finished with its development:%

\index{*rev (constant)|)}\index{append function|)}

*}

end