theory CookBook

imports Main

uses "~/Theorem-Provers/Isabelle/Isabelle-CVS/Doc/antiquote_setup.ML"

("comp_simproc")

begin

(*<*)

ML {*

local structure O = ThyOutput

in

  fun check_exists f = 

    if File.exists (Path.explode ("~~/src/" ^ f)) then ()

    else error ("Source file " ^ quote f ^ " does not exist.")

  val _ = O.add_commands

   [("ML_file", O.args (Scan.lift Args.name) (O.output (fn _ => fn name =>

         (check_exists name; Pretty.str name))))];

end

*}

(*>*)

section {* Introduction *}

text {*

  The purpose of this document is to guide the reader through the

  first steps in Isabelle programming, and to provide recipes for

  solving common problems. 

*}

subsection {* Intended Audience and Prior Knowledge *}

text {* 

  This cookbook targets an audience who already knows how to use the Isabelle

  system to write theories and proofs, but without using ML.

  You should also be familiar with the \emph{Standard ML} programming

  language, which is  used for Isabelle programming. If you are unfamiliar with any of

  these two subjects, you should first work through the Isabelle/HOL

  tutorial \cite{isa-tutorial} and Paulson's book on Standard ML

  \cite{paulson-ml2}.

*}

subsection {* Primary Documentation *}

text {*

  \begin{description}

  \item[The Implementation Manual \cite{isa-imp}] describes Isabelle

  from a programmer's perspective, documenting both the underlying

  concepts and the concrete interfaces. 

  \item[The Isabelle Reference Manual \cite{isabelle-ref}] is an older document that used

  to be the main reference, when all reasoning happened on the ML

  level. Many parts of it are outdated now, but some parts, mainly the

  chapters on tactics, are still useful.

  \item[The code] is of course the ultimate reference for how

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

  way things are actually implemented. More importantly, it is often

  good to look at code that does similar things as you want to do, to

  learn from other people's code.

  \end{description}

  Since Isabelle is not a finished product, these manuals, just like

  the implementation itself, are always under construction. This can

  be dificult and frustrating at times, when interfaces are changing

  frequently. But it is a reality that progress means changing

  things (FIXME: need some short and convincing comment that this

  is a strategy, not a problem that should be solved).

*}

section {* First Steps *}

text {* 

  Isabelle programming happens in an enhanced dialect of Standard ML,

  which adds antiquotations containing references to the logical

  context.

  Just like all lemmas or proofs, all ML code that you write lives in

  a theory, where it is embedded using the \isacommand{ML} command:

*}

ML {*

  3 + 4

*}

text {*

  The \isacommand{ML} command takes an arbitrary ML expression, which

  is evaluated. It can also contain value or function bindings.

*}

subsection {* Antiquotations *}

text {*

  The main advantage of embedding all code in a theory is that the

  code can contain references to entities that are defined in the

  theory. Let us for example, print out the name of the current

  theory:

*}

ML {* Context.theory_name @{theory} *}

text {* The @{text "@{theory}"} antiquotation is substituted with the

  current theory, whose name can then be extracted using a the

  function @{ML "Context.theory_name"}. Note that antiquotations are

  statically scoped. The function

*}

ML {* 

  fun current_thyname () = Context.theory_name @{theory}

*}

text {*

  does \emph{not} return the name of the current theory. Instead, we have

  defined the constant function that always returns the string @{ML "\"CookBook\""}, which is

  the name of @{text "@{theory}"} at the point where the code

  is embedded. Operationally speaking,  @{text "@{theory}"} is \emph{not}

  replaced with code that will look up the current theory in some

  (destructive) data structure and return it. Instead, it is really

  replaced with the theory value.

  In the course of this introduction, we will learn about more of

  these antoquotations, which greatly simplify programming, since you

  can directly access all kinds of logical elements from ML.

*}

subsection {* Terms *}

text {*

  We can simply quote Isabelle terms from ML using the @{text "@{term \<dots>}"} 

  antiquotation:

*}

ML {* @{term "(a::nat) + b = c"} *}

text {*

  This shows the term @{term "(a::nat) + b = c"} in the internal

  representation, with all gory details. Terms are just an ML

  datatype, and they are defined in @{ML_file "Pure/term.ML"}.

  The representation of terms uses deBruin indices: Bound variables

  are represented by the constructor @{ML Bound}, and the index refers to

  the number of lambdas we have to skip until we hit the lambda that

  binds the variable. The names of bound variables are kept at the

  abstractions, but they are just comments. 

  See \ichcite{ch:logic} for more details.

  \begin{readmore}

  Terms are described in detail in \ichcite{ch:logic}. Their

  definition and many useful operations can be found in @{ML_file "Pure/term.ML"}.

  \end{readmore}

  In a similar way we can quote types and theorems:

*}

ML {* @{typ "(int * nat) list"} *}

ML {* @{thm allI} *}

text {* 

  In the default setup, types and theorems are printed as strings. 

*}

text {*

  Sometimes the internal representation can be surprisingly different

  from what you see at the user level, because the layer of

  parsing/type checking/pretty printing can be quite thick. 

\begin{exercise}

  Look at the internal term representation of the following terms, and

  find out why they are represented like this.

  \begin{itemize}

  \item @{term "case x of 0 \<Rightarrow> 0 | Suc y \<Rightarrow> y"}  

  \item @{term "\<lambda>(x,y). P y x"}  

  \item @{term "{ [x::int] | x. x \<le> -2 }"}  

  \end{itemize}

  Hint: The third term is already quite big, and the pretty printer

  may omit parts of it by default. If you want to see all of it, you

  can use @{ML "print_depth 50"} to set the limit to a value high enough.

\end{exercise}

*}

subsection {* Type checking *}

text {* 

  We can freely construct and manipulate terms, since they are just

  arbitrary unchecked trees. However, we eventually want to see if a

  term is wellformed in a certain context.

  Type checking is done via @{ML cterm_of}, which turns a @{ML_type

  term} into a  @{ML_type cterm}, a \emph{certified} term. Unlike

  @{ML_type term}s, which are just trees, @{ML_type

  "cterm"}s are abstract objects that are guaranteed to be

  type-correct, and can only be constructed via the official

  interfaces.

  Type

  checking is always relative to a theory context. For now we can use

  the @{ML "@{theory}"} antiquotation to get hold of the theory at the current

  point:

*}

ML {*

  let

    val natT = @{typ "nat"}

    val zero = @{term "0::nat"}(*Const ("HOL.zero_class.zero", natT)*)

  in

    cterm_of @{theory} 

        (Const ("HOL.plus_class.plus", natT --> natT --> natT) 

         $ zero $ zero)

  end

*}

ML {*

  @{const_name plus}

*}

ML {*

  @{term "{ [x::int] | x. x \<le> -2 }"}

*}

text {*

  The internal names of constants like @{term "zero"} or @{text "+"} are

  often more complex than one first expects. Here, the extra prefixes

  @{text zero_class} and @{text plus_class} are present because the

  constants are defined within a type class. Guessing such internal

  names can be extremely hard, which is why the system provides

  another antiquotation: @{ML "@{const_name plus}"} gives just this

  name.

  \begin{exercise}

  Write a function @{ML_text "rev_sum : term -> term"} that takes a

  term of the form @{text "t\<^isub>1 + t\<^isub>2 + \<dots> + t\<^isub>n"}

  and returns the reversed sum @{text "t\<^isub>n + \<dots> + t\<^isub>2 + t\<^isub>1"}.

  Note that @{text "+"} associates to the left.

  Try your function on some examples, and see if the result typechecks.

  \end{exercise}

  \begin{exercise}

  Write a function which takes two terms representing natural numbers

  in unary (like @{term "Suc (Suc (Suc 0))"}), and produce the unary

  number representing their sum.

  \end{exercise}

  \begin{exercise}

  Look at the functions defined in @{ML_file "Pure/logic.ML"} and

  @{ML_file "HOL/hologic.ML"} and see if they can make your life

  easier.

  \end{exercise}

*}

subsection {* Theorems *}

text {*

  Just like @{ML_type cterm}s, theorems (of type @{ML_type thm}) are

  abstract objects that can only be built by going through the kernel

  interfaces, which means that all your proofs will be checked. The

  basic rules of the Isabelle/Pure logical framework are defined in

  @{ML_file "Pure/thm.ML"}. 

  Using these rules, which are just ML functions, you can do simple

  natural deduction proofs on the ML level. For example, the statement

  @{prop "(\<And>(x::nat). P x \<Longrightarrow> Q x) \<Longrightarrow> P t \<Longrightarrow> Q t"} can be proved like

  this\footnote{Note that @{text "|>"} is just reverse

  application. This combinator, and several variants are defined in

  @{ML_file "Pure/General/basics.ML"}}:

*}

ML {*

let

  val thy = @{theory}

  val nat = HOLogic.natT

  val x = Free ("x", nat)

  val t = Free ("t", nat)

  val P = Free ("P", nat --> HOLogic.boolT)

  val Q = Free ("Q", nat --> HOLogic.boolT)

  val A1 = Logic.all x 

           (Logic.mk_implies (HOLogic.mk_Trueprop (P $ x),

                              HOLogic.mk_Trueprop (Q $ x)))

           |> cterm_of thy

  val A2 = HOLogic.mk_Trueprop (P $ t)

           |> cterm_of thy

  val Pt_implies_Qt = 

        assume A1

        |> forall_elim (cterm_of thy t)

  val Qt = implies_elim Pt_implies_Qt (assume A2)

in

  Qt 

  |> implies_intr A2

  |> implies_intr A1

end

*}

subsection {* Tactical reasoning *}

text {*

  The goal-oriented tactical style is similar to the @{text apply}

  style at the user level. Reasoning is centered around a \emph{goal},

  which is modified in a sequence of proof steps until it is solved.

  A goal (or goal state) is a special @{ML_type thm}, which by

  convention is an implication:

  @{text[display] "A\<^isub>1 \<Longrightarrow> \<dots> \<Longrightarrow> A\<^isub>n \<Longrightarrow> #(C)"}

  Since the final result @{term C} could again be an implication, there is the

  @{text "#"} around the final result, which protects its premises from being

  misinterpreted as open subgoals. The protection @{text "# :: prop \<Rightarrow>

  prop"} is just the identity and used as a syntactic marker.

  Now tactics are just functions that map a goal state to a (lazy)

  sequence of successor states, hence the type of a tactic is

  @{ML_type[display] "thm -> thm Seq.seq"}

  See @{ML_file "Pure/General/seq.ML"} for the implementation of lazy

  sequences.

  Of course, tactics are expected to behave nicely and leave the final

  conclusion @{term C} intact. In order to start a tactical proof for

  @{term A}, we

  just set up the trivial goal @{text "A \<Longrightarrow> #(A)"} and run the tactic

  on it. When the subgoal is solved, we have just @{text "#(A)"} and

  can remove the protection.

  The operations in @{ML_file "Pure/goal.ML"} do just that and we can use

  them.

  Let us transcribe a simple apply style proof from the

  tutorial\cite{isa-tutorial} into ML:

*}

lemma disj_swap: "P \<or> Q \<Longrightarrow> Q \<or> P"

apply (erule disjE)

 apply (rule disjI2)

 apply assumption

apply (rule disjI1)

apply assumption

done

ML {*

let

  val ctxt = @{context}

  val goal = @{prop "P \<or> Q \<Longrightarrow> Q \<or> P"}

in

  Goal.prove ctxt ["P", "Q"] [] goal (fn _ => 

    eresolve_tac [disjE] 1

    THEN resolve_tac [disjI2] 1

    THEN assume_tac 1

    THEN resolve_tac [disjI1] 1

    THEN assume_tac 1)

end

*}

text {*

  Tactics that affect only a certain subgoal, take a subgoal number as

  an integer parameter. Here we always work on the first subgoal,

  following exactly the @{text "apply"} script.

*}

section {* Case Study: Relation Composition *}

text {*

  \emph{Note: This is completely unfinished. I hoped to have a section

  with a nontrivial example, but I ran into several problems.}

  Recall that HOL has special syntax for set comprehensions:

  @{term "{ f x y |x y. P x y}"} abbreviates 

  @{term[source] "{u. \<exists>x y. u = f x y \<and> P x y}"}. 

  We will automatically prove statements of the following form:

  @{lemma[display] "{(l\<^isub>1 x, r\<^isub>1 x) |x. P\<^isub>1 x} O {(l\<^isub>2 x, r\<^isub>2 x) |x. P\<^isub>2 x}

  = {(l\<^isub>2 x, r\<^isub>1 y) |x y. r\<^isub>2 x = l\<^isub>1 y \<and>

  P\<^isub>2 x \<and> P\<^isub>1 y}" by auto}

  In Isabelle, relation composition is defined to be consistent with

  function composition, that is, the relation applied ``first'' is

  written on the right hand side. This different from what many

  textbooks do.

  The above statement about composition is not proved automatically by

  @{method simp}, and it cannot be solved by a fixed set of rewrite

  rules, since the number of (implicit) quantifiers may vary. Here, we

  only have one bound variable in each comprehension, but in general

  there can be more. On the other hand, @{method auto} proves the

  above statement quickly, by breaking the equality into two parts and

  proving them separately. However, if e.g.\ @{term "P\<^isub>1"} is a

  complicated expression, the automated tools may get confused.

  Our goal is now to develop a small procedure that can compute (with proof) the

  composition of two relation comprehensions, which can be used to

  extend the simplifier.

*}

subsection {*A tactic *}

text {* Let's start with a step-by-step proof of the above statement *}

lemma "{(l\<^isub>1 x, r\<^isub>1 x) |x. P\<^isub>1 x} O {(l\<^isub>2

  x, r\<^isub>2 x) |x. P\<^isub>2 x}

  = {(l\<^isub>2 x, r\<^isub>1 y) |x y. r\<^isub>2 x = l\<^isub>1 y \<and> P\<^isub>2 x \<and> P\<^isub>1 y}"

apply (rule set_ext)

apply (rule iffI)

 apply (erule rel_compE)  -- {* @{text "\<subseteq>"} *}

 apply (erule CollectE)     -- {* eliminate @{text "Collect"}, @{text "\<exists>"}, @{text "\<and>"}, and pairs *}

 apply (erule CollectE)

 apply (erule exE)

 apply (erule exE)

 apply (erule conjE)

 apply (erule conjE)