src/Doc/Prog_Prove/Basics.thy

+(*<*)
+theory Basics
+imports Main
+begin
+(*>*)
+text{*
+This chapter introduces HOL as a functional programming language and shows
+how to prove properties of functional programs by induction.
+
+\section{Basics}
+
+\subsection{Types, Terms and Formulas}
+\label{sec:TypesTermsForms}
+
+HOL is a typed logic whose type system resembles that of functional
+programming languages. Thus there are
+\begin{description}
+\item[base types,] 
+in particular @{typ bool}, the type of truth values,
+@{typ nat}, the type of natural numbers ($\mathbb{N}$), and \indexed{@{typ int}}{int},
+the type of mathematical integers ($\mathbb{Z}$).
+\item[type constructors,]
+ in particular @{text list}, the type of
+lists, and @{text set}, the type of sets. Type constructors are written
+postfix, i.e., after their arguments. For example,
+@{typ "nat list"} is the type of lists whose elements are natural numbers.
+\item[function types,]
+denoted by @{text"\<Rightarrow>"}.
+\item[type variables,]
+  denoted by @{typ 'a}, @{typ 'b} etc., just like in ML\@.
+\end{description}
+Note that @{typ"'a \<Rightarrow> 'b list"} means @{typ[source]"'a \<Rightarrow> ('b list)"},
+not @{typ"('a \<Rightarrow> 'b) list"}: postfix type constructors have precedence
+over @{text"\<Rightarrow>"}.
+
+\conceptidx{Terms}{term} are formed as in functional programming by
+applying functions to arguments. If @{text f} is a function of type
+@{text"\<tau>\<^sub>1 \<Rightarrow> \<tau>\<^sub>2"} and @{text t} is a term of type
+@{text"\<tau>\<^sub>1"} then @{term"f t"} is a term of type @{text"\<tau>\<^sub>2"}. We write @{text "t :: \<tau>"} to mean that term @{text t} has type @{text \<tau>}.
+
+\begin{warn}
+There are many predefined infix symbols like @{text "+"} and @{text"\<le>"}.
+The name of the corresponding binary function is @{term"op +"},
+not just @{text"+"}. That is, @{term"x + y"} is nice surface syntax
+(``syntactic sugar'') for \noquotes{@{term[source]"op + x y"}}.
+\end{warn}
+
+HOL also supports some basic constructs from functional programming:
+\begin{quote}
+@{text "(if b then t\<^sub>1 else t\<^sub>2)"}\\
+@{text "(let x = t in u)"}\\
+@{text "(case t of pat\<^sub>1 \<Rightarrow> t\<^sub>1 | \<dots> | pat\<^sub>n \<Rightarrow> t\<^sub>n)"}
+\end{quote}
+\begin{warn}
+The above three constructs must always be enclosed in parentheses
+if they occur inside other constructs.
+\end{warn}
+Terms may also contain @{text "\<lambda>"}-abstractions. For example,
+@{term "\<lambda>x. x"} is the identity function.
+
+\conceptidx{Formulas}{formula} are terms of type @{text bool}.
+There are the basic constants @{term True} and @{term False} and
+the usual logical connectives (in decreasing order of precedence):
+@{text"\<not>"}, @{text"\<and>"}, @{text"\<or>"}, @{text"\<longrightarrow>"}.
+
+\conceptidx{Equality}{equality} is available in the form of the infix function @{text "="}
+of type @{typ "'a \<Rightarrow> 'a \<Rightarrow> bool"}. It also works for formulas, where
+it means ``if and only if''.
+
+\conceptidx{Quantifiers}{quantifier} are written @{prop"\<forall>x. P"} and @{prop"\<exists>x. P"}.
+
+Isabelle automatically computes the type of each variable in a term. This is
+called \concept{type inference}.  Despite type inference, it is sometimes
+necessary to attach an explicit \concept{type constraint} (or \concept{type
+annotation}) to a variable or term.  The syntax is @{text "t :: \<tau>"} as in
+\mbox{\noquotes{@{prop[source] "m < (n::nat)"}}}. Type constraints may be
+needed to
+disambiguate terms involving overloaded functions such as @{text "+"}, @{text
+"*"} and @{text"\<le>"}.
+
+Finally there are the universal quantifier @{text"\<And>"}\index{$4@\isasymAnd} and the implication
+@{text"\<Longrightarrow>"}\index{$3@\isasymLongrightarrow}. They are part of the Isabelle framework, not the logic
+HOL. Logically, they agree with their HOL counterparts @{text"\<forall>"} and
+@{text"\<longrightarrow>"}, but operationally they behave differently. This will become
+clearer as we go along.
+\begin{warn}
+Right-arrows of all kinds always associate to the right. In particular,
+the formula
+@{text"A\<^sub>1 \<Longrightarrow> A\<^sub>2 \<Longrightarrow> A\<^sub>3"} means @{text "A\<^sub>1 \<Longrightarrow> (A\<^sub>2 \<Longrightarrow> A\<^sub>3)"}.
+The (Isabelle specific) notation \mbox{@{text"\<lbrakk> A\<^sub>1; \<dots>; A\<^sub>n \<rbrakk> \<Longrightarrow> A"}}
+is short for the iterated implication \mbox{@{text"A\<^sub>1 \<Longrightarrow> \<dots> \<Longrightarrow> A\<^sub>n \<Longrightarrow> A"}}.
+Sometimes we also employ inference rule notation:
+\inferrule{\mbox{@{text "A\<^sub>1"}}\\ \mbox{@{text "\<dots>"}}\\ \mbox{@{text "A\<^sub>n"}}}
+{\mbox{@{text A}}}
+\end{warn}
+
+
+\subsection{Theories}
+\label{sec:Basic:Theories}
+
+Roughly speaking, a \concept{theory} is a named collection of types,
+functions, and theorems, much like a module in a programming language.
+All the Isabelle text that you ever type needs to go into a theory.
+The general format of a theory @{text T} is
+\begin{quote}
+\indexed{\isacom{theory}}{theory} @{text T}\\
+\indexed{\isacom{imports}}{imports} @{text "T\<^sub>1 \<dots> T\<^sub>n"}\\
+\isacom{begin}\\
+\emph{definitions, theorems and proofs}\\
+\isacom{end}
+\end{quote}
+where @{text "T\<^sub>1 \<dots> T\<^sub>n"} are the names of existing
+theories that @{text T} is based on. The @{text "T\<^sub>i"} are the
+direct \conceptidx{parent theories}{parent theory} of @{text T}.
+Everything defined in the parent theories (and their parents, recursively) is
+automatically visible. Each theory @{text T} must
+reside in a \concept{theory file} named @{text "T.thy"}.
+
+\begin{warn}
+HOL contains a theory @{theory Main}\index{Main@@{theory Main}}, the union of all the basic
+predefined theories like arithmetic, lists, sets, etc.
+Unless you know what you are doing, always include @{text Main}
+as a direct or indirect parent of all your theories.
+\end{warn}
+
+In addition to the theories that come with the Isabelle/HOL distribution
+(see @{url "http://isabelle.in.tum.de/library/HOL/"})
+there is also the \emph{Archive of Formal Proofs}
+at @{url "http://afp.sourceforge.net"}, a growing collection of Isabelle theories
+that everybody can contribute to.
+
+\subsection{Quotation Marks}
+
+The textual definition of a theory follows a fixed syntax with keywords like
+\isacommand{begin} and \isacommand{datatype}.  Embedded in this syntax are
+the types and formulas of HOL.  To distinguish the two levels, everything
+HOL-specific (terms and types) must be enclosed in quotation marks:
+\texttt{"}\dots\texttt{"}. To lessen this burden, quotation marks around a
+single identifier can be dropped.  When Isabelle prints a syntax error
+message, it refers to the HOL syntax as the \concept{inner syntax} and the
+enclosing theory language as the \concept{outer syntax}.
+*}
+(*<*)
+end
+(*>*)