theory Preface
imports Base Main
begin
chapter {* Preface *}
text {*
The \emph{Isabelle} system essentially provides a generic
infrastructure for building deductive systems (programmed in
Standard ML), with a special focus on interactive theorem proving in
higher-order logics. Many years ago, even end-users would refer to
certain ML functions (goal commands, tactics, tacticals etc.) to
pursue their everyday theorem proving tasks.
In contrast \emph{Isar} provides an interpreted language environment
of its own, which has been specifically tailored for the needs of
theory and proof development. Compared to raw ML, the Isabelle/Isar
top-level provides a more robust and comfortable development
platform, with proper support for theory development graphs, managed
transactions with unlimited undo etc.
In its pioneering times, the Isabelle/Isar version of the
\emph{Proof~General} user interface
\cite{proofgeneral,Aspinall:TACAS:2000} has contributed to the
success of for interactive theory and proof development in this
advanced theorem proving environment, even though it was somewhat
biased towards old-style proof scripts. The more recent
Isabelle/jEdit Prover IDE \cite{Wenzel:2012} emphasizes the
document-oriented approach of Isabelle/Isar again more explicitly.
\medskip Apart from the technical advances over bare-bones ML
programming, the main purpose of the Isar language is to provide a
conceptually different view on machine-checked proofs
\cite{Wenzel:1999:TPHOL,Wenzel-PhD}. \emph{Isar} stands for
\emph{Intelligible semi-automated reasoning}. Drawing from both the
traditions of informal mathematical proof texts and high-level
programming languages, Isar offers a versatile environment for
structured formal proof documents. Thus properly written Isar
proofs become accessible to a broader audience than unstructured
tactic scripts (which typically only provide operational information
for the machine). Writing human-readable proof texts certainly
requires some additional efforts by the writer to achieve a good
presentation, both of formal and informal parts of the text. On the
other hand, human-readable formal texts gain some value in their own
right, independently of the mechanic proof-checking process.
Despite its grand design of structured proof texts, Isar is able to
assimilate the old tactical style as an ``improper'' sub-language.
This provides an easy upgrade path for existing tactic scripts, as
well as some means for interactive experimentation and debugging of
structured proofs. Isabelle/Isar supports a broad range of proof
styles, both readable and unreadable ones.
\medskip The generic Isabelle/Isar framework (see
\chref{ch:isar-framework}) works reasonably well for any Isabelle
object-logic that conforms to the natural deduction view of the
Isabelle/Pure framework. Specific language elements introduced by
Isabelle/HOL are described in \partref{part:hol}. Although the main
language elements are already provided by the Isabelle/Pure
framework, examples given in the generic parts will usually refer to
Isabelle/HOL.
\medskip Isar commands may be either \emph{proper} document
constructors, or \emph{improper commands}. Some proof methods and
attributes introduced later are classified as improper as well.
Improper Isar language elements, which are marked by ``@{text
"\<^sup>*"}'' in the subsequent chapters; they are often helpful
when developing proof documents, but their use is discouraged for
the final human-readable outcome. Typical examples are diagnostic
commands that print terms or theorems according to the current
context; other commands emulate old-style tactical theorem proving.
*}
end