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      <h2>Isabelle's Logics</h2>
      <p>Isabelle can be viewed from two main
      perspectives. On the one hand it may serve as a generic framework for rapid
      prototyping of deductive systems. On the other hand, major existing logics
      like <a href="#isabelle_hol"><em>Isabelle/HOL</em></a>
      provide a theorem proving environment
      ready to use for sizable applications.</p>    
      <p>The Isabelle distribution includes a large body of
      object logics and other examples (see the <a href=
      "library/index.html">Isabelle theory library</a>).</p>
        <dt id="isabelle_hol"><a href="library/HOL/index.html">Isabelle/HOL</a></dt>
        <dd>is a version of classical higher-order logic resembling that of the
        <a href="">HOL System</a>. The
        main libraries of the HOL 4 System are now <a href=
        "library/HOL/HOL-Complex/HOL4/index.html">available in Isabelle</a>.</dd>
        <dt><a href=
        <dd>adds Scott's Logic for Computable Functions (domain theory) to
        <dt><a href="library/FOL/index.html">Isabelle/FOL</a></dt>
        <dd>provides basic classical and intuitionistic first-order logic. It is
        <dt><a href="library/ZF/index.html">Isabelle/ZF</a></dt>
            <dd>offers a formulation of Zermelo-Fraenkel set theory on top of FOL.</dd>

      <p><em>Isabelle/HOL</em> is currently the best developed object logic, including an
      extensive library of (concrete) mathematics, and various packages for
      advanced definitional concepts like (co-)inductive sets and types,
      well-founded recursion etc. The distribution also includes some large
      applications, for example correctness proofs of cryptographic protocols
      (<a href="library/HOL/Auth/index.html">HOL/Auth</a>) or communication
      protocols (<a href="library/HOLCF/IOA/index.html">HOLCF/IOA</a>).</p>
      <p><em>Isabelle/ZF</em> provides another starting point for applications, with a
      slightly less developed library. Its definitional packages are similar to
      those of Isabelle/HOL. Untyped ZF provides more advanced constructions for
      sets than simply-typed HOL.</p>
      <p>There are a few minor object logics that may serve as further examples:
      <a href="library/CTT/index.html">CTT</a> is an extensional version of
      Martin-L&ouml;f's Type Theory, <a href="library/Cube/index.html">Cube</a> is
      Barendregt's Lambda Cube. There are also some sequent calculus examples under
      <a href="library/Sequents/index.html">Sequents</a>, including modal and
      linear logics. Again see the <a href="library/index.html">Isabelle theory
      library</a> for other examples.</p>
      <h2>Defining Logics</h2>

      <p>Logics are not hard-wired into Isabelle, but
      formulated within Isabelle's meta logic: <em>Isabelle/Pure</em>.
      There are quite a lot of syntactic and deductive tools available in generic
      Isabelle. Thus defining new logics or extending existing ones basically works
      as follows:</p>
        <li>declare concrete syntax (via mixfix grammar and syntax macros)</li>
        <li>declare abstract syntax (as higher-order constants)</li>
        <li>declare inference rules (as meta-logical propositions)</li>
        <li>instantiate generic automatic proof tools (simplifier, classical
        tableau prover etc.)</li>
        <li>manually code special proof procedures (via tacticals or hand-written
      <p>The first three steps above are fully declarative and involve no ML
      programming at all. Thus one already gets a decent deductive environment
      based on primitive inferences (by employing the built-in mechanisms of
      <em>Isabelle/Pure</em>, in particular higher-order unification and resolution). For
      sizable applications some degree of automated reasoning is essential.
      Instantiating existing tools like the classical tableau prover involves only
      minimal ML-based setup. One may also write arbitrary proof procedures or even
      theory extension packages in ML, without breaking system soundness (Isabelle
      follows the well-known <em>LCF system approach</em> to achieve a secure
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