Theory Locales (Isabelle2005: October 2005)

# Theory Locales

Up to index of Isabelle/HOL/ex

theory Locales
imports Main
begin

    (*  Title:      HOL/ex/Locales.thy
        ID:         $Id: Locales.thy,v 1.19 2005/09/14 20:08:08 wenzelm Exp $
        Author:     Markus Wenzel, LMU München
    *)

    header {* Using locales in Isabelle/Isar -- outdated version! *}

    theory Locales imports Main begin

    text_raw {*
      \newcommand{\isasyminv}{\isasyminverse}
      \newcommand{\isasymIN}{\isatext{\isakeyword{in}}}
      \newcommand{\isasymINCLUDES}{\isatext{\isakeyword{includes}}}
    *}

    subsection {* Overview *}

    text {*
      Locales provide a mechanism for encapsulating local contexts.  The
      original version due to Florian Kammüller \cite{Kamm-et-al:1999}
      refers directly to Isabelle's meta-logic \cite{Paulson:1989}, which
      is minimal higher-order logic with connectives @{text "!!"}
      (universal quantification), @{text "==>"} (implication), and @{text
      "≡"} (equality).

      From this perspective, a locale is essentially a meta-level
      predicate, together with some infrastructure to manage the
      abstracted parameters (@{text "!!"}), assumptions (@{text "==>"}), and
      definitions for (@{text "≡"}) in a reasonable way during the proof
      process.  This simple predicate view also provides a solid
      semantical basis for our specification concepts to be developed
      later.

      \medskip The present version of locales for Isabelle/Isar builds on
      top of the rich infrastructure of proof contexts
      \cite{Wenzel:1999,Wenzel:2001:isar-ref,Wenzel:2001:Thesis}, which in
      turn is based on the same meta-logic.  Thus we achieve a tight
      integration with Isar proof texts, and a slightly more abstract view
      of the underlying logical concepts.  An Isar proof context
      encapsulates certain language elements that correspond to @{text
      "!!/==>/≡"} at the level of structure proof texts.  Moreover, there are
      extra-logical concepts like term abbreviations or local theorem
      attributes (declarations of simplification rules etc.) that are
      useful in applications (e.g.\ consider standard simplification rules
      declared in a group context).

      Locales also support concrete syntax, i.e.\ a localized version of
      the existing concept of mixfix annotations of Isabelle
      \cite{Paulson:1994:Isabelle}.  Furthermore, there is a separate
      concept of ``implicit structures'' that admits to refer to
      particular locale parameters in a casual manner (basically a
      simplified version of the idea of ``anti-quotations'', or
      generalized de-Bruijn indexes as demonstrated elsewhere
      \cite[\S13--14]{Wenzel:2001:Isar-examples}).

      Implicit structures work particular well together with extensible
      records in HOL \cite{Naraschewski-Wenzel:1998} (without the
      ``object-oriented'' features discussed there as well).  Thus we
      achieve a specification technique where record type schemes
      represent polymorphic signatures of mathematical structures, and
      actual locales describe the corresponding logical properties.
      Semantically speaking, such abstract mathematical structures are
      just predicates over record types.  Due to type inference of
      simply-typed records (which subsumes structural subtyping) we arrive
      at succinct specification texts --- ``signature morphisms''
      degenerate to implicit type-instantiations.  Additional eye-candy is
      provided by the separate concept of ``indexed concrete syntax'' used
      for record selectors, so we get close to informal mathematical
      notation.

      \medskip Operations for building up locale contexts from existing
      ones include \emph{merge} (disjoint union) and \emph{rename} (of
      term parameters only, types are inferred automatically).  Here we
      draw from existing traditions of algebraic specification languages.
      A structured specification corresponds to a directed acyclic graph
      of potentially renamed nodes (due to distributivity renames may
      pushed inside of merges).  The result is a ``flattened'' list of
      primitive context elements in canonical order (corresponding to
      left-to-right reading of merges, while suppressing duplicates).

      \medskip The present version of Isabelle/Isar locales still lacks
      some important specification concepts.

      \begin{itemize}

      \item Separate language elements for \emph{instantiation} of
      locales.

      Currently users may simulate this to some extend by having primitive
      Isabelle/Isar operations (@{text of} for substitution and @{text OF}
      for composition, \cite{Wenzel:2001:isar-ref}) act on the
      automatically exported results stemming from different contexts.

      \item Interpretation of locales (think of ``views'', ``functors''
      etc.).

      In principle one could directly work with functions over structures
      (extensible records), and predicates being derived from locale
      definitions.

      \end{itemize}

      \medskip Subsequently, we demonstrate some readily available
      concepts of Isabelle/Isar locales by some simple examples of
      abstract algebraic reasoning.
    *}

    subsection {* Local contexts as mathematical structures *}

    text {*
      The following definitions of @{text group_context} and @{text
      abelian_group_context} merely encapsulate local parameters (with
      private syntax) and assumptions; local definitions of derived
      concepts could be given, too, but are unused below.
    *}

    locale group_context =
      fixes prod :: "'a => 'a => 'a"    (infixl "·" 70)
        and inv :: "'a => 'a"    ("(_\<inv>)" [1000] 999)
        and one :: 'a    ("\<one>")
      assumes assoc: "(x · y) · z = x · (y · z)"
        and left_inv: "x\<inv> · x = \<one>"
        and left_one: "\<one> · x = x"

    locale abelian_group_context = group_context +
      assumes commute: "x · y = y · x"

    text {*
      \medskip We may now prove theorems within a local context, just by
      including a directive ``@{text "(\<IN> name)"}'' in the goal
      specification.  The final result will be stored within the named
      locale, still holding the context; a second copy is exported to the
      enclosing theory context (with qualified name).
    *}

    theorem (in group_context)
      right_inv: "x · x\<inv> = \<one>"
    proof -
      have "x · x\<inv> = \<one> · (x · x\<inv>)" by (simp only: left_one)
      also have "… = \<one> · x · x\<inv>" by (simp only: assoc)
      also have "… = (x\<inv>)\<inv> · x\<inv> · x · x\<inv>" by (simp only: left_inv)
      also have "… = (x\<inv>)\<inv> · (x\<inv> · x) · x\<inv>" by (simp only: assoc)
      also have "… = (x\<inv>)\<inv> · \<one> · x\<inv>" by (simp only: left_inv)
      also have "… = (x\<inv>)\<inv> · (\<one> · x\<inv>)" by (simp only: assoc)
      also have "… = (x\<inv>)\<inv> · x\<inv>" by (simp only: left_one)
      also have "… = \<one>" by (simp only: left_inv)
      finally show ?thesis .
    qed

    theorem (in group_context)
      right_one: "x · \<one> = x"
    proof -
      have "x · \<one> = x · (x\<inv> · x)" by (simp only: left_inv)
      also have "… = x · x\<inv> · x" by (simp only: assoc)
      also have "… = \<one> · x" by (simp only: right_inv)
      also have "… = x" by (simp only: left_one)
      finally show ?thesis .
    qed

    text {*
      Facts like @{text right_one} are available @{text group_context} as
      stated above.  The exported version looses the additional
      infrastructure of Isar proof contexts (syntax etc.) retaining only
      the pure logical content: @{thm [source] group_context.right_one}
      becomes @{thm group_context.right_one} (in Isabelle outermost @{text
      !!} quantification is replaced by schematic variables).

      \medskip Apart from a named locale we may also refer to further
      context elements (parameters, assumptions, etc.) in an ad-hoc
      fashion, just for this particular statement.  In the result (local
      or global), any additional elements are discharged as usual.
    *}

    theorem (in group_context)
      assumes eq: "e · x = x"
      shows one_equality: "\<one> = e"
    proof -
      have "\<one> = x · x\<inv>" by (simp only: right_inv)
      also have "… = (e · x) · x\<inv>" by (simp only: eq)
      also have "… = e · (x · x\<inv>)" by (simp only: assoc)
      also have "… = e · \<one>" by (simp only: right_inv)
      also have "… = e" by (simp only: right_one)
      finally show ?thesis .
    qed

    theorem (in group_context)
      assumes eq: "x' · x = \<one>"
      shows inv_equality: "x\<inv> = x'"
    proof -
      have "x\<inv> = \<one> · x\<inv>" by (simp only: left_one)
      also have "… = (x' · x) · x\<inv>" by (simp only: eq)
      also have "… = x' · (x · x\<inv>)" by (simp only: assoc)
      also have "… = x' · \<one>" by (simp only: right_inv)
      also have "… = x'" by (simp only: right_one)
      finally show ?thesis .
    qed

    theorem (in group_context)
      inv_prod: "(x · y)\<inv> = y\<inv> · x\<inv>"
    proof (rule inv_equality)
      show "(y\<inv> · x\<inv>) · (x · y) = \<one>"
      proof -
        have "(y\<inv> · x\<inv>) · (x · y) = (y\<inv> · (x\<inv> · x)) · y" by (simp only: assoc)
        also have "… = (y\<inv> · \<one>) · y" by (simp only: left_inv)
        also have "… = y\<inv> · y" by (simp only: right_one)
        also have "… = \<one>" by (simp only: left_inv)
        finally show ?thesis .
      qed
    qed

    text {*
      \medskip Established results are automatically propagated through
      the hierarchy of locales.  Below we establish a trivial fact in
      commutative groups, while referring both to theorems of @{text
      group} and the additional assumption of @{text abelian_group}.
    *}

    theorem (in abelian_group_context)
      inv_prod': "(x · y)\<inv> = x\<inv> · y\<inv>"
    proof -
      have "(x · y)\<inv> = y\<inv> · x\<inv>" by (rule inv_prod)
      also have "… = x\<inv> · y\<inv>" by (rule commute)
      finally show ?thesis .
    qed

    text {*
      We see that the initial import of @{text group} within the
      definition of @{text abelian_group} is actually evaluated
      dynamically.  Thus any results in @{text group} are made available
      to the derived context of @{text abelian_group} as well.  Note that
      the alternative context element @{text \<INCLUDES>} would import
      existing locales in a static fashion, without participating in
      further facts emerging later on.

      \medskip Some more properties of inversion in general group theory
      follow.
    *}

    theorem (in group_context)
      inv_inv: "(x\<inv>)\<inv> = x"
    proof (rule inv_equality)
      show "x · x\<inv> = \<one>" by (simp only: right_inv)
    qed

    theorem (in group_context)
      assumes eq: "x\<inv> = y\<inv>"
      shows inv_inject: "x = y"
    proof -
      have "x = x · \<one>" by (simp only: right_one)
      also have "… = x · (y\<inv> · y)" by (simp only: left_inv)
      also have "… = x · (x\<inv> · y)" by (simp only: eq)
      also have "… = (x · x\<inv>) · y" by (simp only: assoc)
      also have "… = \<one> · y" by (simp only: right_inv)
      also have "… = y" by (simp only: left_one)
      finally show ?thesis .
    qed

    text {*
      \bigskip We see that this representation of structures as local
      contexts is rather light-weight and convenient to use for abstract
      reasoning.  Here the ``components'' (the group operations) have been
      exhibited directly as context parameters; logically this corresponds
      to a curried predicate definition:

      @{thm [display] group_context_def [no_vars]}

      The corresponding introduction rule is as follows:

      @{thm [display, mode = no_brackets] group_context.intro [no_vars]}

      Occasionally, this ``externalized'' version of the informal idea of
      classes of tuple structures may cause some inconveniences,
      especially in meta-theoretical studies (involving functors from
      groups to groups, for example).

      Another minor drawback of the naive approach above is that concrete
      syntax will get lost on any kind of operation on the locale itself
      (such as renaming, copying, or instantiation).  Whenever the
      particular terminology of local parameters is affected the
      associated syntax would have to be changed as well, which is hard to
      achieve formally.
    *}

    subsection {* Explicit structures referenced implicitly *}

    text {*
      We introduce the same hierarchy of basic group structures as above,
      this time using extensible record types for the signature part,
      together with concrete syntax for selector functions.
    *}

    record 'a semigroup =
      prod :: "'a => 'a => 'a"    (infixl "·\<index>" 70)

    record 'a group = "'a semigroup" +
      inv :: "'a => 'a"    ("(_\<inv>\<index>)" [1000] 999)
      one :: 'a    ("\<one>\<index>")

    text {*
      The mixfix annotations above include a special ``structure index
      indicator'' @{text \<index>} that makes grammar productions dependent on
      certain parameters that have been declared as ``structure'' in a
      locale context later on.  Thus we achieve casual notation as
      encountered in informal mathematics, e.g.\ @{text "x · y"} for
      @{text "prod G x y"}.

      \medskip The following locale definitions introduce operate on a
      single parameter declared as ``\isakeyword{structure}''.  Type
      inference takes care to fill in the appropriate record type schemes
      internally.
    *}

    locale semigroup =
      fixes S    (structure)
      assumes assoc: "(x · y) · z = x · (y · z)"

    locale group = semigroup G +
      assumes left_inv: "x\<inv> · x = \<one>"
        and left_one: "\<one> · x = x"

    declare semigroup.intro [intro?]
      group.intro [intro?] group_axioms.intro [intro?]

    text {*
      Note that we prefer to call the @{text group} record structure
      @{text G} rather than @{text S} inherited from @{text semigroup}.
      This does not affect our concrete syntax, which is only dependent on
      the \emph{positional} arrangements of currently active structures
      (actually only one above), rather than names.  In fact, these
      parameter names rarely occur in the term language at all (due to the
      ``indexed syntax'' facility of Isabelle).  On the other hand, names
      of locale facts will get qualified accordingly, e.g. @{text S.assoc}
      versus @{text G.assoc}.

      \medskip We may now proceed to prove results within @{text group}
      just as before for @{text group}.  The subsequent proof texts are
      exactly the same as despite the more advanced internal arrangement.
    *}

    theorem (in group)
      right_inv: "x · x\<inv> = \<one>"
    proof -
      have "x · x\<inv> = \<one> · (x · x\<inv>)" by (simp only: left_one)
      also have "… = \<one> · x · x\<inv>" by (simp only: assoc)
      also have "… = (x\<inv>)\<inv> · x\<inv> · x · x\<inv>" by (simp only: left_inv)
      also have "… = (x\<inv>)\<inv> · (x\<inv> · x) · x\<inv>" by (simp only: assoc)
      also have "… = (x\<inv>)\<inv> · \<one> · x\<inv>" by (simp only: left_inv)
      also have "… = (x\<inv>)\<inv> · (\<one> · x\<inv>)" by (simp only: assoc)
      also have "… = (x\<inv>)\<inv> · x\<inv>" by (simp only: left_one)
      also have "… = \<one>" by (simp only: left_inv)
      finally show ?thesis .
    qed

    theorem (in group)
      right_one: "x · \<one> = x"
    proof -
      have "x · \<one> = x · (x\<inv> · x)" by (simp only: left_inv)
      also have "… = x · x\<inv> · x" by (simp only: assoc)
      also have "… = \<one> · x" by (simp only: right_inv)
      also have "… = x" by (simp only: left_one)
      finally show ?thesis .
    qed

    text {*
      \medskip Several implicit structures may be active at the same time.
      The concrete syntax facility for locales actually maintains indexed
      structures that may be references implicitly --- via mixfix
      annotations that have been decorated by an ``index argument''
      (@{text \<index>}).

      The following synthetic example demonstrates how to refer to several
      structures of type @{text group} succinctly.  We work with two
      versions of the @{text group} locale above.
    *}

    lemma
      includes group G
      includes group H
      shows "x · y · \<one> = prod G (prod G x y) (one G)"
        and "x ·2 y ·2 \<one>2 = prod H (prod H x y) (one H)"
        and "x · \<one>2 = prod G x (one H)"
      by (rule refl)+

    text {*
      Note that the trivial statements above need to be given as a
      simultaneous goal in order to have type-inference make the implicit
      typing of structures @{text G} and @{text H} agree.
    *}

    subsection {* Simple meta-theory of structures *}

    text {*
      The packaging of the logical specification as a predicate and the
      syntactic structure as a record type provides a reasonable starting
      point for simple meta-theoretic studies of mathematical structures.
      This includes operations on structures (also known as ``functors''),
      and statements about such constructions.

      For example, the direct product of semigroups works as follows.
    *}

    constdefs
      semigroup_product :: "'a semigroup => 'b semigroup => ('a × 'b) semigroup"
      "semigroup_product S T ≡
        (|prod = λp q. (prod S (fst p) (fst q), prod T (snd p) (snd q))|)),"

    lemma semigroup_product [intro]:
      assumes S: "semigroup S"
        and T: "semigroup T"
      shows "semigroup (semigroup_product S T)"
    proof
      fix p q r :: "'a × 'b"
      have "prod S (prod S (fst p) (fst q)) (fst r) =
          prod S (fst p) (prod S (fst q) (fst r))"
        by (rule semigroup.assoc [OF S])
      moreover have "prod T (prod T (snd p) (snd q)) (snd r) =
          prod T (snd p) (prod T (snd q) (snd r))"
        by (rule semigroup.assoc [OF T])
      ultimately
      show "prod (semigroup_product S T) (prod (semigroup_product S T) p q) r =
          prod (semigroup_product S T) p (prod (semigroup_product S T) q r)"
        by (simp add: semigroup_product_def)
    qed

    text {*
      The above proof is fairly easy, but obscured by the lack of concrete
      syntax.  In fact, we didn't make use of the infrastructure of
      locales, apart from the raw predicate definition of @{text
      semigroup}.

      The alternative version below uses local context expressions to
      achieve a succinct proof body.  The resulting statement is exactly
      the same as before, even though its specification is a bit more
      complex.
    *}

    lemma
      includes semigroup S + semigroup T
      fixes U    (structure)
      defines "U ≡ semigroup_product S T"
      shows "semigroup U"
    proof
      fix p q r :: "'a × 'b"
      have "(fst p ·1 fst q) ·1 fst r = fst p ·1 (fst q ·1 fst r)"
        by (rule S.assoc)
      moreover have "(snd p ·2 snd q) ·2 snd r = snd p ·2 (snd q ·2 snd r)"
        by (rule T.assoc)
      ultimately show "(p ·3 q) ·3 r = p ·3 (q ·3 r)"
        by (simp add: U_def semigroup_product_def semigroup.defs)
    qed

    text {*
      Direct products of group structures may be defined in a similar
      manner, taking two further operations into account.  Subsequently,
      we use high-level record operations to convert between different
      signature types explicitly; see also
      \cite[\S8.3]{isabelle-hol-book}.
    *}

    constdefs
      group_product :: "'a group => 'b group => ('a × 'b) group"
      "group_product G H ≡
        semigroup.extend
          (semigroup_product (semigroup.truncate G) (semigroup.truncate H))
          (group.fields (λp. (inv G (fst p), inv H (snd p))) (one G, one H))"

    lemma group_product_aux:
      includes group G + group H
      fixes I    (structure)
      defines "I ≡ group_product G H"
      shows "group I"
    proof
      show "semigroup I"
      proof -
        let ?G' = "semigroup.truncate G" and ?H' = "semigroup.truncate H"
        have "prod (semigroup_product ?G' ?H') = prod I"
          by (simp add: I_def group_product_def group.defs
            semigroup_product_def semigroup.defs)
        moreover
        have "semigroup ?G'" and "semigroup ?H'"
          using prems by (simp_all add: semigroup_def semigroup.defs)
        then have "semigroup (semigroup_product ?G' ?H')" ..
        ultimately show ?thesis by (simp add: I_def semigroup_def)
      qed
      show "group_axioms I"
      proof
        fix p :: "'a × 'b"
        have "(fst p)\<inv>1 ·1 fst p = \<one>1"
          by (rule G.left_inv)
        moreover have "(snd p)\<inv>2 ·2 snd p = \<one>2"
          by (rule H.left_inv)
        ultimately show "p\<inv>3 ·3 p = \<one>3"
          by (simp add: I_def group_product_def group.defs
            semigroup_product_def semigroup.defs)
        have "\<one>1 ·1 fst p = fst p" by (rule G.left_one)
        moreover have "\<one>2 ·2 snd p = snd p" by (rule H.left_one)
        ultimately show "\<one>3 ·3 p = p"
          by (simp add: I_def group_product_def group.defs
            semigroup_product_def semigroup.defs)
      qed
    qed

    theorem group_product: "group G ==> group H ==> group (group_product G H)"
      by (rule group_product_aux) (assumption | rule group.axioms)+

    end

### Overview

### Local contexts as mathematical structures

theorem right_inv:

      group_context prod inv one ==> prod x (inv x) = one

theorem right_one:

      group_context prod inv one ==> prod x one = x

theorem one_equality:

      [| group_context prod inv one; prod e x = x |] ==> one = e

theorem inv_equality:

      [| group_context prod inv one; prod x' x = one |] ==> inv x = x'

theorem inv_prod:

      group_context prod inv one ==> inv (prod x y) = prod (inv y) (inv x)

theorem inv_prod':

      abelian_group_context prod inv one ==> inv (prod x y) = prod (inv x) (inv y)

theorem inv_inv:

      group_context prod inv one ==> inv (inv x) = x

theorem inv_inject:

      [| group_context prod inv one; inv x = inv y |] ==> x = y

### Explicit structures referenced implicitly

theorem right_inv:

      group G ==> x ·G group.inv G x = \<one>G

theorem right_one:

      group G ==> x ·G \<one>G = x

lemma

      [| group G; group H |] ==> x ·G y ·G \<one>G = x ·G y ·G \<one>G

and

      [| group G; group H |] ==> x ·H y ·H \<one>H = x ·H y ·H \<one>H

and

      [| group G; group H |] ==> x ·G \<one>H = x ·G \<one>H

### Simple meta-theory of structures

lemma semigroup_product:

      [| semigroup S; semigroup T |] ==> semigroup (semigroup_product S T)

lemma

      [| semigroup S; semigroup T |] ==> semigroup (semigroup_product S T)

lemma group_product_aux:

      [| group G; group H |] ==> group (group_product G H)

theorem group_product:

      [| group G; group H |] ==> group (group_product G H)