src/HOL/Library/Nested_Environment.thy

updated

(*  Title:      HOL/Library/Nested_Environment.thy
    ID:         $Id$
    Author:     Markus Wenzel, TU Muenchen
*)

header {* Nested environments *}

theory Nested_Environment
imports Main
begin

text {*
  Consider a partial function @{term [source] "e :: 'a => 'b option"};
  this may be understood as an \emph{environment} mapping indexes
  @{typ 'a} to optional entry values @{typ 'b} (cf.\ the basic theory
  @{text Map} of Isabelle/HOL).  This basic idea is easily generalized
  to that of a \emph{nested environment}, where entries may be either
  basic values or again proper environments.  Then each entry is
  accessed by a \emph{path}, i.e.\ a list of indexes leading to its
  position within the structure.
*}

datatype ('a, 'b, 'c) env =
    Val 'a
  | Env 'b  "'c => ('a, 'b, 'c) env option"

text {*
  \medskip In the type @{typ "('a, 'b, 'c) env"} the parameter @{typ
  'a} refers to basic values (occurring in terminal positions), type
  @{typ 'b} to values associated with proper (inner) environments, and
  type @{typ 'c} with the index type for branching.  Note that there
  is no restriction on any of these types.  In particular, arbitrary
  branching may yield rather large (transfinite) tree structures.
*}


subsection {* The lookup operation *}

text {*
  Lookup in nested environments works by following a given path of
  index elements, leading to an optional result (a terminal value or
  nested environment).  A \emph{defined position} within a nested
  environment is one where @{term lookup} at its path does not yield
  @{term None}.
*}

consts
  lookup :: "('a, 'b, 'c) env => 'c list => ('a, 'b, 'c) env option"
  lookup_option :: "('a, 'b, 'c) env option => 'c list => ('a, 'b, 'c) env option"

primrec (lookup)
  "lookup (Val a) xs = (if xs = [] then Some (Val a) else None)"
  "lookup (Env b es) xs =
    (case xs of
      [] => Some (Env b es)
    | y # ys => lookup_option (es y) ys)"
  "lookup_option None xs = None"
  "lookup_option (Some e) xs = lookup e xs"

hide const lookup_option

text {*
  \medskip The characteristic cases of @{term lookup} are expressed by
  the following equalities.
*}

theorem lookup_nil: "lookup e [] = Some e"
  by (cases e) simp_all

theorem lookup_val_cons: "lookup (Val a) (x # xs) = None"
  by simp

theorem lookup_env_cons:
  "lookup (Env b es) (x # xs) =
    (case es x of
      None => None
    | Some e => lookup e xs)"
  by (cases "es x") simp_all

lemmas lookup.simps [simp del]
  and lookup_simps [simp] = lookup_nil lookup_val_cons lookup_env_cons

theorem lookup_eq:
  "lookup env xs =
    (case xs of
      [] => Some env
    | x # xs =>
      (case env of
        Val a => None
      | Env b es =>
          (case es x of
            None => None
          | Some e => lookup e xs)))"
  by (simp split: list.split env.split)

text {*
  \medskip Displaced @{term lookup} operations, relative to a certain
  base path prefix, may be reduced as follows.  There are two cases,
  depending whether the environment actually extends far enough to
  follow the base path.
*}

theorem lookup_append_none:
  assumes "lookup env xs = None"
  shows "lookup env (xs @ ys) = None"
  using prems
proof (induct xs arbitrary: env)
  case Nil
  then have False by simp
  then show ?case ..
next
  case (Cons x xs)
  show ?case
  proof (cases env)
    case Val
    then show ?thesis by simp
  next
    case (Env b es)
    show ?thesis
    proof (cases "es x")
      case None
      with Env show ?thesis by simp
    next
      case (Some e)
      note es = `es x = Some e`
      show ?thesis
      proof (cases "lookup e xs")
        case None
        then have "lookup e (xs @ ys) = None" by (rule Cons.hyps)
        with Env Some show ?thesis by simp
      next
        case Some
        with Env es have False using Cons.prems by simp
        then show ?thesis ..
      qed
    qed
  qed
qed

theorem lookup_append_some:
  assumes "lookup env xs = Some e"
  shows "lookup env (xs @ ys) = lookup e ys"
  using prems
proof (induct xs arbitrary: env e)
  case Nil
  then have "env = e" by simp
  then show "lookup env ([] @ ys) = lookup e ys" by simp
next
  case (Cons x xs)
  note asm = `lookup env (x # xs) = Some e`
  show "lookup env ((x # xs) @ ys) = lookup e ys"
  proof (cases env)
    case (Val a)
    with asm have False by simp
    then show ?thesis ..
  next
    case (Env b es)
    show ?thesis
    proof (cases "es x")
      case None
      with asm Env have False by simp
      then show ?thesis ..
    next
      case (Some e')
      note es = `es x = Some e'`
      show ?thesis
      proof (cases "lookup e' xs")
        case None
        with asm Env es have False by simp
        then show ?thesis ..
      next
        case Some
        with asm Env es have "lookup e' xs = Some e"
          by simp
        then have "lookup e' (xs @ ys) = lookup e ys" by (rule Cons.hyps)
        with Env es show ?thesis by simp
      qed
    qed
  qed
qed

text {*
  \medskip Successful @{term lookup} deeper down an environment
  structure means we are able to peek further up as well.  Note that
  this is basically just the contrapositive statement of @{thm
  [source] lookup_append_none} above.
*}

theorem lookup_some_append:
  assumes "lookup env (xs @ ys) = Some e"
  shows "\<exists>e. lookup env xs = Some e"
proof -
  from prems have "lookup env (xs @ ys) \<noteq> None" by simp
  then have "lookup env xs \<noteq> None"
    by (rule contrapos_nn) (simp only: lookup_append_none)
  then show ?thesis by (simp)
qed

text {*
  The subsequent statement describes in more detail how a successful
  @{term lookup} with a non-empty path results in a certain situation
  at any upper position.
*}

theorem lookup_some_upper:
  assumes "lookup env (xs @ y # ys) = Some e"
  shows "\<exists>b' es' env'.
    lookup env xs = Some (Env b' es') \<and>
    es' y = Some env' \<and>
    lookup env' ys = Some e"
  using prems
proof (induct xs arbitrary: env e)
  case Nil
  from Nil.prems have "lookup env (y # ys) = Some e"
    by simp
  then obtain b' es' env' where
      env: "env = Env b' es'" and
      es': "es' y = Some env'" and
      look': "lookup env' ys = Some e"
    by (auto simp add: lookup_eq split: option.splits env.splits)
  from env have "lookup env [] = Some (Env b' es')" by simp
  with es' look' show ?case by blast
next
  case (Cons x xs)
  from Cons.prems
  obtain b' es' env' where
      env: "env = Env b' es'" and
      es': "es' x = Some env'" and
      look': "lookup env' (xs @ y # ys) = Some e"
    by (auto simp add: lookup_eq split: option.splits env.splits)
  from Cons.hyps [OF look'] obtain b'' es'' env'' where
      upper': "lookup env' xs = Some (Env b'' es'')" and
      es'': "es'' y = Some env''" and
      look'': "lookup env'' ys = Some e"
    by blast
  from env es' upper' have "lookup env (x # xs) = Some (Env b'' es'')"
    by simp
  with es'' look'' show ?case by blast
qed


subsection {* The update operation *}

text {*
  Update at a certain position in a nested environment may either
  delete an existing entry, or overwrite an existing one.  Note that
  update at undefined positions is simple absorbed, i.e.\ the
  environment is left unchanged.
*}

consts
  update :: "'c list => ('a, 'b, 'c) env option
    => ('a, 'b, 'c) env => ('a, 'b, 'c) env"
  update_option :: "'c list => ('a, 'b, 'c) env option
    => ('a, 'b, 'c) env option => ('a, 'b, 'c) env option"

primrec (update)
  "update xs opt (Val a) =
    (if xs = [] then (case opt of None => Val a | Some e => e)
    else Val a)"
  "update xs opt (Env b es) =
    (case xs of
      [] => (case opt of None => Env b es | Some e => e)
    | y # ys => Env b (es (y := update_option ys opt (es y))))"
  "update_option xs opt None =
    (if xs = [] then opt else None)"
  "update_option xs opt (Some e) =
    (if xs = [] then opt else Some (update xs opt e))"

hide const update_option

text {*
  \medskip The characteristic cases of @{term update} are expressed by
  the following equalities.
*}

theorem update_nil_none: "update [] None env = env"
  by (cases env) simp_all

theorem update_nil_some: "update [] (Some e) env = e"
  by (cases env) simp_all

theorem update_cons_val: "update (x # xs) opt (Val a) = Val a"
  by simp

theorem update_cons_nil_env:
    "update [x] opt (Env b es) = Env b (es (x := opt))"
  by (cases "es x") simp_all

theorem update_cons_cons_env:
  "update (x # y # ys) opt (Env b es) =
    Env b (es (x :=
      (case es x of
        None => None
      | Some e => Some (update (y # ys) opt e))))"
  by (cases "es x") simp_all

lemmas update.simps [simp del]
  and update_simps [simp] = update_nil_none update_nil_some
    update_cons_val update_cons_nil_env update_cons_cons_env

lemma update_eq:
  "update xs opt env =
    (case xs of
      [] =>
        (case opt of
          None => env
        | Some e => e)
    | x # xs =>
        (case env of
          Val a => Val a
        | Env b es =>
            (case xs of
              [] => Env b (es (x := opt))
            | y # ys =>
                Env b (es (x :=
                  (case es x of
                    None => None
                  | Some e => Some (update (y # ys) opt e)))))))"
  by (simp split: list.split env.split option.split)

text {*
  \medskip The most basic correspondence of @{term lookup} and @{term
  update} states that after @{term update} at a defined position,
  subsequent @{term lookup} operations would yield the new value.
*}

theorem lookup_update_some:
  assumes "lookup env xs = Some e"
  shows "lookup (update xs (Some env') env) xs = Some env'"
  using prems
proof (induct xs arbitrary: env e)
  case Nil
  then have "env = e" by simp
  then show ?case by simp
next
  case (Cons x xs)
  note hyp = Cons.hyps
    and asm = `lookup env (x # xs) = Some e`
  show ?case
  proof (cases env)
    case (Val a)
    with asm have False by simp
    then show ?thesis ..
  next
    case (Env b es)
    show ?thesis
    proof (cases "es x")
      case None
      with asm Env have False by simp
      then show ?thesis ..
    next
      case (Some e')
      note es = `es x = Some e'`
      show ?thesis
      proof (cases xs)
        case Nil
        with Env show ?thesis by simp
      next
        case (Cons x' xs')
        from asm Env es have "lookup e' xs = Some e" by simp
        then have "lookup (update xs (Some env') e') xs = Some env'" by (rule hyp)
        with Env es Cons show ?thesis by simp
      qed
    qed
  qed
qed

text {*
  \medskip The properties of displaced @{term update} operations are
  analogous to those of @{term lookup} above.  There are two cases:
  below an undefined position @{term update} is absorbed altogether,
  and below a defined positions @{term update} affects subsequent
  @{term lookup} operations in the obvious way.
*}

theorem update_append_none:
  assumes "lookup env xs = None"
  shows "update (xs @ y # ys) opt env = env"
  using prems
proof (induct xs arbitrary: env)
  case Nil
  then have False by simp
  then show ?case ..
next
  case (Cons x xs)
  note hyp = Cons.hyps
    and asm = `lookup env (x # xs) = None`
  show "update ((x # xs) @ y # ys) opt env = env"
  proof (cases env)
    case (Val a)
    then show ?thesis by simp
  next
    case (Env b es)
    show ?thesis
    proof (cases "es x")
      case None
      note es = `es x = None`
      show ?thesis
        by (cases xs) (simp_all add: es Env fun_upd_idem_iff)
    next
      case (Some e)
      note es = `es x = Some e`
      show ?thesis
      proof (cases xs)
        case Nil
        with asm Env Some have False by simp
        then show ?thesis ..
      next
        case (Cons x' xs')
        from asm Env es have "lookup e xs = None" by simp
        then have "update (xs @ y # ys) opt e = e" by (rule hyp)
        with Env es Cons show "update ((x # xs) @ y # ys) opt env = env"
          by (simp add: fun_upd_idem_iff)
      qed
    qed
  qed
qed

theorem update_append_some:
  assumes "lookup env xs = Some e"
  shows "lookup (update (xs @ y # ys) opt env) xs = Some (update (y # ys) opt e)"
  using prems
proof (induct xs arbitrary: env e)
  case Nil
  then have "env = e" by simp
  then show ?case by simp
next
  case (Cons x xs)
  note hyp = Cons.hyps
    and asm = `lookup env (x # xs) = Some e`
  show "lookup (update ((x # xs) @ y # ys) opt env) (x # xs) =
      Some (update (y # ys) opt e)"
  proof (cases env)
    case (Val a)
    with asm have False by simp
    then show ?thesis ..
  next
    case (Env b es)
    show ?thesis
    proof (cases "es x")
      case None
      with asm Env have False by simp
      then show ?thesis ..
    next
      case (Some e')
      note es = `es x = Some e'`
      show ?thesis
      proof (cases xs)
        case Nil
        with asm Env es have "e = e'" by simp
        with Env es Nil show ?thesis by simp
      next
        case (Cons x' xs')
        from asm Env es have "lookup e' xs = Some e" by simp
        then have "lookup (update (xs @ y # ys) opt e') xs =
          Some (update (y # ys) opt e)" by (rule hyp)
        with Env es Cons show ?thesis by simp
      qed
    qed
  qed
qed

text {*
  \medskip Apparently, @{term update} does not affect the result of
  subsequent @{term lookup} operations at independent positions, i.e.\
  in case that the paths for @{term update} and @{term lookup} fork at
  a certain point.
*}

theorem lookup_update_other:
  assumes neq: "y \<noteq> (z::'c)"
  shows "lookup (update (xs @ z # zs) opt env) (xs @ y # ys) =
    lookup env (xs @ y # ys)"
proof (induct xs arbitrary: env)
  case Nil
  show ?case
  proof (cases env)
    case Val
    then show ?thesis by simp
  next
    case Env
    show ?thesis
    proof (cases zs)
      case Nil
      with neq Env show ?thesis by simp
    next
      case Cons
      with neq Env show ?thesis by simp
    qed
  qed
next
  case (Cons x xs)
  note hyp = Cons.hyps
  show ?case
  proof (cases env)
    case Val
    then show ?thesis by simp
  next
    case (Env y es)
    show ?thesis
    proof (cases xs)
      case Nil
      show ?thesis
      proof (cases "es x")
        case None
        with Env Nil show ?thesis by simp
      next
        case Some
        with neq hyp and Env Nil show ?thesis by simp
      qed
    next
      case (Cons x' xs')
      show ?thesis
      proof (cases "es x")
        case None
        with Env Cons show ?thesis by simp
      next
        case Some
        with neq hyp and Env Cons show ?thesis by simp
      qed
    qed
  qed
qed

end