(* 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:
"!!env. lookup env xs = None ==> lookup env (xs @ ys) = None"
(is "PROP ?P xs")
proof (induct xs)
fix env :: "('a, 'b, 'c) env"
{
assume "lookup env [] = None"
hence False by simp
thus "lookup env ([] @ ys) = None" ..
next
fix x xs
assume hyp: "PROP ?P xs"
assume asm: "lookup env (x # xs) = None"
show "lookup env ((x # xs) @ ys) = None"
proof (cases env)
case Val
thus ?thesis by simp
next
fix b es assume env: "env = Env b es"
show ?thesis
proof (cases "es x")
assume "es x = None"
with env show ?thesis by simp
next
fix e assume es: "es x = Some e"
show ?thesis
proof (cases "lookup e xs")
case None
hence "lookup e (xs @ ys) = None" by (rule hyp)
with env es show ?thesis by simp
next
case Some
with asm env es have False by simp
thus ?thesis ..
qed
qed
qed
}
qed
theorem lookup_append_some:
"!!env e. lookup env xs = Some e ==> lookup env (xs @ ys) = lookup e ys"
(is "PROP ?P xs")
proof (induct xs)
fix env e :: "('a, 'b, 'c) env"
{
assume "lookup env [] = Some e"
hence "env = e" by simp
thus "lookup env ([] @ ys) = lookup e ys" by simp
next
fix x xs
assume hyp: "PROP ?P xs"
assume asm: "lookup env (x # xs) = Some e"
show "lookup env ((x # xs) @ ys) = lookup e ys"
proof (cases env)
fix a assume "env = Val a"
with asm have False by simp
thus ?thesis ..
next
fix b es assume env: "env = Env b es"
show ?thesis
proof (cases "es x")
assume "es x = None"
with asm env have False by simp
thus ?thesis ..
next
fix e' assume es: "es x = Some e'"
show ?thesis
proof (cases "lookup e' xs")
case None
with asm env es have False by simp
thus ?thesis ..
next
case Some
with asm env es have "lookup e' xs = Some e"
by simp
hence "lookup e' (xs @ ys) = lookup e ys" by (rule hyp)
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:
"lookup env (xs @ ys) = Some e ==> \<exists>e. lookup env xs = Some e"
proof -
assume "lookup env (xs @ ys) = Some e"
hence "lookup env (xs @ ys) \<noteq> None" by simp
hence "lookup env xs \<noteq> None"
by (rule contrapos_nn) (simp only: lookup_append_none)
thus ?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: "!!env e.
lookup env (xs @ y # ys) = Some e ==>
\<exists>b' es' env'.
lookup env xs = Some (Env b' es') \<and>
es' y = Some env' \<and>
lookup env' ys = Some e"
(is "PROP ?P xs" is "!!env e. ?A env e xs ==> ?C env e xs")
proof (induct xs)
fix env e let ?A = "?A env e" and ?C = "?C env e"
{
assume "?A []"
hence "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 "?C []" by blast
next
fix x xs
assume hyp: "PROP ?P xs"
assume "?A (x # xs)"
then 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 hyp [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 "?C (x # xs)" 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:
"!!env e. lookup env xs = Some e ==>
lookup (update xs (Some env') env) xs = Some env'"
(is "PROP ?P xs")
proof (induct xs)
fix env e :: "('a, 'b, 'c) env"
{
assume "lookup env [] = Some e"
hence "env = e" by simp
thus "lookup (update [] (Some env') env) [] = Some env'"
by simp
next
fix x xs
assume hyp: "PROP ?P xs"
assume asm: "lookup env (x # xs) = Some e"
show "lookup (update (x # xs) (Some env') env) (x # xs) = Some env'"
proof (cases env)
fix a assume "env = Val a"
with asm have False by simp
thus ?thesis ..
next
fix b es assume env: "env = Env b es"
show ?thesis
proof (cases "es x")
assume "es x = None"
with asm env have False by simp
thus ?thesis ..
next
fix e' assume es: "es x = Some e'"
show ?thesis
proof (cases xs)
assume "xs = []"
with env show ?thesis by simp
next
fix x' xs' assume xs: "xs = x' # xs'"
from asm env es have "lookup e' xs = Some e" by simp
hence "lookup (update xs (Some env') e') xs = Some env'" by (rule hyp)
with env es xs 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:
"!!env. lookup env xs = None ==> update (xs @ y # ys) opt env = env"
(is "PROP ?P xs")
proof (induct xs)
fix env :: "('a, 'b, 'c) env"
{
assume "lookup env [] = None"
hence False by simp
thus "update ([] @ y # ys) opt env = env" ..
next
fix x xs
assume hyp: "PROP ?P xs"
assume asm: "lookup env (x # xs) = None"
show "update ((x # xs) @ y # ys) opt env = env"
proof (cases env)
fix a assume "env = Val a"
thus ?thesis by simp
next
fix b es assume env: "env = Env b es"
show ?thesis
proof (cases "es x")
assume es: "es x = None"
show ?thesis
by (cases xs) (simp_all add: es env fun_upd_idem_iff)
next
fix e assume es: "es x = Some e"
show ?thesis
proof (cases xs)
assume "xs = []"
with asm env es have False by simp
thus ?thesis ..
next
fix x' xs' assume xs: "xs = x' # xs'"
from asm env es have "lookup e xs = None" by simp
hence "update (xs @ y # ys) opt e = e" by (rule hyp)
with env es xs show "update ((x # xs) @ y # ys) opt env = env"
by (simp add: fun_upd_idem_iff)
qed
qed
qed
}
qed
theorem update_append_some:
"!!env e. lookup env xs = Some e ==>
lookup (update (xs @ y # ys) opt env) xs = Some (update (y # ys) opt e)"
(is "PROP ?P xs")
proof (induct xs)
fix env e :: "('a, 'b, 'c) env"
{
assume "lookup env [] = Some e"
hence "env = e" by simp
thus "lookup (update ([] @ y # ys) opt env) [] = Some (update (y # ys) opt e)"
by simp
next
fix x xs
assume hyp: "PROP ?P xs"
assume 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)
fix a assume "env = Val a"
with asm have False by simp
thus ?thesis ..
next
fix b es assume env: "env = Env b es"
show ?thesis
proof (cases "es x")
assume "es x = None"
with asm env have False by simp
thus ?thesis ..
next
fix e' assume es: "es x = Some e'"
show ?thesis
proof (cases xs)
assume xs: "xs = []"
from asm env es xs have "e = e'" by simp
with env es xs show ?thesis by simp
next
fix x' xs' assume xs: "xs = x' # xs'"
from asm env es have "lookup e' xs = Some e" by simp
hence "lookup (update (xs @ y # ys) opt e') xs =
Some (update (y # ys) opt e)" by (rule hyp)
with env es xs 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:
"!!env. y \<noteq> (z::'c) ==> lookup (update (xs @ z # zs) opt env) (xs @ y # ys) =
lookup env (xs @ y # ys)"
(is "PROP ?P xs")
proof (induct xs)
fix env :: "('a, 'b, 'c) env"
assume neq: "y \<noteq> z"
{
show "lookup (update ([] @ z # zs) opt env) ([] @ y # ys) =
lookup env ([] @ y # ys)"
proof (cases env)
case Val
thus ?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
fix x xs
assume hyp: "PROP ?P xs"
show "lookup (update ((x # xs) @ z # zs) opt env) ((x # xs) @ y # ys) =
lookup env ((x # xs) @ y # ys)"
proof (cases env)
case Val
thus ?thesis by simp
next
fix y es assume env: "env = Env y es"
show ?thesis
proof (cases xs)
assume xs: "xs = []"
show ?thesis
proof (cases "es x")
case None
with env xs show ?thesis by simp
next
case Some
with hyp env xs and neq show ?thesis by simp
qed
next
fix x' xs' assume xs: "xs = x' # xs'"
show ?thesis
proof (cases "es x")
case None
with env xs show ?thesis by simp
next
case Some
with hyp env xs neq show ?thesis by simp
qed
qed
qed
}
qed
end