(* Title: HOL/Lambda/StrongNorm.thy
ID: $Id$
Author: Stefan Berghofer
Copyright 2000 TU Muenchen
*)
header {* Strong normalization for simply-typed lambda calculus *}
theory StrongNorm = Type + InductTermi:
text {*
Formalization by Stefan Berghofer. Partly based on a paper proof by
Felix Joachimski and Ralph Matthes \cite{Matthes-Joachimski-AML}.
*}
subsection {* Properties of @{text IT} *}
lemma lift_IT [intro!]: "t \<in> IT \<Longrightarrow> (\<And>i. lift t i \<in> IT)"
apply (induct set: IT)
apply (simp (no_asm))
apply (rule conjI)
apply
(rule impI,
rule IT.Var,
erule lists.induct,
simp (no_asm),
rule lists.Nil,
simp (no_asm),
erule IntE,
rule lists.Cons,
blast,
assumption)+
apply auto
done
lemma lifts_IT: "ts \<in> lists IT \<Longrightarrow> map (\<lambda>t. lift t 0) ts \<in> lists IT"
by (induct ts) auto
lemma subst_Var_IT: "r \<in> IT \<Longrightarrow> (\<And>i j. r[Var i/j] \<in> IT)"
apply (induct set: IT)
txt {* Case @{term Var}: *}
apply (simp (no_asm) add: subst_Var)
apply
((rule conjI impI)+,
rule IT.Var,
erule lists.induct,
simp (no_asm),
rule lists.Nil,
simp (no_asm),
erule IntE,
erule CollectE,
rule lists.Cons,
fast,
assumption)+
txt {* Case @{term Lambda}: *}
apply atomize
apply simp
apply (rule IT.Lambda)
apply fast
txt {* Case @{term Beta}: *}
apply atomize
apply (simp (no_asm_use) add: subst_subst [symmetric])
apply (rule IT.Beta)
apply auto
done
lemma Var_IT: "Var n \<in> IT"
apply (subgoal_tac "Var n \<degree>\<degree> [] \<in> IT")
apply simp
apply (rule IT.Var)
apply (rule lists.Nil)
done
lemma app_Var_IT: "t \<in> IT \<Longrightarrow> t \<degree> Var i \<in> IT"
apply (induct set: IT)
apply (subst app_last)
apply (rule IT.Var)
apply simp
apply (rule lists.Cons)
apply (rule Var_IT)
apply (rule lists.Nil)
apply (rule IT.Beta [where ?ss = "[]", unfolded foldl_Nil [THEN eq_reflection]])
apply (erule subst_Var_IT)
apply (rule Var_IT)
apply (subst app_last)
apply (rule IT.Beta)
apply (subst app_last [symmetric])
apply assumption
apply assumption
done
subsection {* Well-typed substitution preserves termination *}
lemma subst_type_IT:
"\<And>t e T u i. t \<in> IT \<Longrightarrow> e\<langle>i:U\<rangle> \<turnstile> t : T \<Longrightarrow>
u \<in> IT \<Longrightarrow> e \<turnstile> u : U \<Longrightarrow> t[u/i] \<in> IT"
(is "PROP ?P U" is "\<And>t e T u i. _ \<Longrightarrow> PROP ?Q t e T u i U")
proof (induct U)
fix T t
assume MI1: "\<And>T1 T2. T = T1 \<Rightarrow> T2 \<Longrightarrow> PROP ?P T1"
assume MI2: "\<And>T1 T2. T = T1 \<Rightarrow> T2 \<Longrightarrow> PROP ?P T2"
assume "t \<in> IT"
thus "\<And>e T' u i. PROP ?Q t e T' u i T"
proof induct
fix e T' u i
assume uIT: "u \<in> IT"
assume uT: "e \<turnstile> u : T"
{
case (Var n rs e_ T'_ u_ i_)
assume nT: "e\<langle>i:T\<rangle> \<turnstile> Var n \<degree>\<degree> rs : T'"
let ?ty = "{t. \<exists>T'. e\<langle>i:T\<rangle> \<turnstile> t : T'}"
let ?R = "\<lambda>t. \<forall>e T' u i.
e\<langle>i:T\<rangle> \<turnstile> t : T' \<longrightarrow> u \<in> IT \<longrightarrow> e \<turnstile> u : T \<longrightarrow> t[u/i] \<in> IT"
show "(Var n \<degree>\<degree> rs)[u/i] \<in> IT"
proof (cases "n = i")
case True
show ?thesis
proof (cases rs)
case Nil
with uIT True show ?thesis by simp
next
case (Cons a as)
with nT have "e\<langle>i:T\<rangle> \<turnstile> Var n \<degree> a \<degree>\<degree> as : T'" by simp
then obtain Ts
where headT: "e\<langle>i:T\<rangle> \<turnstile> Var n \<degree> a : Ts \<Rrightarrow> T'"
and argsT: "e\<langle>i:T\<rangle> \<tturnstile> as : Ts"
by (rule list_app_typeE)
from headT obtain T''
where varT: "e\<langle>i:T\<rangle> \<turnstile> Var n : T'' \<Rightarrow> Ts \<Rrightarrow> T'"
and argT: "e\<langle>i:T\<rangle> \<turnstile> a : T''"
by cases simp_all
from varT True have T: "T = T'' \<Rightarrow> Ts \<Rrightarrow> T'"
by cases auto
with uT have uT': "e \<turnstile> u : T'' \<Rightarrow> Ts \<Rrightarrow> T'" by simp
from T have "(Var 0 \<degree>\<degree> map (\<lambda>t. lift t 0)
(map (\<lambda>t. t[u/i]) as))[(u \<degree> a[u/i])/0] \<in> IT"
proof (rule MI2)
from T have "(lift u 0 \<degree> Var 0)[a[u/i]/0] \<in> IT"
proof (rule MI1)
have "lift u 0 \<in> IT" by (rule lift_IT)
thus "lift u 0 \<degree> Var 0 \<in> IT" by (rule app_Var_IT)
show "e\<langle>0:T''\<rangle> \<turnstile> lift u 0 \<degree> Var 0 : Ts \<Rrightarrow> T'"
proof (rule typing.App)
show "e\<langle>0:T''\<rangle> \<turnstile> lift u 0 : T'' \<Rightarrow> Ts \<Rrightarrow> T'"
by (rule lift_type) (rule uT')
show "e\<langle>0:T''\<rangle> \<turnstile> Var 0 : T''"
by (rule typing.Var) simp
qed
from Var have "?R a" by cases (simp_all add: Cons)
with argT uIT uT show "a[u/i] \<in> IT" by simp
from argT uT show "e \<turnstile> a[u/i] : T''"
by (rule subst_lemma) simp
qed
thus "u \<degree> a[u/i] \<in> IT" by simp
from Var have "as \<in> lists {t. ?R t}"
by cases (simp_all add: Cons)
moreover from argsT have "as \<in> lists ?ty"
by (rule lists_typings)
ultimately have "as \<in> lists ({t. ?R t} \<inter> ?ty)"
by (rule lists_IntI)
hence "map (\<lambda>t. lift t 0) (map (\<lambda>t. t[u/i]) as) \<in> lists IT"
(is "(?ls as) \<in> _")
proof induct
case Nil
show ?case by fastsimp
next
case (Cons b bs)
hence I: "?R b" by simp
from Cons obtain U where "e\<langle>i:T\<rangle> \<turnstile> b : U" by fast
with uT uIT I have "b[u/i] \<in> IT" by simp
hence "lift (b[u/i]) 0 \<in> IT" by (rule lift_IT)
hence "lift (b[u/i]) 0 # ?ls bs \<in> lists IT"
by (rule lists.Cons) (rule Cons)
thus ?case by simp
qed
thus "Var 0 \<degree>\<degree> ?ls as \<in> IT" by (rule IT.Var)
have "e\<langle>0:Ts \<Rrightarrow> T'\<rangle> \<turnstile> Var 0 : Ts \<Rrightarrow> T'"
by (rule typing.Var) simp
moreover from uT argsT have "e \<tturnstile> map (\<lambda>t. t[u/i]) as : Ts"
by (rule substs_lemma)
hence "e\<langle>0:Ts \<Rrightarrow> T'\<rangle> \<tturnstile> ?ls as : Ts"
by (rule lift_types)
ultimately show "e\<langle>0:Ts \<Rrightarrow> T'\<rangle> \<turnstile> Var 0 \<degree>\<degree> ?ls as : T'"
by (rule list_app_typeI)
from argT uT have "e \<turnstile> a[u/i] : T''"
by (rule subst_lemma) (rule refl)
with uT' show "e \<turnstile> u \<degree> a[u/i] : Ts \<Rrightarrow> T'"
by (rule typing.App)
qed
with Cons True show ?thesis
by (simp add: map_compose [symmetric] o_def)
qed
next
case False
from Var have "rs \<in> lists {t. ?R t}" by simp
moreover from nT obtain Ts where "e\<langle>i:T\<rangle> \<tturnstile> rs : Ts"
by (rule list_app_typeE)
hence "rs \<in> lists ?ty" by (rule lists_typings)
ultimately have "rs \<in> lists ({t. ?R t} \<inter> ?ty)"
by (rule lists_IntI)
hence "map (\<lambda>x. x[u/i]) rs \<in> lists IT"
proof induct
case Nil
show ?case by fastsimp
next
case (Cons a as)
hence I: "?R a" by simp
from Cons obtain U where "e\<langle>i:T\<rangle> \<turnstile> a : U" by fast
with uT uIT I have "a[u/i] \<in> IT" by simp
hence "(a[u/i] # map (\<lambda>t. t[u/i]) as) \<in> lists IT"
by (rule lists.Cons) (rule Cons)
thus ?case by simp
qed
with False show ?thesis by (auto simp add: subst_Var)
qed
next
case (Lambda r e_ T'_ u_ i_)
assume "e\<langle>i:T\<rangle> \<turnstile> Abs r : T'"
and "\<And>e T' u i. PROP ?Q r e T' u i T"
with uIT uT show "Abs r[u/i] \<in> IT"
by fastsimp
next
case (Beta r a as e_ T'_ u_ i_)
assume T: "e\<langle>i:T\<rangle> \<turnstile> Abs r \<degree> a \<degree>\<degree> as : T'"
assume SI1: "\<And>e T' u i. PROP ?Q (r[a/0] \<degree>\<degree> as) e T' u i T"
assume SI2: "\<And>e T' u i. PROP ?Q a e T' u i T"
have "Abs (r[lift u 0/Suc i]) \<degree> a[u/i] \<degree>\<degree> map (\<lambda>t. t[u/i]) as \<in> IT"
proof (rule IT.Beta)
have "Abs r \<degree> a \<degree>\<degree> as \<rightarrow>\<^sub>\<beta> r[a/0] \<degree>\<degree> as"
by (rule apps_preserves_beta) (rule beta.beta)
with T have "e\<langle>i:T\<rangle> \<turnstile> r[a/0] \<degree>\<degree> as : T'"
by (rule subject_reduction)
hence "(r[a/0] \<degree>\<degree> as)[u/i] \<in> IT"
by (rule SI1)
thus "r[lift u 0/Suc i][a[u/i]/0] \<degree>\<degree> map (\<lambda>t. t[u/i]) as \<in> IT"
by (simp del: subst_map add: subst_subst subst_map [symmetric])
from T obtain U where "e\<langle>i:T\<rangle> \<turnstile> Abs r \<degree> a : U"
by (rule list_app_typeE) fast
then obtain T'' where "e\<langle>i:T\<rangle> \<turnstile> a : T''" by cases simp_all
thus "a[u/i] \<in> IT" by (rule SI2)
qed
thus "(Abs r \<degree> a \<degree>\<degree> as)[u/i] \<in> IT" by simp
}
qed
qed
subsection {* Well-typed terms are strongly normalizing *}
lemma type_implies_IT: "e \<turnstile> t : T \<Longrightarrow> t \<in> IT"
proof -
assume "e \<turnstile> t : T"
thus ?thesis
proof induct
case Var
show ?case by (rule Var_IT)
next
case Abs
show ?case by (rule IT.Lambda)
next
case (App T U e s t)
have "(Var 0 \<degree> lift t 0)[s/0] \<in> IT"
proof (rule subst_type_IT)
have "lift t 0 \<in> IT" by (rule lift_IT)
hence "[lift t 0] \<in> lists IT" by (rule lists.Cons) (rule lists.Nil)
hence "Var 0 \<degree>\<degree> [lift t 0] \<in> IT" by (rule IT.Var)
also have "Var 0 \<degree>\<degree> [lift t 0] = Var 0 \<degree> lift t 0" by simp
finally show "\<dots> \<in> IT" .
have "e\<langle>0:T \<Rightarrow> U\<rangle> \<turnstile> Var 0 : T \<Rightarrow> U"
by (rule typing.Var) simp
moreover have "e\<langle>0:T \<Rightarrow> U\<rangle> \<turnstile> lift t 0 : T"
by (rule lift_type)
ultimately show "e\<langle>0:T \<Rightarrow> U\<rangle> \<turnstile> Var 0 \<degree> lift t 0 : U"
by (rule typing.App)
qed
thus ?case by simp
qed
qed
theorem type_implies_termi: "e \<turnstile> t : T \<Longrightarrow> t \<in> termi beta"
proof -
assume "e \<turnstile> t : T"
hence "t \<in> IT" by (rule type_implies_IT)
thus ?thesis by (rule IT_implies_termi)
qed
end