src/HOL/ex/Higher_Order_Logic.thy

tuned proofs;

(*  Title:      HOL/ex/Higher_Order_Logic.thy
    Author:     Gertrud Bauer and Markus Wenzel, TU Muenchen
*)

section \<open>Foundations of HOL\<close>

theory Higher_Order_Logic
imports Pure
begin

text \<open>
  The following theory development demonstrates Higher-Order Logic
  itself, represented directly within the Pure framework of Isabelle.
  The ``HOL'' logic given here is essentially that of Gordon
  @{cite "Gordon:1985:HOL"}, although we prefer to present basic concepts
  in a slightly more conventional manner oriented towards plain
  Natural Deduction.
\<close>


subsection \<open>Pure Logic\<close>

class type
default_sort type

typedecl o
instance o :: type ..
instance "fun" :: (type, type) type ..


subsubsection \<open>Basic logical connectives\<close>

judgment Trueprop :: "o \<Rightarrow> prop"    ("_" 5)

axiomatization
  imp :: "o \<Rightarrow> o \<Rightarrow> o"    (infixr "\<longrightarrow>" 25) and
  All :: "('a \<Rightarrow> o) \<Rightarrow> o"    (binder "\<forall>" 10)
where
  impI [intro]: "(A \<Longrightarrow> B) \<Longrightarrow> A \<longrightarrow> B" and
  impE [dest, trans]: "A \<longrightarrow> B \<Longrightarrow> A \<Longrightarrow> B" and
  allI [intro]: "(\<And>x. P x) \<Longrightarrow> \<forall>x. P x" and
  allE [dest]: "\<forall>x. P x \<Longrightarrow> P a"


subsubsection \<open>Extensional equality\<close>

axiomatization
  equal :: "'a \<Rightarrow> 'a \<Rightarrow> o"   (infixl "=" 50)
where
  refl [intro]: "x = x" and
  subst: "x = y \<Longrightarrow> P x \<Longrightarrow> P y"

axiomatization where
  ext [intro]: "(\<And>x. f x = g x) \<Longrightarrow> f = g" and
  iff [intro]: "(A \<Longrightarrow> B) \<Longrightarrow> (B \<Longrightarrow> A) \<Longrightarrow> A = B"

theorem sym [sym]: "x = y \<Longrightarrow> y = x"
proof -
  assume "x = y"
  then show "y = x" by (rule subst) (rule refl)
qed

lemma [trans]: "x = y \<Longrightarrow> P y \<Longrightarrow> P x"
  by (rule subst) (rule sym)

lemma [trans]: "P x \<Longrightarrow> x = y \<Longrightarrow> P y"
  by (rule subst)

theorem trans [trans]: "x = y \<Longrightarrow> y = z \<Longrightarrow> x = z"
  by (rule subst)

theorem iff1 [elim]: "A = B \<Longrightarrow> A \<Longrightarrow> B"
  by (rule subst)

theorem iff2 [elim]: "A = B \<Longrightarrow> B \<Longrightarrow> A"
  by (rule subst) (rule sym)


subsubsection \<open>Derived connectives\<close>

definition false :: o  ("\<bottom>") where "\<bottom> \<equiv> \<forall>A. A"

definition true :: o  ("\<top>") where "\<top> \<equiv> \<bottom> \<longrightarrow> \<bottom>"

definition not :: "o \<Rightarrow> o"  ("\<not> _" [40] 40)
  where "not \<equiv> \<lambda>A. A \<longrightarrow> \<bottom>"

definition conj :: "o \<Rightarrow> o \<Rightarrow> o"  (infixr "\<and>" 35)
  where "conj \<equiv> \<lambda>A B. \<forall>C. (A \<longrightarrow> B \<longrightarrow> C) \<longrightarrow> C"

definition disj :: "o \<Rightarrow> o \<Rightarrow> o"  (infixr "\<or>" 30)
  where "disj \<equiv> \<lambda>A B. \<forall>C. (A \<longrightarrow> C) \<longrightarrow> (B \<longrightarrow> C) \<longrightarrow> C"

definition Ex :: "('a \<Rightarrow> o) \<Rightarrow> o"  (binder "\<exists>" 10)
  where "\<exists>x. P x \<equiv> \<forall>C. (\<forall>x. P x \<longrightarrow> C) \<longrightarrow> C"

abbreviation not_equal :: "'a \<Rightarrow> 'a \<Rightarrow> o"  (infixl "\<noteq>" 50)
  where "x \<noteq> y \<equiv> \<not> (x = y)"

theorem falseE [elim]: "\<bottom> \<Longrightarrow> A"
proof (unfold false_def)
  assume "\<forall>A. A"
  then show A ..
qed

theorem trueI [intro]: \<top>
proof (unfold true_def)
  show "\<bottom> \<longrightarrow> \<bottom>" ..
qed

theorem notI [intro]: "(A \<Longrightarrow> \<bottom>) \<Longrightarrow> \<not> A"
proof (unfold not_def)
  assume "A \<Longrightarrow> \<bottom>"
  then show "A \<longrightarrow> \<bottom>" ..
qed

theorem notE [elim]: "\<not> A \<Longrightarrow> A \<Longrightarrow> B"
proof (unfold not_def)
  assume "A \<longrightarrow> \<bottom>" and A
  then have \<bottom> ..
  then show B ..
qed

lemma notE': "A \<Longrightarrow> \<not> A \<Longrightarrow> B"
  by (rule notE)

lemmas contradiction = notE notE'  -- \<open>proof by contradiction in any order\<close>

theorem conjI [intro]: "A \<Longrightarrow> B \<Longrightarrow> A \<and> B"
proof (unfold conj_def)
  assume A and B
  show "\<forall>C. (A \<longrightarrow> B \<longrightarrow> C) \<longrightarrow> C"
  proof
    fix C show "(A \<longrightarrow> B \<longrightarrow> C) \<longrightarrow> C"
    proof
      assume "A \<longrightarrow> B \<longrightarrow> C"
      also note \<open>A\<close>
      also note \<open>B\<close>
      finally show C .
    qed
  qed
qed

theorem conjE [elim]: "A \<and> B \<Longrightarrow> (A \<Longrightarrow> B \<Longrightarrow> C) \<Longrightarrow> C"
proof (unfold conj_def)
  assume c: "\<forall>C. (A \<longrightarrow> B \<longrightarrow> C) \<longrightarrow> C"
  assume "A \<Longrightarrow> B \<Longrightarrow> C"
  moreover {
    from c have "(A \<longrightarrow> B \<longrightarrow> A) \<longrightarrow> A" ..
    also have "A \<longrightarrow> B \<longrightarrow> A"
    proof
      assume A
      then show "B \<longrightarrow> A" ..
    qed
    finally have A .
  } moreover {
    from c have "(A \<longrightarrow> B \<longrightarrow> B) \<longrightarrow> B" ..
    also have "A \<longrightarrow> B \<longrightarrow> B"
    proof
      show "B \<longrightarrow> B" ..
    qed
    finally have B .
  } ultimately show C .
qed

theorem disjI1 [intro]: "A \<Longrightarrow> A \<or> B"
proof (unfold disj_def)
  assume A
  show "\<forall>C. (A \<longrightarrow> C) \<longrightarrow> (B \<longrightarrow> C) \<longrightarrow> C"
  proof
    fix C show "(A \<longrightarrow> C) \<longrightarrow> (B \<longrightarrow> C) \<longrightarrow> C"
    proof
      assume "A \<longrightarrow> C"
      also note \<open>A\<close>
      finally have C .
      then show "(B \<longrightarrow> C) \<longrightarrow> C" ..
    qed
  qed
qed

theorem disjI2 [intro]: "B \<Longrightarrow> A \<or> B"
proof (unfold disj_def)
  assume B
  show "\<forall>C. (A \<longrightarrow> C) \<longrightarrow> (B \<longrightarrow> C) \<longrightarrow> C"
  proof
    fix C show "(A \<longrightarrow> C) \<longrightarrow> (B \<longrightarrow> C) \<longrightarrow> C"
    proof
      show "(B \<longrightarrow> C) \<longrightarrow> C"
      proof
        assume "B \<longrightarrow> C"
        also note \<open>B\<close>
        finally show C .
      qed
    qed
  qed
qed

theorem disjE [elim]: "A \<or> B \<Longrightarrow> (A \<Longrightarrow> C) \<Longrightarrow> (B \<Longrightarrow> C) \<Longrightarrow> C"
proof (unfold disj_def)
  assume c: "\<forall>C. (A \<longrightarrow> C) \<longrightarrow> (B \<longrightarrow> C) \<longrightarrow> C"
  assume r1: "A \<Longrightarrow> C" and r2: "B \<Longrightarrow> C"
  from c have "(A \<longrightarrow> C) \<longrightarrow> (B \<longrightarrow> C) \<longrightarrow> C" ..
  also have "A \<longrightarrow> C"
  proof
    assume A
    then show C by (rule r1)
  qed
  also have "B \<longrightarrow> C"
  proof
    assume B
    then show C by (rule r2)
  qed
  finally show C .
qed

theorem exI [intro]: "P a \<Longrightarrow> \<exists>x. P x"
proof (unfold Ex_def)
  assume "P a"
  show "\<forall>C. (\<forall>x. P x \<longrightarrow> C) \<longrightarrow> C"
  proof
    fix C show "(\<forall>x. P x \<longrightarrow> C) \<longrightarrow> C"
    proof
      assume "\<forall>x. P x \<longrightarrow> C"
      then have "P a \<longrightarrow> C" ..
      also note \<open>P a\<close>
      finally show C .
    qed
  qed
qed

theorem exE [elim]: "\<exists>x. P x \<Longrightarrow> (\<And>x. P x \<Longrightarrow> C) \<Longrightarrow> C"
proof (unfold Ex_def)
  assume c: "\<forall>C. (\<forall>x. P x \<longrightarrow> C) \<longrightarrow> C"
  assume r: "\<And>x. P x \<Longrightarrow> C"
  from c have "(\<forall>x. P x \<longrightarrow> C) \<longrightarrow> C" ..
  also have "\<forall>x. P x \<longrightarrow> C"
  proof
    fix x show "P x \<longrightarrow> C"
    proof
      assume "P x"
      then show C by (rule r)
    qed
  qed
  finally show C .
qed


subsection \<open>Classical logic\<close>

locale classical =
  assumes classical: "(\<not> A \<Longrightarrow> A) \<Longrightarrow> A"

theorem (in classical) Peirce's_Law: "((A \<longrightarrow> B) \<longrightarrow> A) \<longrightarrow> A"
proof
  assume a: "(A \<longrightarrow> B) \<longrightarrow> A"
  show A
  proof (rule classical)
    assume "\<not> A"
    have "A \<longrightarrow> B"
    proof
      assume A
      with \<open>\<not> A\<close> show B by (rule contradiction)
    qed
    with a show A ..
  qed
qed

theorem (in classical) double_negation: "\<not> \<not> A \<Longrightarrow> A"
proof -
  assume "\<not> \<not> A"
  show A
  proof (rule classical)
    assume "\<not> A"
    with \<open>\<not> \<not> A\<close> show ?thesis by (rule contradiction)
  qed
qed

theorem (in classical) tertium_non_datur: "A \<or> \<not> A"
proof (rule double_negation)
  show "\<not> \<not> (A \<or> \<not> A)"
  proof
    assume "\<not> (A \<or> \<not> A)"
    have "\<not> A"
    proof
      assume A then have "A \<or> \<not> A" ..
      with \<open>\<not> (A \<or> \<not> A)\<close> show \<bottom> by (rule contradiction)
    qed
    then have "A \<or> \<not> A" ..
    with \<open>\<not> (A \<or> \<not> A)\<close> show \<bottom> by (rule contradiction)
  qed
qed

theorem (in classical) classical_cases: "(A \<Longrightarrow> C) \<Longrightarrow> (\<not> A \<Longrightarrow> C) \<Longrightarrow> C"
proof -
  assume r1: "A \<Longrightarrow> C" and r2: "\<not> A \<Longrightarrow> C"
  from tertium_non_datur show C
  proof
    assume A
    then show ?thesis by (rule r1)
  next
    assume "\<not> A"
    then show ?thesis by (rule r2)
  qed
qed

lemma (in classical) "(\<not> A \<Longrightarrow> A) \<Longrightarrow> A"  (* FIXME *)
proof -
  assume r: "\<not> A \<Longrightarrow> A"
  show A
  proof (rule classical_cases)
    assume A then show A .
  next
    assume "\<not> A" then show A by (rule r)
  qed
qed

end