Theory Typedef

(*  Title:      HOL/Typedef.thy
    Author:     Markus Wenzel, TU Munich
*)

section ‹HOL type definitions›

theory Typedef
imports Set
keywords
  "typedef" :: thy_goal_defn and
  "morphisms" :: quasi_command
begin

locale type_definition =
  fixes Rep and Abs and A
  assumes Rep: "Rep x ∈ A"
    and Rep_inverse: "Abs (Rep x) = x"
    and Abs_inverse: "y ∈ A ⟹ Rep (Abs y) = y"
  ― ‹This will be axiomatized for each typedef!›
begin

lemma Rep_inject: "Rep x = Rep y ⟷ x = y"
proof
  assume "Rep x = Rep y"
  then have "Abs (Rep x) = Abs (Rep y)" by (simp only:)
  also have "Abs (Rep x) = x" by (rule Rep_inverse)
  also have "Abs (Rep y) = y" by (rule Rep_inverse)
  finally show "x = y" .
next
  show "x = y ⟹ Rep x = Rep y" by (simp only:)
qed

lemma Abs_inject:
  assumes "x ∈ A" and "y ∈ A"
  shows "Abs x = Abs y ⟷ x = y"
proof
  assume "Abs x = Abs y"
  then have "Rep (Abs x) = Rep (Abs y)" by (simp only:)
  also from ‹x ∈ A› have "Rep (Abs x) = x" by (rule Abs_inverse)
  also from ‹y ∈ A› have "Rep (Abs y) = y" by (rule Abs_inverse)
  finally show "x = y" .
next
  show "x = y ⟹ Abs x = Abs y" by (simp only:)
qed

lemma Rep_cases [cases set]:
  assumes "y ∈ A"
    and hyp: "⋀x. y = Rep x ⟹ P"
  shows P
proof (rule hyp)
  from ‹y ∈ A› have "Rep (Abs y) = y" by (rule Abs_inverse)
  then show "y = Rep (Abs y)" ..
qed

lemma Abs_cases [cases type]:
  assumes r: "⋀y. x = Abs y ⟹ y ∈ A ⟹ P"
  shows P
proof (rule r)
  have "Abs (Rep x) = x" by (rule Rep_inverse)
  then show "x = Abs (Rep x)" ..
  show "Rep x ∈ A" by (rule Rep)
qed

lemma Rep_induct [induct set]:
  assumes y: "y ∈ A"
    and hyp: "⋀x. P (Rep x)"
  shows "P y"
proof -
  have "P (Rep (Abs y))" by (rule hyp)
  also from y have "Rep (Abs y) = y" by (rule Abs_inverse)
  finally show "P y" .
qed

lemma Abs_induct [induct type]:
  assumes r: "⋀y. y ∈ A ⟹ P (Abs y)"
  shows "P x"
proof -
  have "Rep x ∈ A" by (rule Rep)
  then have "P (Abs (Rep x))" by (rule r)
  also have "Abs (Rep x) = x" by (rule Rep_inverse)
  finally show "P x" .
qed

lemma Rep_range: "range Rep = A"
proof
  show "range Rep ⊆ A" using Rep by (auto simp add: image_def)
  show "A ⊆ range Rep"
  proof
    fix x assume "x ∈ A"
    then have "x = Rep (Abs x)" by (rule Abs_inverse [symmetric])
    then show "x ∈ range Rep" by (rule range_eqI)
  qed
qed

lemma Abs_image: "Abs ` A = UNIV"
proof
  show "Abs ` A ⊆ UNIV" by (rule subset_UNIV)
  show "UNIV ⊆ Abs ` A"
  proof
    show "x ∈ Abs ` A" for x
    proof (rule image_eqI)
      show "x = Abs (Rep x)" by (rule Rep_inverse [symmetric])
      show "Rep x ∈ A" by (rule Rep)
    qed
  qed
qed

end

ML_file ‹Tools/typedef.ML›

end