src/HOL/W0/Type.thy

* Pure: method 'atomize' presents local goal premises as object-level
statements (atomic meta-level propositions); setup controlled via
rewrite rules declarations of 'atomize' attribute; example
application: 'induct' method with proper rule statements in improper
proof *scripts*;

(* Title:     HOL/W0/Type.thy
   ID:        $Id$
   Author:    Dieter Nazareth and Tobias Nipkow
   Copyright  1995 TU Muenchen

MiniML-types and type substitutions.
*)

Type = Maybe + 

(* new class for structures containing type variables *)
classes
        type_struct < term 

(* type expressions *)
datatype
        typ = TVar nat | "->" typ typ (infixr 70)

(* type variable substitution *)
types
        subst = nat => typ

arities
        typ::type_struct
        list::(type_struct)type_struct
        fun::(term,type_struct)type_struct

(* substitutions *)

(* identity *)
constdefs
        id_subst :: subst
        "id_subst == (%n. TVar n)"

(* extension of substitution to type structures *)
consts
        app_subst :: [subst, 'a::type_struct] => 'a::type_struct ("$")

primrec
  app_subst_TVar  "$ s (TVar n) = s n" 
  app_subst_Fun   "$ s (t1 -> t2) = ($ s t1) -> ($ s t2)" 

defs
        app_subst_list  "$ s == map ($ s)"
  
(* free_tv s: the type variables occuring freely in the type structure s *)
consts
        free_tv :: ['a::type_struct] => nat set

primrec
  "free_tv (TVar m) = {m}"
  "free_tv (t1 -> t2) = (free_tv t1) Un (free_tv t2)"

primrec
  "free_tv [] = {}"
  "free_tv (x#l) = (free_tv x) Un (free_tv l)"

(* domain of a substitution *)
constdefs
        dom :: subst => nat set
        "dom s == {n. s n ~= TVar n}" 

(* codomain of a substitutions: the introduced variables *)
constdefs
        cod :: subst => nat set
        "cod s == (UN m:dom s. free_tv (s m))"

defs
        free_tv_subst   "free_tv s == (dom s) Un (cod s)"

(* new_tv s n computes whether n is a new type variable w.r.t. a type 
   structure s, i.e. whether n is greater than any type variable 
   occuring in the type structure *)
constdefs
        new_tv :: [nat,'a::type_struct] => bool
        "new_tv n ts == ! m. m:free_tv ts --> m<n"

(* unification algorithm mgu *)
consts
        mgu :: [typ,typ] => subst maybe
rules
        mgu_eq   "mgu t1 t2 = Ok u ==> $u t1 = $u t2"
        mgu_mg   "[| (mgu t1 t2) = Ok u; $s t1 = $s t2 |] ==>
                  ? r. s = $r o u"
        mgu_Ok   "$s t1 = $s t2 ==> ? u. mgu t1 t2 = Ok u"
        mgu_free "mgu t1 t2 = Ok u ==> free_tv u <= free_tv t1 Un free_tv t2"

end