(* Author: Florian Haftmann, TU Muenchen *)

section \<open>Reflecting Pure types into HOL\<close>

theory Typerep
imports String
begin

datatype typerep = Typerep String.literal "typerep list"

class typerep =
  fixes typerep :: "'a itself \<Rightarrow> typerep"
begin

definition typerep_of :: "'a \<Rightarrow> typerep" where
  [simp]: "typerep_of x = typerep TYPE('a)"

end

syntax
  "_TYPEREP" :: "type => logic"  ("(1TYPEREP/(1'(_')))")

parse_translation \<open>
  let
    fun typerep_tr (*"_TYPEREP"*) [ty] =
          Syntax.const \<^const_syntax>\<open>typerep\<close> $
            (Syntax.const \<^syntax_const>\<open>_constrain\<close> $ Syntax.const \<^const_syntax>\<open>Pure.type\<close> $
              (Syntax.const \<^type_syntax>\<open>itself\<close> $ ty))
      | typerep_tr (*"_TYPEREP"*) ts = raise TERM ("typerep_tr", ts);
  in [(\<^syntax_const>\<open>_TYPEREP\<close>, K typerep_tr)] end
\<close>

typed_print_translation \<open>
  let
    fun typerep_tr' ctxt (*"typerep"*)
            (Type (\<^type_name>\<open>fun\<close>, [Type (\<^type_name>\<open>itself\<close>, [T]), _]))
            (Const (\<^const_syntax>\<open>Pure.type\<close>, _) :: ts) =
          Term.list_comb
            (Syntax.const \<^syntax_const>\<open>_TYPEREP\<close> $ Syntax_Phases.term_of_typ ctxt T, ts)
      | typerep_tr' _ T ts = raise Match;
  in [(\<^const_syntax>\<open>typerep\<close>, typerep_tr')] end
\<close>

setup \<open>
let

fun add_typerep tyco thy =
  let
    val sorts = replicate (Sign.arity_number thy tyco) \<^sort>\<open>typerep\<close>;
    val vs = Name.invent_names Name.context "'a" sorts;
    val ty = Type (tyco, map TFree vs);
    val lhs = Const (\<^const_name>\<open>typerep\<close>, Term.itselfT ty --> \<^typ>\<open>typerep\<close>)
      $ Free ("T", Term.itselfT ty);
    val rhs = \<^term>\<open>Typerep\<close> $ HOLogic.mk_literal tyco
      $ HOLogic.mk_list \<^typ>\<open>typerep\<close> (map (HOLogic.mk_typerep o TFree) vs);
    val eq = HOLogic.mk_Trueprop (HOLogic.mk_eq (lhs, rhs));
  in
    thy
    |> Class.instantiation ([tyco], vs, \<^sort>\<open>typerep\<close>)
    |> `(fn lthy => Syntax.check_term lthy eq)
    |-> (fn eq => Specification.definition NONE [] [] (Binding.empty_atts, eq))
    |> snd
    |> Class.prove_instantiation_exit (fn ctxt => Class.intro_classes_tac ctxt [])
  end;

fun ensure_typerep tyco thy =
  if not (Sorts.has_instance (Sign.classes_of thy) tyco \<^sort>\<open>typerep\<close>)
    andalso Sorts.has_instance (Sign.classes_of thy) tyco \<^sort>\<open>type\<close>
  then add_typerep tyco thy else thy;

in

add_typerep \<^type_name>\<open>fun\<close>
#> Typedef.interpretation (Local_Theory.background_theory o ensure_typerep)
#> Code.type_interpretation ensure_typerep

end
\<close>

lemma [code]:
  "HOL.equal (Typerep tyco1 tys1) (Typerep tyco2 tys2) \<longleftrightarrow> HOL.equal tyco1 tyco2
     \<and> list_all2 HOL.equal tys1 tys2"
  by (auto simp add: eq_equal [symmetric] list_all2_eq [symmetric])

lemma [code nbe]:
  "HOL.equal (x :: typerep) x \<longleftrightarrow> True"
  by (fact equal_refl)

code_printing
  type_constructor typerep \<rightharpoonup> (Eval) "Term.typ"
| constant Typerep \<rightharpoonup> (Eval) "Term.Type/ (_, _)"

code_reserved Eval Term

hide_const (open) typerep Typerep

end