(* Title: HOL/Library/Phantom_Type.thy
Author: Andreas Lochbihler
*)
header {* A generic phantom type *}
theory Phantom_Type
imports Main
begin
datatype ('a, 'b) phantom = phantom 'b
primrec of_phantom :: "('a, 'b) phantom \<Rightarrow> 'b"
where "of_phantom (phantom x) = x"
lemma of_phantom_phantom [simp]: "phantom (of_phantom x) = x"
by(cases x) simp
lemma type_definition_phantom': "type_definition of_phantom phantom UNIV"
by(unfold_locales) simp_all
lemma phantom_comp_of_phantom [simp]: "phantom \<circ> of_phantom = id"
and of_phantom_comp_phantom [simp]: "of_phantom \<circ> phantom = id"
by(simp_all add: o_def id_def)
syntax "_Phantom" :: "type \<Rightarrow> logic" ("(1Phantom/(1'(_')))")
translations
"Phantom('t)" => "CONST phantom :: _ \<Rightarrow> ('t, _) phantom"
typed_print_translation {*
let
fun phantom_tr' ctxt (Type (@{type_name fun}, [_, Type (@{type_name phantom}, [T, _])])) ts =
list_comb
(Syntax.const @{syntax_const "_Phantom"} $ Syntax_Phases.term_of_typ ctxt T, ts)
| phantom_tr' _ _ _ = raise Match;
in [(@{const_syntax phantom}, phantom_tr')] end
*}
end