(* Title: HOL/Library/Type_Length.thy
Author: John Matthews, Galois Connections, Inc., Copyright 2006
*)
section \<open>Assigning lengths to types by type classes\<close>
theory Type_Length
imports Numeral_Type
begin
text \<open>
The aim of this is to allow any type as index type, but to provide a
default instantiation for numeral types. This independence requires
some duplication with the definitions in \<^file>\<open>Numeral_Type.thy\<close>.
\<close>
class len0 =
fixes len_of :: "'a itself \<Rightarrow> nat"
syntax "_type_length" :: "type \<Rightarrow> nat" (\<open>(1LENGTH/(1'(_')))\<close>)
translations "LENGTH('a)" \<rightharpoonup>
"CONST len_of (CONST Pure.type :: 'a itself)"
print_translation \<open>
let
fun len_of_itself_tr' ctxt [Const (\<^const_syntax>\<open>Pure.type\<close>, Type (_, [T]))] =
Syntax.const \<^syntax_const>\<open>_type_length\<close> $ Syntax_Phases.term_of_typ ctxt T
in [(\<^const_syntax>\<open>len_of\<close>, len_of_itself_tr')] end
\<close>
text \<open>Some theorems are only true on words with length greater 0.\<close>
class len = len0 +
assumes len_gt_0 [iff]: "0 < LENGTH('a)"
begin
lemma len_not_eq_0 [simp]:
"LENGTH('a) \<noteq> 0"
by simp
end
instantiation num0 and num1 :: len0
begin
definition len_num0: "len_of (_ :: num0 itself) = 0"
definition len_num1: "len_of (_ :: num1 itself) = 1"
instance ..
end
instantiation bit0 and bit1 :: (len0) len0
begin
definition len_bit0: "len_of (_ :: 'a::len0 bit0 itself) = 2 * LENGTH('a)"
definition len_bit1: "len_of (_ :: 'a::len0 bit1 itself) = 2 * LENGTH('a) + 1"
instance ..
end
lemmas len_of_numeral_defs [simp] = len_num0 len_num1 len_bit0 len_bit1
instance num1 :: len
by standard simp
instance bit0 :: (len) len
by standard simp
instance bit1 :: (len0) len
by standard simp
end