src/HOL/Wfrec.thy
 author haftmann Sun Sep 21 16:56:11 2014 +0200 (2014-09-21) changeset 58410 6d46ad54a2ab parent 58184 db1381d811ab child 58889 5b7a9633cfa8 permissions -rw-r--r--
explicit separation of signed and unsigned numerals using existing lexical categories num and xnum
```     1 (*  Title:      HOL/Wfrec.thy
```
```     2     Author:     Tobias Nipkow
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```     3     Author:     Lawrence C Paulson
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```     4     Author:     Konrad Slind
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```     5 *)
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```     6
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```     7 header {* Well-Founded Recursion Combinator *}
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```     8
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```     9 theory Wfrec
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```    10 imports Wellfounded
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```    11 begin
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```    12
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```    13 inductive wfrec_rel :: "('a \<times> 'a) set \<Rightarrow> (('a \<Rightarrow> 'b) \<Rightarrow> ('a \<Rightarrow> 'b)) \<Rightarrow> 'a \<Rightarrow> 'b \<Rightarrow> bool" for R F where
```
```    14   wfrecI: "(\<And>z. (z, x) \<in> R \<Longrightarrow> wfrec_rel R F z (g z)) \<Longrightarrow> wfrec_rel R F x (F g x)"
```
```    15
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```    16 definition cut :: "('a \<Rightarrow> 'b) \<Rightarrow> ('a \<times> 'a) set \<Rightarrow> 'a \<Rightarrow> 'a \<Rightarrow> 'b" where
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```    17   "cut f R x = (\<lambda>y. if (y, x) \<in> R then f y else undefined)"
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```    18
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```    19 definition adm_wf :: "('a \<times> 'a) set \<Rightarrow> (('a \<Rightarrow> 'b) \<Rightarrow> ('a \<Rightarrow> 'b)) \<Rightarrow> bool" where
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```    20   "adm_wf R F \<longleftrightarrow> (\<forall>f g x. (\<forall>z. (z, x) \<in> R \<longrightarrow> f z = g z) \<longrightarrow> F f x = F g x)"
```
```    21
```
```    22 definition wfrec :: "('a \<times> 'a) set \<Rightarrow> (('a \<Rightarrow> 'b) \<Rightarrow> ('a \<Rightarrow> 'b)) \<Rightarrow> ('a \<Rightarrow> 'b)" where
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```    23   "wfrec R F = (\<lambda>x. THE y. wfrec_rel R (\<lambda>f x. F (cut f R x) x) x y)"
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```    24
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```    25 lemma cuts_eq: "(cut f R x = cut g R x) \<longleftrightarrow> (\<forall>y. (y, x) \<in> R \<longrightarrow> f y = g y)"
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```    26   by (simp add: fun_eq_iff cut_def)
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```    27
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```    28 lemma cut_apply: "(x, a) \<in> R \<Longrightarrow> cut f R a x = f x"
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```    29   by (simp add: cut_def)
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```    30
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```    31 text{*Inductive characterization of wfrec combinator; for details see:
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```    32 John Harrison, "Inductive definitions: automation and application"*}
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```    33
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```    34 lemma theI_unique: "\<exists>!x. P x \<Longrightarrow> P x \<longleftrightarrow> x = The P"
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```    35   by (auto intro: the_equality[symmetric] theI)
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```    36
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```    37 lemma wfrec_unique: assumes "adm_wf R F" "wf R" shows "\<exists>!y. wfrec_rel R F x y"
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```    38   using `wf R`
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```    39 proof induct
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```    40   def f \<equiv> "\<lambda>y. THE z. wfrec_rel R F y z"
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```    41   case (less x)
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```    42   then have "\<And>y z. (y, x) \<in> R \<Longrightarrow> wfrec_rel R F y z \<longleftrightarrow> z = f y"
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```    43     unfolding f_def by (rule theI_unique)
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```    44   with `adm_wf R F` show ?case
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```    45     by (subst wfrec_rel.simps) (auto simp: adm_wf_def)
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```    46 qed
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```    47
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```    48 lemma adm_lemma: "adm_wf R (\<lambda>f x. F (cut f R x) x)"
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```    49   by (auto simp add: adm_wf_def
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```    50            intro!: arg_cong[where f="\<lambda>x. F x y" for y] cuts_eq[THEN iffD2])
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```    51
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```    52 lemma wfrec: "wf R \<Longrightarrow> wfrec R F a = F (cut (wfrec R F) R a) a"
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```    53 apply (simp add: wfrec_def)
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```    54 apply (rule adm_lemma [THEN wfrec_unique, THEN the1_equality], assumption)
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```    55 apply (rule wfrec_rel.wfrecI)
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```    56 apply (erule adm_lemma [THEN wfrec_unique, THEN theI'])
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```    57 done
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```    58
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```    59
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```    60 text{** This form avoids giant explosions in proofs.  NOTE USE OF ==*}
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```    61 lemma def_wfrec: "f \<equiv> wfrec R F \<Longrightarrow> wf R \<Longrightarrow> f a = F (cut f R a) a"
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```    62  by (auto intro: wfrec)
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```    63
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```    64
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```    65 subsubsection {* Well-founded recursion via genuine fixpoints *}
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```    66
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```    67 lemma wfrec_fixpoint:
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```    68   assumes WF: "wf R" and ADM: "adm_wf R F"
```
```    69   shows "wfrec R F = F (wfrec R F)"
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```    70 proof (rule ext)
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```    71   fix x
```
```    72   have "wfrec R F x = F (cut (wfrec R F) R x) x"
```
```    73     using wfrec[of R F] WF by simp
```
```    74   also
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```    75   { have "\<And> y. (y,x) \<in> R \<Longrightarrow> (cut (wfrec R F) R x) y = (wfrec R F) y"
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```    76       by (auto simp add: cut_apply)
```
```    77     hence "F (cut (wfrec R F) R x) x = F (wfrec R F) x"
```
```    78       using ADM adm_wf_def[of R F] by auto }
```
```    79   finally show "wfrec R F x = F (wfrec R F) x" .
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```    80 qed
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```    81
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```    82 subsection {* Wellfoundedness of @{text same_fst} *}
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```    83
```
```    84 definition same_fst :: "('a \<Rightarrow> bool) \<Rightarrow> ('a \<Rightarrow> ('b \<times> 'b) set) \<Rightarrow> (('a \<times> 'b) \<times> ('a \<times> 'b)) set" where
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```    85   "same_fst P R = {((x', y'), (x, y)) . x' = x \<and> P x \<and> (y',y) \<in> R x}"
```
```    86    --{*For @{const wfrec} declarations where the first n parameters
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```    87        stay unchanged in the recursive call. *}
```
```    88
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```    89 lemma same_fstI [intro!]: "P x \<Longrightarrow> (y', y) \<in> R x \<Longrightarrow> ((x, y'), (x, y)) \<in> same_fst P R"
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```    90   by (simp add: same_fst_def)
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```    91
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```    92 lemma wf_same_fst:
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```    93   assumes prem: "\<And>x. P x \<Longrightarrow> wf (R x)"
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```    94   shows "wf (same_fst P R)"
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```    95 apply (simp cong del: imp_cong add: wf_def same_fst_def)
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```    96 apply (intro strip)
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```    97 apply (rename_tac a b)
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```    98 apply (case_tac "wf (R a)")
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```    99  apply (erule_tac a = b in wf_induct, blast)
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```   100 apply (blast intro: prem)
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```   101 done
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```   102
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```   103 end
```