2 ||M|| This file is part of HELM, an Hypertextual, Electronic
3 ||A|| Library of Mathematics, developed at the Computer Science
4 ||T|| Department of the University of Bologna, Italy.
8 \ / This file is distributed under the terms of the
9 \ / GNU General Public License Version 2
10 V_____________________________________________________________*)
12 include "basics/vectors.ma".
13 (* include "basics/relations.ma". *)
15 record tape (sig:FinSet): Type[0] ≝
20 inductive move : Type[0] ≝
25 (* We do not distinuish an input tape *)
27 record TM (sig:FinSet): Type[1] ≝
29 trans : states × (option sig) → states × (option (sig × move));
34 record config (sig:FinSet) (M:TM sig): Type[0] ≝
35 { cstate : states sig M;
39 definition option_hd ≝ λA.λl:list A.
45 definition tape_move ≝ λsig.λt: tape sig.λm:option (sig × move).
50 [ R ⇒ mk_tape sig ((\fst m1)::(left ? t)) (tail ? (right ? t))
51 | L ⇒ mk_tape sig (tail ? (left ? t)) ((\fst m1)::(right ? t))
55 definition step ≝ λsig.λM:TM sig.λc:config sig M.
56 let current_char ≝ option_hd ? (right ? (ctape ?? c)) in
57 let 〈news,mv〉 ≝ trans sig M 〈cstate ?? c,current_char〉 in
58 mk_config ?? news (tape_move sig (ctape ?? c) mv).
60 let rec loop (A:Type[0]) n (f:A→A) p a on n ≝
63 | S m ⇒ if p a then (Some ? a) else loop A m f p (f a)
66 definition initc ≝ λsig.λM:TM sig.λt.
67 mk_config sig M (start sig M) t.
69 definition Realize ≝ λsig.λM:TM sig.λR:relation (tape sig).
71 loop ? i (step sig M) (λc.halt sig M (cstate ?? c)) (initc sig M t) = Some ? outc ∧
76 definition seq_trans ≝ λsig. λM1,M2 : TM sig.
80 if halt sig M1 s1 then 〈inr … (start sig M2), None ?〉
82 let 〈news1,m〉 ≝ trans sig M1 〈s1,a〉 in
85 let 〈news2,m〉 ≝ trans sig M2 〈s2,a〉 in
89 definition seq ≝ λsig. λM1,M2 : TM sig.
91 (FinSum (states sig M1) (states sig M2))
93 (inl … (start sig M1))
95 [ inl _ ⇒ false | inr s2 ⇒ halt sig M2 s2]).
97 definition Rcomp ≝ λA.λR1,R2:relation A.λa1,a2.
98 ∃am.R1 a1 am ∧ R2 am a2.
101 definition injectRl ≝ λsig.λM1.λM2.λR.
103 inl … (cstate sig M1 c11) = cstate sig (seq sig M1 M2) c1 ∧
104 inl … (cstate sig M1 c12) = cstate sig (seq sig M1 M2) c2 ∧
105 ctape sig M1 c11 = ctape sig (seq sig M1 M2) c1 ∧
106 ctape sig M1 c12 = ctape sig (seq sig M1 M2) c2 ∧
109 definition injectRr ≝ λsig.λM1.λM2.λR.
111 inr … (cstate sig M2 c21) = cstate sig (seq sig M1 M2) c1 ∧
112 inr … (cstate sig M2 c22) = cstate sig (seq sig M1 M2) c2 ∧
113 ctape sig M2 c21 = ctape sig (seq sig M1 M2) c1 ∧
114 ctape sig M2 c22 = ctape sig (seq sig M1 M2) c2 ∧
117 definition Rlink ≝ λsig.λM1,M2.λc1,c2.
118 ctape sig (seq sig M1 M2) c1 = ctape sig (seq sig M1 M2) c2 ∧
119 cstate sig (seq sig M1 M2) c1 = inl … (halt sig M1) ∧
120 cstate sig (seq sig M1 M2) c2 = inr … (start sig M2). *)
122 interpretation "relation composition" 'compose R1 R2 = (Rcomp ? R1 R2).
124 theorem sem_seq: ∀sig,M1,M2,R1,R2.
125 Realize sig M1 R1 → Realize sig M2 R2 →
126 Realize sig (seq sig M1 M2) (R1 ∘ R2).
127 #sig #M1 #M2 #R1 #R2 #HR1 #HR2 #t
128 cases (HR1 t) #k1 * #outc1 * #Hloop1 #HM1
129 cases (HR2 (ctape sig M1 outc1)) #k2 * #outc2 * #Hloop2 #HM2
130 @(ex_intro … (S(k1+k2))) @
135 definition empty_tapes ≝ λsig.λn.
136 mk_Vector ? n (make_list (tape sig) (mk_tape sig [] []) n) ?.
137 elim n // normalize //
140 definition init ≝ λsig.λM:TM sig.λi:(list sig).
143 (vec_cons ? (mk_tape sig [] i) ? (empty_tapes sig (tapes_no sig M)))
146 definition stop ≝ λsig.λM:TM sig.λc:config sig M.
147 halt sig M (state sig M c).
149 let rec loop (A:Type[0]) n (f:A→A) p a on n ≝
152 | S m ⇒ if p a then (Some ? a) else loop A m f p (f a)
155 (* Compute ? M f states that f is computed by M *)
156 definition Compute ≝ λsig.λM:TM sig.λf:(list sig) → (list sig).
158 loop ? i (step sig M) (stop sig M) (init sig M l) = Some ? c ∧
161 (* for decision problems, we accept a string if on termination
162 output is not empty *)
164 definition ComputeB ≝ λsig.λM:TM sig.λf:(list sig) → bool.
166 loop ? i (step sig M) (stop sig M) (init sig M l) = Some ? c ∧
167 (isnilb ? (out ?? c) = false).
169 (* alternative approach.
170 We define the notion of computation. The notion must be constructive,
171 since we want to define functions over it, like lenght and size
173 Perche' serve Type[2] se sposto a e b a destra? *)
175 inductive cmove (A:Type[0]) (f:A→A) (p:A →bool) (a,b:A): Type[0] ≝
176 mk_move: p a = false → b = f a → cmove A f p a b.
178 inductive cstar (A:Type[0]) (M:A→A→Type[0]) : A →A → Type[0] ≝
179 | empty : ∀a. cstar A M a a
180 | more : ∀a,b,c. M a b → cstar A M b c → cstar A M a c.
182 definition computation ≝ λsig.λM:TM sig.
183 cstar ? (cmove ? (step sig M) (stop sig M)).
185 definition Compute_expl ≝ λsig.λM:TM sig.λf:(list sig) → (list sig).
186 ∀l.∃c.computation sig M (init sig M l) c →
187 (stop sig M c = true) ∧ out ?? c = f l.
189 definition ComputeB_expl ≝ λsig.λM:TM sig.λf:(list sig) → bool.
190 ∀l.∃c.computation sig M (init sig M l) c →
191 (stop sig M c = true) ∧ (isnilb ? (out ?? c) = false).