4 This file provides a basic theory of sequences.
7 (** {2 Sequences and basic operations} *)
13 (** the polymorphic type of sequences *)
16 (** `seq 'a` is an infinite type *)
17 meta "infinite_type" type seq
19 val function length (seq 'a) : int
21 axiom length_nonnegative:
22 forall s: seq 'a. 0 <= length s
24 val function get (seq 'a) int : 'a
25 (* FIXME requires { 0 <= i < length s } *)
26 (** `get s i` is the `i+1`-th element of sequence `s`
27 (the first element has index 0) *)
29 let function ([]) (s: seq 'a) (i: int) : 'a =
32 (** equality is extensional *)
33 val predicate (==) (s1 s2: seq 'a)
34 ensures { result <-> length s1 = length s2 &&
35 forall i: int. 0 <= i < length s1 -> s1[i] = s2[i] }
36 ensures { result -> s1 = s2 }
38 (** sequence comprehension *)
39 val function create (len: int) (f: int -> 'a) : seq 'a
41 ensures { length result = len }
42 ensures { forall i. 0 <= i < len -> result[i] = f i }
44 (*** FIXME: could be defined, but let constant does
46 (*** let constant empty : seq 'a
47 ensures { length result = 0 }
48 = while false do variant { 0 } () done;
49 create 0 (fun _ requires { false } -> absurd)
53 val constant empty : seq 'a
54 ensures { length result = 0 }
56 (** `set s i v` is a new sequence `u` such that
57 `u[i] = v` and `u[j] = s[j]` otherwise *)
58 let function set (s:seq 'a) (i:int) (v:'a) : seq 'a
59 requires { 0 <= i < length s }
60 ensures { length result = length s }
61 ensures { result[i] = v }
62 ensures { forall j. 0 <= j < length s /\ j <> i -> result[j] = s[j] }
63 = while false do variant { 0 } () done;
64 create s.length (fun j -> if j = i then v else s[j])
66 (* FIXME: not a real alias because of spec, but should be. *)
67 let function ([<-]) (s: seq 'a) (i: int) (v: 'a) : seq 'a
68 requires { 0 <= i < length s }
71 (** singleton sequence *)
72 let function singleton (v:'a) : seq 'a
73 ensures { length result = 1 }
74 ensures { result[0] = v }
75 = while false do variant { 0 } () done;
78 (** insertion of elements on both sides *)
79 let function cons (x:'a) (s:seq 'a) : seq 'a
80 ensures { length result = 1 + length s }
81 ensures { result[0] = x }
82 ensures { forall i. 0 < i <= length s -> result[i] = s[i-1] }
83 = while false do variant { 0 } () done;
84 create (1 + length s) (fun i -> if i = 0 then x else s[i-1])
86 let function snoc (s:seq 'a) (x:'a) : seq 'a
87 ensures { length result = 1 + length s }
88 ensures { result[length s] = x }
89 ensures { forall i. 0 <= i < length s -> result[i] = s[i] }
90 = while false do variant { 0 } () done;
91 create (1 + length s) (fun i -> if i = length s then x else s[i])
93 (** `s[i..j]` is the sub-sequence of `s` from element `i` included
94 to element `j` excluded *)
95 let function ([..]) (s:seq 'a) (i:int) (j:int) : seq 'a
96 requires { 0 <= i <= j <= length s }
97 ensures { length result = j - i }
98 ensures { forall k. 0 <= k < j - i -> result[k] = s[i + k] }
99 = while false do variant { 0 } () done;
100 create (j-i) (fun k -> s[i+k])
102 (* FIXME: spec/alias *)
103 let function ([_..]) (s: seq 'a) (i: int) : seq 'a
104 requires { 0 <= i <= length s }
107 (* FIXME: spec/alias *)
108 let function ([.._]) (s: seq 'a) (j: int) : seq 'a
109 requires { 0 <= j <= length s }
113 let function (++) (s1:seq 'a) (s2:seq 'a) : seq 'a
114 ensures { length result = length s1 + length s2 }
115 ensures { forall i. 0 <= i < length s1 -> result[i] = s1[i] }
116 ensures { forall i. length s1 <= i < length result ->
117 result[i] = s2[i - length s1] }
118 = while false do variant { 0 } () done;
120 create (l + length s2)
121 (fun i -> if i < l then s1[i] else s2[i-l])
125 (** {2 Lemma library about algebraic interactions between
126 `empty`/`singleton`/`cons`/`snoc`/`++`/`[ .. ]`} *)
133 (* Monoidal properties/simplification. *)
135 let lemma associative (s1 s2 s3:seq 'a)
136 ensures { s1 ++ (s2 ++ s3) = (s1 ++ s2) ++ s3 }
137 = if not (s1 ++ s2) ++ s3 == s1 ++ (s2 ++ s3) then absurd
138 meta rewrite axiom associative
140 let lemma left_neutral (s:seq 'a)
141 ensures { empty ++ s = s }
142 = if not empty ++ s == s then absurd
143 meta rewrite axiom left_neutral
145 let lemma right_neutral (s:seq 'a)
146 ensures { s ++ empty = s }
147 = if not s ++ empty == s then absurd
148 meta rewrite axiom right_neutral
150 let lemma cons_def (x:'a) (s:seq 'a)
151 ensures { cons x s = singleton x ++ s }
152 = if not cons x s == singleton x ++ s then absurd
153 meta rewrite axiom cons_def
155 let lemma snoc_def (s:seq 'a) (x:'a)
156 ensures { snoc s x = s ++ singleton x }
157 = if not snoc s x == s ++ singleton x then absurd
158 meta rewrite axiom snoc_def
160 let lemma double_sub_sequence (s:seq 'a) (i j k l:int)
161 requires { 0 <= i <= j <= length s }
162 requires { 0 <= k <= l <= j - i }
163 ensures { s[i .. j][k .. l] = s[k+i .. l+i] }
164 = if not s[i .. j][k .. l] == s[k+i .. l+i] then absurd
166 (* Inverting cons/snoc/catenation *)
168 let lemma cons_back (x:'a) (s:seq 'a)
169 ensures { (cons x s)[1 ..] = s }
170 = if not (cons x s)[1 ..] == s then absurd
172 let lemma snoc_back (s:seq 'a) (x:'a)
173 ensures { (snoc s x)[.. length s] = s }
174 = if not (snoc s x)[.. length s] == s then absurd
176 let lemma cat_back (s1 s2:seq 'a)
177 ensures { (s1 ++ s2)[.. length s1] = s1 }
178 ensures { (s1 ++ s2)[length s1 ..] = s2 }
179 = let c = s1 ++ s2 in let l = length s1 in
180 if not (c[.. l] == s1 || c[l ..] == s2) then absurd
182 (* Decomposing sequences as cons/snoc/catenation/empty/singleton *)
184 let lemma cons_dec (s:seq 'a)
185 requires { length s >= 1 }
186 ensures { s = cons s[0] s[1 ..] }
187 = if not s == cons s[0] s[1 ..] then absurd
189 let lemma snoc_dec (s:seq 'a)
190 requires { length s >= 1 }
191 ensures { s = snoc s[.. length s - 1] s[length s - 1] }
192 = if not s == snoc s[.. length s - 1] s[length s - 1] then absurd
194 let lemma cat_dec (s:seq 'a) (i:int)
195 requires { 0 <= i <= length s }
196 ensures { s = s[.. i] ++ s[i ..] }
197 = if not s == s[.. i] ++ s[i ..] then absurd
199 let lemma empty_dec (s:seq 'a)
200 requires { length s = 0 }
201 ensures { s = empty }
202 = if not s == empty then absurd
204 let lemma singleton_dec (s:seq 'a)
205 requires { length s = 1 }
206 ensures { s = singleton s[0] }
207 = if not s == singleton s[0] then absurd
216 val function to_list (a: seq 'a) : list 'a
219 to_list (empty: seq 'a) = (Nil: list 'a)
222 forall s: seq 'a. 0 < length s ->
223 to_list s = Cons s[0] (to_list s[1 ..])
225 use list.Length as ListLength
227 lemma to_list_length:
228 forall s: seq 'a. ListLength.length (to_list s) = length s
230 use list.Nth as ListNth
234 forall s: seq 'a, i: int. 0 <= i < length s ->
235 ListNth.nth i (to_list s) = Some s[i]
237 let rec lemma to_list_def_cons (s: seq 'a) (x: 'a)
239 ensures { to_list (cons x s) = Cons x (to_list s) }
240 = assert { (cons x s)[1 ..] == s }
253 let rec function of_list (l: list 'a) : seq 'a = match l with
255 | Cons x r -> cons x (of_list r)
258 lemma length_of_list:
259 forall l: list 'a. length (of_list l) = L.length l
261 predicate point_wise (s: seq 'a) (l: list 'a) =
262 forall i. 0 <= i < L.length l -> Some (get s i) = nth i l
264 lemma elts_seq_of_list: forall l: list 'a.
265 point_wise (of_list l) l
267 lemma is_of_list: forall l: list 'a, s: seq 'a.
268 L.length l = length s -> point_wise s l -> s == of_list l
270 let rec lemma of_list_app (l1 l2: list 'a)
271 ensures { of_list (l1 ++ l2) == Seq.(++) (of_list l1) (of_list l2) }
275 | Cons _ r -> of_list_app r l2
278 lemma of_list_app_length: forall l1 [@induction] l2: list 'a.
279 length (of_list (l1 ++ l2)) = L.length l1 + L.length l2
281 let rec lemma of_list_snoc (l: list 'a) (x: 'a)
283 ensures { of_list (l ++ Cons x Nil) == snoc (of_list l) x }
285 | Nil -> assert { snoc empty x = cons x empty }
286 | Cons _ r -> of_list_snoc r x;
289 meta coercion function of_list
293 lemma convolution_to_of_list: forall l: list 'a.
294 to_list (of_list l) = l
303 predicate mem (x: 'a) (s: seq 'a) =
304 exists i: int. 0 <= i < length s && s[i] = x
306 lemma mem_append : forall x: 'a, s1 s2.
307 mem x (s1 ++ s2) <-> mem x s1 \/ mem x s2
309 lemma mem_tail: forall x: 'a, s.
311 mem x s <-> (x = s[0] \/ mem x s[1 .. ])
319 predicate distinct (s : seq 'a) =
320 forall i j. 0 <= i < length s -> 0 <= j < length s ->
321 i <> j -> s[i] <> s[j]
330 let function reverse (s: seq 'a) : seq 'a =
331 create (length s) (fun i -> s[length s - 1 - i])
341 val function to_set (s: seq 'a) : fset 'a
343 axiom to_set_empty: to_set (empty: seq 'a) = (Fset.empty: fset 'a)
345 axiom to_set_add: forall s: seq 'a. length s > 0 ->
346 to_set s = add s[0] (to_set s[1 ..])
348 lemma to_set_cardinal: forall s: seq 'a.
349 cardinal (to_set s) <= length s
351 lemma to_set_mem: forall s: seq 'a, e: 'a.
352 mem e s <-> Fset.mem e (to_set s)
354 lemma to_set_snoc: forall s: seq 'a, x: 'a.
355 to_set (snoc s x) = add x (to_set s)
359 lemma to_set_cardinal_distinct: forall s: seq 'a. distinct s ->
360 cardinal (to_set s) = length s
364 (** {2 Sorted Sequences} *)
373 clone relations.TotalPreOrder as TO with
374 type t = t, predicate rel = le, axiom .
376 predicate sorted_sub (s: seq t) (l u: int) =
377 forall i1 i2. l <= i1 <= i2 < u -> le s[i1] s[i2]
378 (** `sorted_sub s l u` is true whenever the sub-sequence `s[l .. u-1]` is
379 sorted w.r.t. order relation `le` *)
381 predicate sorted (s: seq t) =
382 sorted_sub s 0 (length s)
383 (** `sorted s` is true whenever the sequence `s` is sorted w.r.t `le` *)
386 forall x: t, s: seq t.
387 (forall i: int. 0 <= i < length s -> le x s[i]) /\ sorted s <->
392 (sorted s1 /\ sorted s2 /\
393 (forall i j: int. 0 <= i < length s1 /\ 0 <= j < length s2 ->
394 le s1[i] s2[j])) <-> sorted (s1 ++ s2)
397 forall x: t, s: seq t.
398 (forall i: int. 0 <= i < length s -> le s[i] x) /\ sorted s <->
403 module SortedInt (** sorted sequences of integers *)
406 clone export Sorted with type t = int, predicate le = (<=), goal .
416 function sum (s: seq int) : int = S.sum (fun i -> s[i]) 0 (length s)
419 forall s x. sum (snoc s x) = sum s + x
421 forall s. length s >= 1 -> sum s = s[0] + sum s[1 .. ]
423 forall s. length s >= 2 -> sum s = s[0] + s[1] + sum s[2 .. ]
427 (** {2 Number of occurrences in a sequence} *)
435 function iseq (x: 'a) (s: seq 'a) : int->bool = fun i -> s[i] = x
437 function occ (x: 'a) (s: seq 'a) (l u: int) : int = N.numof (iseq x s) l u
439 function occ_all (x: 'a) (s: seq 'a) : int =
443 forall k: 'a, s: seq 'a, x: 'a.
444 (occ_all k (cons x s) =
445 if k = x then 1 + occ_all k s else occ_all k s
446 ) by (cons x s == (cons x empty) ++ s)
449 forall k: 'a, s: seq 'a, x: 'a.
450 occ_all k (snoc s x) =
451 if k = x then 1 + occ_all k s else occ_all k s
454 forall k: 'a, s: seq 'a.
457 if k = s[0] then (occ_all k s) - 1 else occ_all k s
458 ) by (s == cons s[0] s[1..])
460 lemma append_num_occ:
461 forall x: 'a, s1 s2: seq 'a.
462 occ_all x (s1 ++ s2) =
463 occ_all x s1 + occ_all x s2
467 (** {2 Sequences Equality} *)
474 predicate seq_eq_sub (s1 s2: seq 'a) (l u: int) =
475 forall i. l <= i < u -> s1[i] = s2[i]
484 predicate exchange (s1 s2: seq 'a) (i j: int) =
485 length s1 = length s2 /\
486 0 <= i < length s1 /\ 0 <= j < length s1 /\
487 s1[i] = s2[j] /\ s1[j] = s2[i] /\
488 (forall k:int. 0 <= k < length s1 -> k <> i -> k <> j -> s1[k] = s2[k])
491 forall s: seq 'a, i j: int.
492 0 <= i < length s -> 0 <= j < length s ->
493 exchange s s[i <- s[j]][j <- s[i]] i j
497 (** {2 Permutation of sequences} *)
507 predicate permut (s1 s2: seq 'a) (l u: int) =
508 length s1 = length s2 /\
509 0 <= l <= length s1 /\ 0 <= u <= length s1 /\
510 forall v: 'a. occ v s1 l u = occ v s2 l u
511 (** `permut s1 s2 l u` is true when the segment `s1[l..u-1]` is a
512 permutation of the segment `s2[l..u-1]`. Values outside this range are
515 predicate permut_sub (s1 s2: seq 'a) (l u: int) =
516 seq_eq_sub s1 s2 0 l /\
518 seq_eq_sub s1 s2 u (length s1)
519 (** `permut_sub s1 s2 l u` is true when the segment `s1[l..u-1]` is a
520 permutation of the segment `s2[l..u-1]` and values outside this range
523 predicate permut_all (s1 s2: seq 'a) =
524 length s1 = length s2 /\ permut s1 s2 0 (length s1)
525 (** `permut_all s1 s2` is true when sequence `s1` is a permutation of
528 lemma exchange_permut_sub:
529 forall s1 s2: seq 'a, i j l u: int.
530 exchange s1 s2 i j -> l <= i < u -> l <= j < u ->
531 0 <= l -> u <= length s1 -> permut_sub s1 s2 l u
533 (** enlarge the interval *)
534 lemma Permut_sub_weakening:
535 forall s1 s2: seq 'a, l1 u1 l2 u2: int.
536 permut_sub s1 s2 l1 u1 -> 0 <= l2 <= l1 -> u1 <= u2 <= length s1 ->
537 permut_sub s1 s2 l2 u2
539 (** {3 Lemmas about permut} *)
541 lemma permut_refl: forall s: seq 'a, l u: int.
542 0 <= l <= length s -> 0 <= u <= length s ->
545 lemma permut_sym: forall s1 s2: seq 'a, l u: int.
546 permut s1 s2 l u -> permut s2 s1 l u
549 forall s1 s2 s3: seq 'a, l u: int.
550 permut s1 s2 l u -> permut s2 s3 l u -> permut s1 s3 l u
553 forall s1 s2: seq 'a, l u i: int.
554 permut s1 s2 l u -> l <= i < u ->
555 exists j: int. l <= j < u /\ s1[j] = s2[i]
557 (** {3 Lemmas about permut_all} *)
561 lemma permut_all_mem: forall s1 s2: seq 'a. permut_all s1 s2 ->
562 forall x. mem x s1 <-> mem x s2
564 lemma exchange_permut_all:
565 forall s1 s2: seq 'a, i j: int.
566 exchange s1 s2 i j -> permut_all s1 s2
575 (** `fold_left f a [b1; ...; bn]` is `f (... (f (f a b1) b2) ...) bn` *)
576 let rec function fold_left (f: 'a -> 'b -> 'a) (acc: 'a) (s: seq 'b) : 'a
578 = if length s = 0 then acc else fold_left f (f acc s[0]) s[1 ..]
580 lemma fold_left_ext: forall f: 'b -> 'a -> 'b, acc: 'b, s1 s2: seq 'a.
581 s1 == s2 -> fold_left f acc s1 = fold_left f acc s2
583 lemma fold_left_cons: forall s: seq 'a, x: 'a, f: 'b -> 'a -> 'b, acc: 'b.
584 fold_left f acc (cons x s) = fold_left f (f acc x) s
586 let rec lemma fold_left_app (s1 s2: seq 'a) (f: 'b -> 'a -> 'b) (acc: 'b)
587 ensures { fold_left f acc (s1 ++ s2) = fold_left f (fold_left f acc s1) s2 }
588 variant { Seq.length s1 }
589 = if Seq.length s1 > 0 then fold_left_app s1[1 ..] s2 f (f acc (Seq.get s1 0))
598 (** `fold_right f [a1; ...; an] b` is `f a1 (f a2 (... (f an b) ...))` *)
599 let rec function fold_right (f: 'b -> 'a -> 'a) (s: seq 'b) (acc: 'a) : 'a
601 = if length s = 0 then acc
602 else let acc = f s[length s - 1] acc in fold_right f s[.. length s - 1] acc
604 lemma fold_right_ext: forall f: 'a -> 'b -> 'b, acc: 'b, s1 s2: seq 'a.
605 s1 == s2 -> fold_right f s1 acc = fold_right f s2 acc
607 lemma fold_right_snoc: forall s: seq 'a, x: 'a, f: 'a -> 'b -> 'b, acc: 'b.
608 fold_right f (snoc s x) acc = fold_right f s (f x acc)
612 (*** TODO / TO DISCUSS
614 - what about s[i..j] when i..j is not a valid range?
615 left undefined? empty sequence?
617 - what about negative index e.g. s[-3..] for the last three elements?
619 - a syntax for cons and snoc?
621 - create: better name? move to a separate theory?
623 - UNPLEASANT: we cannot write s[1..] because 1. is recognized as a float
624 so we have to write s[1 ..]
626 - UNPLEASANT: when using both arrays and sequences, the lack of overloading
627 is a pain; see for instance vstte12_ring_buffer.mlw