random_access_list.mlw 3 KB
 Jean-Christophe Filliatre committed May 02, 2015 1 2 `````` (** Random Access Lists. `````` Jean-Christophe Filliatre committed May 20, 2015 3 `````` (Okasaki, "Purely Functional Data Structures", 10.1.2.) `````` Jean-Christophe Filliatre committed May 02, 2015 4 5 6 7 `````` The code below uses polymorphic recursion (both in the logic and in the programs). `````` Jean-Christophe Filliatre committed May 20, 2015 8 `````` Author: Jean-Christophe Filliâtre (CNRS) `````` Jean-Christophe Filliatre committed May 02, 2015 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 `````` *) module RandomAccessList use import int.Int use import int.ComputerDivision use import list.List use import list.Length use import list.Nth use import option.Option type ral 'a = | Empty | Zero (ral ('a, 'a)) | One 'a (ral ('a, 'a)) `````` Jean-Christophe Filliatre committed May 20, 2015 25 `````` function flatten (l: list ('a, 'a)) : list 'a `````` Jean-Christophe Filliatre committed May 02, 2015 26 27 28 29 30 `````` = match l with | Nil -> Nil | Cons (x, y) l1 -> Cons x (Cons y (flatten l1)) end `````` Jean-Christophe Filliatre committed May 20, 2015 31 `````` let rec lemma length_flatten (l:list ('a, 'a)) `````` Martin Clochard committed May 04, 2015 32 33 34 35 36 37 38 `````` ensures { length (flatten l) = 2 * length l } variant { l } = match l with | Cons (_,_) q -> length_flatten q | Nil -> () end `````` Jean-Christophe Filliatre committed May 02, 2015 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 `````` function elements (l: ral 'a) : list 'a = match l with | Empty -> Nil | Zero l1 -> flatten (elements l1) | One x l1 -> Cons x (flatten (elements l1)) end let rec size (l: ral 'a) : int variant { l } ensures { result = length (elements l) } = match l with | Empty -> 0 | Zero l1 -> 2 * size l1 | One _ l1 -> 1 + 2 * size l1 end let rec add (x: 'a) (l: ral 'a) : ral 'a variant { l } ensures { elements result = Cons x (elements l) } = match l with | Empty -> One x Empty | Zero l1 -> One x l1 | One y l1 -> Zero (add (x, y) l1) end let rec lemma nth_flatten (i: int) (l: list ('a, 'a)) requires { 0 <= i < length l } variant { l } ensures { match nth i l with | None -> false | Some (x0, x1) -> Some x0 = nth (2 * i) (flatten l) /\ Some x1 = nth (2 * i + 1) (flatten l) end } = match l with | Nil -> () | Cons _ r -> if i > 0 then nth_flatten (i-1) r end let rec get (i: int) (l: ral 'a) : 'a requires { 0 <= i < length (elements l) } variant { i, l } ensures { nth i (elements l) = Some result } = match l with | Empty -> absurd | One x l1 -> if i = 0 then x else get (i-1) (Zero l1) | Zero l1 -> let (x0, x1) = get (div i 2) l1 in if mod i 2 = 0 then x0 else x1 end end `````` Jean-Christophe Filliatre committed May 02, 2015 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 `````` (** A straightforward encapsulation with a list ghost model (in anticipation of module refinement) *) module RAL use import int.Int use import RandomAccessList as R use import list.List use import list.Length use import option.Option use import list.Nth type t 'a = { r: ral 'a; ghost l: list 'a } invariant { self.l = elements self.r } let empty () : t 'a ensures { result.l = Nil } = { r = Empty; l = Nil } let size (t: t 'a) : int ensures { result = length t.l } = size t.r let cons (x: 'a) (s: t 'a) : t 'a ensures { result.l = Cons x s.l } = { r = add x s.r; l = Cons x s.l } let get (i: int) (s: t 'a) : 'a requires { 0 <= i < length s.l } ensures { Some result = nth i s.l } = get i s.r end ``````