OCaml pattern matching Tuples and lists CS235 Languages and Automata Department of Computer Science Wellesley College Lists in Java and OCaml Java OCaml empty() [] preprend(x, ys) x :: ys head(xs) List.hd(xs) tail(xs) List.tl(xs) isEmpty(xs) xs = [] *Note that [exp1; exp2; … ; expn] is an OCaml abbreviation for exp1 :: exp2 :: … :: expn :: []. Pattern matching 8-2 1 Summing an IntList in Java // Returns the integer sum of all element in ns public static int sumList (IntList ns) { if (IntList.isEmpty(ns) { return 0; } else { return IntList.head(ns) + sum(IntList.tail(ns));; } } s1 91 0 -22 Ø Pattern matching 8-3 Summing an integer list in OCaml # let rec sum ns = if ns = [] then 0 else List.hd(ns) + (sum (List.tl ns));; Pattern matching 8-4 2 Summing an integer list in OCaml # let rec sum ns = match ns with [] -> 0 | (n :: ns’) -> n + sum(ns’);; # sum [];; # sum [3];; # sum [3;2;7;5];; Pattern matching 8-5 range lo hi # range 3 7;; _: int list = [3;4;5;6;7] # range 5 5;; _: int list = [5] # range 6 5;; _: int list = [] *Returns a list of integers from lo up to hi, inclusive. The list is empty is lo > hi. Pattern matching 8-6 3 squares ns # squares [3;1;5;4;2];; _: int list = [9; 1; 25; 16; 4] # squares [5];; _: int list = [25] # squares [];; _: int list = [] # squares (range 3 6);; _: int list = [9; 16; 25; 36] Pattern matching 8-7 map f # let square x = x*x;; val square : int -> int = <fun> # square 3;; _: int = 9 # let rec map f = function [] -> [] | x :: xs -> f x :: map f xs;; val map : ('a -> 'b) -> 'a list -> 'b list = <fun> # map square (range 3 6);; - : int list = [9; 16; 25; 36] Pattern matching 8-8 4 isSorted xs # isSorted [];; _: bool = true # isSorted [3];; _: bool = true # isSorted [3;1;4;2];; _: bool = false # isSorted [1;2;3;4;5;6];; _: bool = true Pattern matching 8-9 Tuples # let t4 = (2 * 3, 4 < 5, "foo" ^ "bar", String.get "baz" 2);; # let w,x,y,z = t4;; # let first (x,_,_,_) = x;; # let last (_,_,_,z) = z;; Pattern matching 8-10 5 OCaml simplicity # let swap (a,b) = (b,a);; # swap (1+2,3=4);; # swap(swap(1+2,3=4));; Pattern matching 8-11 isMember (x,ys) # isMember(3, [5;2;3;1;4]);; _: bool = true # isMember(6, [5;2;3;1;4]);; _: bool = false # isMember(“be”, [“to”;”be”;”or”;”not”;”to”;”be”]);; _: bool = true # isMember((2,”too”),[(3,”three”);(1,”one”);(2,”two”)]);; _: bool = false Pattern matching 8-12 6.