Programming Language Concepts (CIS 635)

Elsa L Gunter

4303 GITC

NJIT, www.cs.njit.edu/~elsa/635

Tuples

- val s = (5,"hi",3.2);

val s = (5,"hi",3.2) : int * string * real

- (* Pattern matching with tuples *)

- val (a,b,c) = s;

val a = 5 : int

val b = "hi" : string

val c = 3.2 : real

Tuples

- val d = ((1,4,62),("bye",15),73.95);

val d = ((1,4,62),("bye",15),73.95) : (int * int * int) * (string * int) * real

- val (p,(st,_),_) = d;

val p = (1,4,6) : int * int * int

val st = "bye" : string

Tuples

- fun fst_of_3 (x,_,_) = x;val fst_of_3 = fn : 'a * 'b * 'c -> 'a- s;val it = (5,"hi",3.2) : int * string * real- fst_of_3 s;val it = 5 : int- fst_of_3 d;val it = (1,4,62) : int * int * int

Records

- val teacher = {Name = "Elsa L. Gunter", ss = (119,73,6244), age = 102};

val teacher = {Name="Elsa L. Gunter",age=102,ss=(119,73,6244)}

: {Name:string, age:int, ss:int * int * int}

Records

- val {ss = (s1,s2,s3), Name = elsa, age = years} = teacher;

val elsa = "Elsa L. Gunter" : string

val years = 102 : int

val s1 = 119 : int

val s2 = 73 : int

val s3 = 6244 : int

Records and Tuples

- val q = (280,{student = {Name = "Joseph Martins",

= ss = (325,40,1276),

= age = 19},

= instructor = teacher});

Records and Tuples

val q = (280, {instructor={Name="Elsa L. Gunter",

age=102,ss=(119,73,6244)}, student={Name="Joseph Martins",

age=19,ss=(325,40,1276)}}) : int * {instructor:{Name:string, age:int, ss:int * int *

int}, student:{Name:string, age:int, ss:int * int *

Tuples are Records

- val strange = (1,"f",2) = {3 = 2, 2 = "f", 1 = 1};;

val strange = true : bool

- val h = #2 s;

val h = "hi" : string

- val fib5 = [8,5,3,2,1,1];val fib5 = [8,5,3,2,1,1] : int list- val fib6 = 13 :: fib5;val fib6 = [13,8,5,3,2,1,1] : int list- (8::5::3::2::1::1::[]) = fib5;val it = true : bool- fib5 @ fib6;val it = [8,5,3,2,1,1,13,8,5,3,2,1,...] : int list

Functions Over Lists

- fun double_up [] = []

= | double_up (x :: xs) = (x :: x :: double_up xs);

val double_up = fn : 'a list -> 'a list

- val fib5_2 = double_up fib5;

val fib5_2 = [8,8,5,5,3,3,2,2,1,1,1,1] : int list

- val silly = double_up ["hi", "there"];

val silly = ["hi","hi","there","there"] : string list

- fun rev [] = []

= | rev (x::xs) = rev xs @ [x];

val rev = fn : 'a list -> 'a list

- rev silly;

val it = ["there","there","hi","hi"] : string list

- fun map f [] = []

= | map f (h::t) = (f h) :: (map f t);

val map = fn : ('a -> 'b) -> 'a list -> 'b list

- map plus_two fib5;

val it = [10,7,5,4,3,3] : int list

- map (fn x => x - 1) fib6;

val it = [12,7,4,2,1,0,0] : int list

Cases and Scoped Bindings

- fun fib_seq n == if n < 0 then [] else= (case fib_seq (n - 1) = of [] => [1]= | (_::[]) => [1,1]= | result as (m1 :: m2 :: _)= => (m1 + m2) :: result);val fib_seq = fn : int -> int list

Cases and Scoped Bindings

- fib_seq 6;

val it = [13,8,5,3,2,1,1] : int list

User-defined Datatypes

- datatype weekday = Monday | Tuesday | Wednesday

= | Thursday | Friday | Saturday | Sunday;

datatype weekday

Functions Over Datatypes

Functions Over Datatypes

- fun days_later 0 day = day= | days_later n day == if n > 0= then day_after (days_later (n - 1) day)= else days_later (n + 7) day;val days_later = fn : int -> weekday -> weekday- days_later ~1 Wednesday;val it = Tuesday : weekday- days_later ~4 Monday;val it = Thursday : weekday

More Datatypes

- datatype Int_Bin_Tree =

= Leaf of int | Node of (Int_Bin_Tree * Int_Bin_Tree);

datatype Int_Bin_Tree = Leaf of int | Node of Int_Bin_Tree * Int_Bin_Tree

- datatype 'a option = NONE | SOME of 'a;

datatype 'a option = NONE | SOME of 'a

Datatype Values

- val bin_tree = Node(Node(Leaf 3, Leaf 6),Leaf ~7);

val bin_tree = Node (Node (Leaf #,Leaf #),Leaf ~7) : Int_Bin_Tree

Functions over Datatypes

- fun first_leaf_value (Leaf n) = n

= | first_leaf_value (Node (left_tree, right_tree)) =

= first_leaf_value left_tree;

val first_leaf_value = fn : Int_Bin_Tree -> int

- val first = first_leaf_value bin_tree;

val first = 3 : int

- fun is_neg n = n < 0;val is_neg = fn : int -> bool

- fun first_neg (Leaf n) == if is_neg n then SOME n else NONE= | first_neg (Node(t1,t2)) == (case first_neg t1= of SOME n => SOME n= | NONE => first_neg t2);

val first_neg = fn : Int_Bin_Tree -> int option

- val n = first_neg bin_tree;

val n = SOME ~7 : int option

Additional Topics

Topics that follow won’t be covered in class

You won’t be tested on them, or need them for homework. They are here in case you are interested

Scoped Bindings

- val fib7 =

= let val (m1::m2::_) = fib6

= in (m1 + m2) :: fib6

= end;

stdIn:26.7-26.29 Warning: binding not exhaustive

m1 :: m2 :: _ = ...

val fib7 = [21,13,8,5,3,2,1,1] : int list

Scoped Bindings

- local= fun fib_pair 0 = (0,1)= | fib_pair n == if n < 0 then (0,0) else= let val (m1,m2) = fib_pair (n -1)= in (m2, m1+m2)= end= in

Scoped Bindings

= fun fib n = #2 (fib_pair n)

= end;

val fib = fn : int -> int

- fib 7;

val it = 21 : int

Exceptions

- exception Cant_handle of string;exception Cant_handle of string- fun head [] = raise Cant_handle "head"= | head (h::tl) = h= fun tail [] = raise Cant_handle "tail"= | tail (_::tl) = tl;val head = fn : 'a list -> 'aval tail = fn : 'a list -> 'a list

Exceptions

- fun fib_seq 0 = [1]

= | fib_seq 1 = [1,1]

= | fib_seq n =

= if n < 0 then raise Cant_handle "fib_seq"

= else

= let val result as (m1::m2::_) = fib_seq (n - 1)

= in (m1 + m2)::result

= end;

Exceptions

stdIn:130.6-130.49 Warning: binding not exhaustive

result as m1 :: m2 :: _ = ...

val fib_seq = fn : int -> int list

- fib_seq (~15);

uncaught exception Cant_handle

raised at: stdIn:128.25-128.46

Exceptions

- fib_seq (~2) handle Cant_handle _ => [];

val it = [] : int list

- fun new_fib_seq n = fib_seq n handle Cant_handle "fib_seq" => [];

val new_fib_seq = fn : int -> int list

- new_fib_seq (~3);

val it = [] : int list

Exceptions

- exception Cant_happen;

exception Cant_happen

- fun fib_seq 0 = [1]

= | fib_seq 1 = [1,1]

= | fib_seq n =

= if n < 0 then raise Cant_handle "fib_seq"

Exceptions

= else

= (case fib_seq (n - 1) of

= result as (r1 :: r2 :: _) => r1 + r2 :: result

= | _ => raise Cant_happen);

val fib_seq = fn : int -> int list

Side Effects and Imperative Programming

- local

= val cell = ref 0

= fun get_count () =

= let val count = !cell

= val _ = cell := count + 1

= in count

= end;

val get_count = fn : unit -> int- get_count();val it = 0 : int- get_count();val it = 1 : int- get_count();val it = 2 : int- (* Example of an integer stream *)

- local= val cell = ref (~1)= in = fun get_count () == (cell := ((!cell) + 1); = !cell)= end;val get_count = fn : unit -> int

- get_count();

val it = 0 : int

- get_count();

val it = 1 : int

- get_count();

val it = 2 : int

- local

= val cell = ref (~1)

= fun get_count () =

= let val _ = cell := !cell + 1

= in !cell

= end;

val get_count = fn : unit -> int

Programming Language Concepts (CIS 635)

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