Cs776 (Prasad)L16rem1 Remaining Features of SML97 Records Exceptions Reference Types Arrays

cs776 (Prasad) L16rem 1

Remaining Features of SML97

• Records

• Exceptions

• Reference Types

•Arrays


Records

type component = { key : int, info : string };

val part = { key = 3, info = “nail” }; (* val part = {info="nail",key=3} : {info:string, key:int} *)

#key part = 3;

part = {info = “na” ^ “il”, key = 3-0 }; (* val it = true : bool *)

• Order of fields is irrelevant when matching.

(2, “two”) = { 1 = 2, 2 = “two” };• Tuples are special records with numbers as field labels.


Pattern Matching for Records

fun info_compare (p1:component) (p2:component) = String.compare (#info p1, #info p2);

fun key_compare ({key=k1,info=_}:component) ({key=k2, ... }:component) = Int.compare (k1, k2);

(* Wildcards : Type name mandatory with ellipses.

*)

fun check_info (c as {key,info}:component) = (#info c = info) ; (* Using a field name to refer to value *)

(* maps a component to true *)


Exceptions

• An exception name is a constructor of the built-in type exn. This datatype is special in that the set of constructors can be extended.

- exception Overflow;- exception message of string;- exception outOfRange of int*int;- Overflow; (* val it = Overflow(-) : exn *)

- message; (* val it = fn : string -> exn *)

- outOfRange; (* val it = fn : int * int -> exn *)

Predefined exceptions : Div, Empty, Match,... etc.


Operations on exception

raiseraise exn-name;

expression handlehandle exn_name1 => … | exn_name2 => … ;

Organization

“Trigger” exception

Handle exception

Raise exception

Define exception

CLIENTCLIENT SERVERSERVER


Motivation for incorporating exception• Robustness

– Language/application specified behavior on error

• Context-sensitive handling– Retry

• to overcome transient errors

– Attempt• another solution strategy (heuristic) on failure

– Rescue• reinstate any required invariant (consistency)

• Dynamic Handling via run-time call stack


Examples

exception fail of string;

fun search name [] = raise (fail name) | search name ((x,v)::xvs) = if (name = x) then v else search name xvs;

datatype employers = wsu | wpafb | ncr | lexis_nexis ;

val name_emp = [(“john”,wsu),(“jane”,ncr)];

(search “jill” name_emp) handle (fail elem) => wpafb;

Server

Client


(cont’d)

val name_car = [(“john”, “acura”),(“jane”, “lexus”)];

(search “jill” name_car) handle (fail elem) => elem ^ “ owns Dodge Stealth!”;

val name_tel = [(“john”,8501),(“jane”,1359)];

(search “jill” name_tel) handle (fail elem) =>

elem ^ “’s phone number is unlisted.”;

(* type error *)


References and Assignments


• (ML) Redefinition val x = [5]; val x = 2;

(* The first list is inaccessible (garbage). *)• (ML) Shadowing

let val x = 5 in (let val x = [2] in (hd x) end) + x end;

• (Pascal) Assignment var i: int; begin i := 0; i := i + 1 end;

Redefinition and shadowing are different from assignment.


• ML supports a separate sub-language to deal with variables and assignments (that can change value bound to a variable).

val x = ref 5;

x := 6; x := 1 + !x;

l-value r-value (address of location) (contents of location)

5

References

x


Iteration : Pascal vs MLvar i, sum : int := 0;

var n : int := 5; while i < n do begin i := i + 1; sum := sum + i

end;

The l-value of a variable on the rhs of an assignment is automatically coerced to its r-value.

val i = ref 0;val sum = ref 0;val n = 5; while !i < n do ( i := !i + 1 ; sum := !sum + !i );

A reference variable on the rhs of an assignment must be explicitly dereferenced, to get its r-value.


Aliasing problemfun rot3 (a,b,c) =

let val t = !a in a := !b; b := !c; c := t end;

val i = ref 0; val j = ref 1; val k = ref 2;

(!i,!j,!k); rot3 (i,j,k); (!i,!j,!k);

(* (0,1,2) , () : unit, (1,2,0) *)

val i = ref 0; val j = ref 1; val k = ref 2;

(!i,!j,!i); rot3 (i,j,i); (!i,!j,!i);

(* (0,1,0) , () : unit, (0,1,0) *)


Encapsulating State : Objectsfun new_counter () = let val cnt = ref 0 fun tick () = ( cnt := !cnt + 1; !cnt ) fun reset () = ( cnt := 0 )in { tick = tick, reset = reset }end;

val c1 = new_counter();val c2 = new_counter();

(#tick c1) (); (* 1 *)(#tick c2) (); (* 1 *)(#tick c1) (); (* 2 *)(#tick c2) (); (* 2 *)

(#reset c1) (); (#tick c1) (); (* 1 *)(#tick c2) (); (* 3 *)

(* Object-based programming *)


Mutable Data Structure :structure Arrayopen Array;val n = 3;val M =

array(n, array(n,0)); (* Constructs array whose top-

level elements share the same inner array. *)

val i = ref 0;while (!i < n) do ( update(M,!i,array(n,0)); i := !i + 1); (* creates n x n locations

initialized to 0 *)

i := 0;val j = ref 0;while (!i < n) do ( while (!j < n) do ( update(sub(M,!i),!j,!i + !j); j := !j + 1 ); j := 0; i := !i + 1);

(* initializes M[i,j] to i+j. *)(* M = [|[|0,1,2|],[|1,2,3|],[|

2,3,4|]|] : int array array *)


Interaction with polymorphism

• Normally, variables/functions can be polymorphicpolymorphic, but not values.

fun id x = x;id 5;( (fn x => (id x)) 5 );

( (fn f => (f 5)) id );(* (fn f => (f 5)) : (int -> 'a) -> 'a *)

(id 5, id true);

( (fn f => (f 5, f true)) id ); (* Type error *)


• Polymorphic references are banned.

val fp = ref id; (* potential type:

’a->’a ref *)

fp := not;!fp 5;

• Polymorphic exceptions are banned.

exception fail of ’a;

(raise fail true) handle

(fail x) => 0 * x;

Value Restriction for polymorphism

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Cs776 (Prasad)L16rem1 Remaining Features of SML97 Records Exceptions Reference Types Arrays