1
CMSC 330: Organization of
Programming Languages
Objects vs. Functional
Programming
CMSC 330 2
OOP vs. FP
Object-oriented programming (OOP)
•Computation as interactions between objects
•Objects encapsulate mutable data (state)
¾Accessed / modified via object’s public methods
Functional programming (FP)
•Computation as evaluation of functions
¾Mutable data used to improve efficiency
•Higher-order functions implemented as closures
¾Closure = function + environment
CMSC 330 3
An Integer “Stack” Abstraction in Java
class Stack {
class Node {
Integer val; Node next;
Node(Integer v, Node n) { val = v; next = n; }
};
private Node theStack;
void push(Integer v) {
theStack = new Node(v, theStack);
}
Integer pop() {
if (theStack == null)
throw new NoSuchElementException();
Integer temp = theStack.val;
theStack = theStack.next;
return temp;
}
}
CMSC 330 4
A “Stack” Abstraction in OCaml
module type STACK =
sig
type 'a stack
val new_stack : unit -> 'a stack
val push : 'a stack -> 'a -> unit
val pop : 'a stack -> 'a
end
module Stack : STACK =
struct
type 'a stack = 'a list ref
let new_stack () = ref []
let push s x = s := (x::!s)
let pop s = match !s with
[] -> failwith "Empty stack"
| (h::t) -> s := t; h
end
CMSC 330 5
Another “Stack” Abstraction in OCaml
let new_stack () =
let this = ref [] in
let push x = this := (x::!this)
and pop () = match !this with
[] -> failwith "Empty stack"
| (h::t) -> this := t; h
in
(push, pop)
# let s = new_stack ();;
val s : ('_a -> unit) * (unit -> '_a) = (<fun>, <fun>)
# Pervasives.fst s 3;; (* applies 1st part of s to 3 *)
- : unit = ()
# Pervasives.snd s ();; (* applies 2nd part of s to () *)
-: int= 3
CMSC 330 6
Two OCaml Stack Implementations
1st implementation (OOP style)
•Based on modules
•Specifies methods for
¾Creating stack
¾Pushing value onto stack parameter
¾Popping value from stack parameter
2nd implementation (FP style)
•Based on closures
•Creating stack returns tuple containing
¾Closure for pushing value onto created stack
¾Closure for popping value from created stack
