"Object Oriented" Programming using ML

Transcription

1 "Object Oriented" Programming using ML Stavros Aronis (based on slides by Kostis Sagonas)

2 OOP Terminology Class Inheritance Encapsulation Polymorphism

3 Java example public class Node { private String data; private Node link; public Node(String thedata, Node thelink) { data = thedata; link = thelink; } public String getdata() { return data; } public Node getlink() { return link; } }

4 Node class Two attributes: data and link Constructor Methods: getdata and getlink

5 Node class Two attributes: data and link Constructor Methods: getdata and getlink On an abstract level, an "object" accepts two types of messages (i.e. "get data" and "get link") and returns a response (e.g. a string or some other value).

6 Node class Two attributes: data and link Constructor Methods: getdata and getlink On an abstract level, an "object" accepts two types of messages (i.e. "get data" and "get link") and returns a response (e.g. a string or some other value). We could describe this behaviour as a function with type: message -> response

7 Object Oriented Programming in ML We can implement this "idea" in ML First we define types for message and response : datatype message = GetData GetLink; datatype response = Data of string Object of message -> response;

8 The constructor is a call to node, passing only attributes. Object Oriented Programming in ML We can implement this "idea" in ML First we define types for message and response : datatype message = GetData GetLink; datatype response = Data of string Object of message -> response; Putting it together: fun node data link GetData = Data data node data link GetLink = Object link;

9 Example of usage - val n1 = node "Hello" null; val n1 = fn : message -> response - val n2 = node "world" n1; val n2 = fn : message -> response - n1 GetData; val it = Data "Hello" : response - n2 GetData; val it = Data "world" : response null should also have the message -> response type e.g: fun null _ = Data "null"

10 Stack class One attribute: top Three methods: empty, push, pop Implemented as a list of node objects.

11 Extending the definitions: datatype message = IsNull GetData GetLink IsEmpty Pop Push of string; datatype response = Data of string Object of message -> response Pred of bool Popped of (message -> response) * string; Base "class" for inheritance: fun root _ = Pred false

12 fun null IsNull = Pred true null message = root message; fun node data link GetData = Data data node data link GetLink = Object link node data link message = root message; fun stack top IsEmpty = let val Pred p = top IsNull in Pred (not p) end stack top (Push data) = Object (stack (node data top)) stack top Pop = let val Object new = top GetLink val Data data = top GetData in Popped (stack new, data) stack _ message = root message;

13 - val a = stack null; val a = fn : message -> response - val Object(b) = a (Push "design"); val b = fn : message -> response - val Object(c) = b (Push "oriented "); val c = fn : message -> response - val Object(d) = c (Push "Object "); val d = fn : message -> response - val Popped(e,s1) = d Remove; val e = fn : message -> response val s1 = "Object " : string - val Popped(f,s2) = e Pop; val f = fn : message -> response val s2 = "oriented " : string - val Popped(_,s3) = f Pop; val s3 = "design." : string - s1^s2^s3; val it = "Object oriented design" : string

14 Inheritance (... or almost!) Extending stack with a peekable stack: fun peekablestack top Peek = top GetData peekablestack top message = stack top message;

15 Inheritance (... or almost!) Extending stack with a peekable stack: fun peekablestack top Peek = top GetData peekablestack top message = stack top message; This style of programming is similar with the one used in Smalltalk: Message-passing If a class does not handle some messages, they are forwarded to a super-class.

16 Concluding remarks This might not be the best way to use ML: Loss of correspondence between message and response types. Lots of "binding not exhaustive" warnings... Still, even in ML we can do Object Oriented programming. Object Oriented programming does NOT require a language with inherent support for it. Objective CAML: ML dialect with OO extensions.

17 Concluding remarks This might not be the best way to use ML: Loss of correspondence between message and response types. Lots of "binding not exhaustive" warnings... Still, even in ML we can do Object Oriented programming.