Polymorphism Chapter Eight Modern Programming Languages 1 Introduction

Introduction Compare these function types n The ML function is more flexible, since it

Polymorphism Functions with that extra flexibility are called polymorphic n A difficult word to

Outline Overloading n Parameter coercion n Parametric polymorphism n Subtype polymorphism n Definitions and

Overloading An overloaded function name or operator is one that has at least two

Predefined Overloaded Operators ML: Pascal: Chapter Eight val x = 1 + 2; val

Adding to Overloaded Operators n Some languages, like C++, allow additional meanings to be

Defining Overloaded Functions n Some languages, like C++, permit the programmer to overload function

Implementing Overloading: Example C++: int shazam(int a, int b) {return a+b; } double shazam(double

Coercion n A coercion is an implicit type conversion, supplied automatically even if the

Parameter Coercion n When a language supports coercion of parameters on a function call

Example: Java void f(double x) { … } f((byte) 1); f((short) 2); f('a'); f(3);

Parametric Polymorphism A function exhibits parametric polymorphism if it has a type that contains

Example: C++ Function Templates template<class X> X max(X a, X b) { return a>b

Example: ML Functions - fun identity x = x; val identity = fn :

Conclusion n We have seen polymorphic functions A function or operator is polymorphic if

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Polymorphism Chapter Eight Modern Programming Languages 1

Introduction Compare these function types n The ML function is more flexible, since it can be applied to any pair of the same (equality-testable) type n Chapter Eight C: int f(char a, char b) { return a==b; } ML: - fun f(a, b) = (a = b); val f = fn : ''a * ''a -> bool Modern Programming Languages 2

Polymorphism Functions with that extra flexibility are called polymorphic n A difficult word to define: n – – – Chapter Eight Applies to a wide variety of language features Most languages have at least a little We will examine four major examples, then return to the problem of finding a definition that covers them Modern Programming Languages 3

Outline Overloading n Parameter coercion n Parametric polymorphism n Subtype polymorphism n Definitions and classifications n Chapter Eight Modern Programming Languages 4

Overloading An overloaded function name or operator is one that has at least two definitions, all of different types n Many languages have overloaded operators n Some also allow the programmer to define new overloaded function names and operators n Chapter Eight Modern Programming Languages 5

Predefined Overloaded Operators ML: Pascal: Chapter Eight val x = 1 + 2; val y = 1. 0 + 2. 0; a b c d : = : = 1 + 2; 1. 0 + 2. 0; "hello " + "there"; ['a'. . 'd'] + ['f'] Modern Programming Languages 6

Adding to Overloaded Operators n Some languages, like C++, allow additional meanings to be defined for operators class complex { double rp, ip; // real part, imaginary part public: complex(double r, double i) {rp=r; ip=i; } friend complex operator+(complex, operator*(complex, complex); }; Chapter Eight void f(complex a, complex b, complex d = a + b * c; … } Modern Programming Languages complex c) { 7

Defining Overloaded Functions n Some languages, like C++, permit the programmer to overload function names int square(int x) { return x*x; } double square(double x) { return x*x; } Chapter Eight Modern Programming Languages 8

Implementing Overloading: Example C++: int shazam(int a, int b) {return a+b; } double shazam(double a, double b) {return a+b; } Assembler: Chapter Eight shazam__Fii: lda $30, -32($30). frame $15, 32, $26, 0 … shazam__Fdd: lda $30, -32($30). frame $15, 32, $26, 0 … Modern Programming Languages 9

Outline Overloading n Parameter coercion n Parametric polymorphism n Subtype polymorphism n Definitions and classifications n Chapter Eight Modern Programming Languages 10

Coercion n A coercion is an implicit type conversion, supplied automatically even if the programmer leaves it out double x; conversion in Java: x = (double) 2; Explicit type Coercion in Java: Chapter Eight double x; x = 2; Modern Programming Languages 11

Parameter Coercion n When a language supports coercion of parameters on a function call (or of operands when an operator is applied), the resulting function (or operator) is polymorphic Chapter Eight Modern Programming Languages 12

Example: Java void f(double x) { … } f((byte) 1); f((short) 2); f('a'); f(3); f(4 L); f(5. 6 F); Chapter Eight This f can be called with any type of parameter Java is willing to coerce to type double Modern Programming Languages 13

Outline Overloading n Parameter coercion n Parametric polymorphism n Subtype polymorphism n Definitions and classifications n Chapter Eight Modern Programming Languages 14

Parametric Polymorphism A function exhibits parametric polymorphism if it has a type that contains one or more type variables n A type with type variables is a polytype n Found in languages including ML, C++ and Ada n Chapter Eight Modern Programming Languages 15

Example: C++ Function Templates template<class X> X max(X a, X b) { return a>b ? a : b; } void g(int a, int b, char c, char d) { int m 1 = max(a, b); char m 2 = max(c, d); } Note that > can be overloaded, so X is not limited to types for which > is predefined. Chapter Eight Modern Programming Languages 16

Example: ML Functions - fun identity x = x; val identity = fn : 'a -> 'a - identity 3; val it = 3 : int - identity "hello"; val it = "hello" : string - fun reverse x = = if null x then nil = else (reverse (tl x)) @ [(hd x)]; val reverse = fn : 'a list -> 'a list Chapter Eight Modern Programming Languages 17

Conclusion n We have seen polymorphic functions A function or operator is polymorphic if it has at least two possible types Overloading – n Each different type requires a separate definition Parameter coercion – Chapter Eight different types can be coerced to a given parameter type Modern Programming Languages 18