All programmers know that different data types can have common attributes. Apples and oranges may not be comparable, but they both have weight, can be peeled, and so on. And if you had a basket of fruit you might want to count the fruit or sort items by weight. Even though the basket could be full of apples, oranges or even bananas, the same operations apply.

This month's lecture covers parametric polymorphism and type classes. We will see how parametricity provides strong guarantees about the behaviour of a function, and how type classes can be used to define functions that apply to disparate types. Type classes will be compared to classes in other languages, and we will meet some of Haskell's common type classes and learn how to use them with our types, as well as how to define new type classes.

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