Effective Haskell

Chapter 3Getting Started with Types

Although a formal definition of types is outside of the scope of this book, informally you can think of a type as a way of identifying some collection of different values. For example, the Bool type is a way of naming the set of values True and False; similarly, the Integer type is a way of naming the set of all the whole numbers. To be precise, you can say the values True and False inhabit the type Bool, or they are its inhabitants, although informally throughout this book we’ll simply say “True is a Bool.”

Every value has a type, whether it’s a number, some text, or a function. The way the value was created doesn’t matter; a literal value of True defined at the top level of the program, returned by a function, ...