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Chapter 1. What is functional programming?

Functional programming (FP) is based on a simple premise with far-reaching implications: we construct our programs using only pure functions—in other words, functions that have no side effects. What are side effects? A function has a side effect if it does something other than simply return a result, for example:

  • Modifying a variable
  • Modifying a data structure in place
  • Setting a field on an object
  • Throwing an exception or halting with an error
  • Printing to the console or reading user input
  • Reading from or writing to a file
  • Drawing on the screen

We’ll provide a more precise definition of side effects later in this chapter, but consider what programming would be like without the ability to do these things, or with significant restrictions on when and how these actions can occur. It may be difficult to imagine. How is it even possible to write useful programs at all? If we can’t reassign variables, how do we write simple programs like loops? What about working with data that changes, or handling errors without throwing exceptions? How can we write programs that must perform I/O, like drawing to the screen or reading from a file?

1.1. The benefits of FP: a simple example

1.2. Exactly what is a (pure) function?

1.3. Referential transparency, purity, and the substitution model

1.4. Summary

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