Functional Programming

Functional programming is a restriction on how we write programs, not on what programs we can express.

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