Side Effects

A function or expression is said to have a side effect:

• If it modifies some state
• Has an observable interaction with calling functions or the outside world

For example, a particular function might:

• Modify a global variable or static variable
• Modify one of its arguments
• Raise an exception
• Write data to a display or file
• Read data
• Call other side-effecting functions

Simply, side effect = changing something somewhere.

Problems with Side Effects

In the presence of side effects, a program’s behavior may depend on history i.e., the order of evaluation.

Understanding and debugging a function with side effects requires knowledge about the context and its possible histories.