Functions

Side Effects

An externally observable effect of a function is called a side effect.
- Displaying characters on the screen (including error messages).
- Updating variables outside the functions.
- Terminating the program.
A function should leave no trace of its existence except for returning a value.
Functions that print values become worthless in certain situations.
- Graphics programs that have no output streams.
- Programs where the user's language is not English.
- Programs that don't want to print the value!
Ideally, a function computes a single value and has no other side effect.