Reusable units.
WHAT A FUNCTION IS
A named unit taking inputs and producing a result.
WHAT DIFFERS BETWEEN LANGUAGES
Whether arguments are passed by value or by reference Whether default values are supported Whether arguments can be named Whether a variable number is permitted
WHY PASSING MATTERS
Passing by reference means the function can change the caller's value.
WHAT THAT CAUSES WHEN MISUNDERSTOOD
Functions modifying data the caller did not expect to change.
WHAT MANY LANGUAGES ACTUALLY DO
Pass references by value, which behaves differently from both simple descriptions.
WHAT THAT MEANS PRACTICALLY
Reassigning the parameter does not affect the caller; modifying what it points to does.
WHAT MAKES A GOOD FUNCTION
One responsibility A name saying what it does Few parameters No dependence on hidden state Predictable output for given input
WHAT TO AVOID
Many parameters, which indicates too many responsibilities Boolean parameters changing behaviour Functions modifying things outside themselves without saying so
WHAT HIGHER-ORDER FUNCTIONS ARE
Functions taking or returning functions.
WHAT THEY ENABLE
Expressing transformation and filtering declaratively.
