Small units with one purpose.
WHAT A FUNCTION SHOULD DO
One thing, at one level of abstraction.
WHAT ONE LEVEL MEANS
Not mixing high-level orchestration with low-level detail in the same function.
WHAT LENGTH SUGGESTS A PROBLEM
Anything that does not fit on a screen, usually.
WHY
It becomes difficult to hold in mind.
WHAT TO DO ABOUT A LONG FUNCTION
Extract the parts that have a name.
WHY THAT TEST
If a block can be named, it is a separate responsibility.
WHAT TO LIMIT
Parameters. Many parameters indicate too many responsibilities.
WHAT TO AVOID
Boolean parameters changing behaviour Output parameters Functions that return a value and also change state, without saying so
WHAT TO PREFER
Early returns for exceptional cases.
WHAT THAT ACHIEVES
The main path at one level of indentation.
WHAT NESTING DEPTH SUGGESTS
Beyond two or three levels, something should be extracted.
WHAT TO MAKE EXPLICIT
What the function requires, and what it guarantees.
WHAT TO AVOID ENTIRELY
Functions that modify global state invisibly.
