Programming Fundamentals › Functional Programming

First-Class Functions

Functions treated as values that can be stored, passed and returned.

Also known as: first-class functions, functions as values, functions as first class citizens

A language has first-class functions when functions are values like any other: you can store them in variables, pass them as arguments, return them from other functions, and keep them in lists.

def shout(text):
    return text.upper() + "!"

f = shout                         # store a function in a variable
f("hi")                           # "HI!"

def apply_twice(func, x):         # pass a function as an argument
    return func(func(x))

apply_twice(shout, "hi")          # "HI!!"

def make_adder(n):                # return a function
    return lambda x: x + n

add5 = make_adder(5)
add5(10)                          # 15

handlers = {"upper": str.upper, "lower": str.lower}     # functions in a dictionary
handlers["upper"]("abc")

Python, JavaScript, Go, Rust and most modern languages work this way.

Why it’s powerful

• Callbacks: tell a library what to do when something happens (button.onclick = save).
• Strategy choices without classes: pick behavior from a dictionary instead of a chain of ifs.
• Configuration with behavior: pass in how to sort, filter or retry.
• Building blocks: functions that make or combine functions (higher-order functions, closures).

Remember

• Pass the function, don’t call it. onclick = save, not onclick = save(), which runs it immediately and passes its result.
• A function passed around keeps access to the variables where it was defined (closures).
• In languages where functions aren’t first-class (older Java, C), you use interfaces or function pointers instead.