Pure functions concept in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a pure function?

A pure function is a function that always returns the same result given the same inputs and has no side effects (does not change anything outside itself).

beginner

Why are pure functions important in programming?

Pure functions are important because they are predictable, easier to test, and help avoid bugs caused by changing external data.

intermediate

Which of these is a side effect that a pure function must NOT have?

Changing a global variable, modifying input arguments, or performing input/output like printing to the screen are side effects pure functions must avoid.

beginner

Example: Is this Ruby method pure?

def add(a, b)
  a + b
end

Yes, this method is pure because it always returns the sum of a and b without changing anything else.

intermediate

Example: Is this Ruby method pure?

def add_and_print(a, b)
  sum = a + b
  puts sum
  sum
end

No, this method is not pure because it prints to the screen, which is a side effect.

What does a pure function always guarantee?

A Depends on external data

B Same output for same inputs and no side effects

C Prints output to the screen

D Changes global variables

Pure functions always return the same result for the same inputs and do not cause side effects.

Which of these is NOT a characteristic of a pure function?

A No side effects

B Returns a value

C Modifies input arguments

D Deterministic output

Pure functions do not modify their input arguments.

In Ruby, which method below is pure?

A def multiply(x, y); x * y; end

B def greet(name); puts "Hello, #{name}"; end

C def save_to_file(data); File.write('file.txt', data); end

D def increment_counter; $counter += 1; end

Only the multiply method is pure because it returns a value without side effects.

Why is it easier to test pure functions?

A Because they depend on external state

B Because they print output

C Because they modify global variables

D Because they always produce the same output for the same input

Pure functions are easier to test since their output depends only on their input.

Which of these is a side effect that breaks purity?

A Changing a global variable

B Returning a calculated value

C Using local variables

D Calling another pure function

Changing a global variable is a side effect and breaks the purity of a function.

Explain what a pure function is and why it is useful.

Think about functions that behave like a math formula.

Give an example of a pure function in Ruby and explain why it is pure.

Use a simple method like adding two numbers.