Why Pure functions concept in Ruby? - Purpose & Use Cases

The Big Idea

What if your code could be as predictable and reliable as a simple math problem?

The Scenario

Imagine you have a recipe book where each recipe changes ingredients randomly every time you cook. You try to follow the recipe, but the results are never the same. This is like writing code where functions change things outside themselves or depend on hidden information.

The Problem

When functions depend on or change things outside their own work, it becomes hard to predict what will happen. Bugs hide easily, and fixing one problem might cause another. It's like trying to fix a leaking pipe but water keeps flowing from unknown places.

The Solution

Pure functions always give the same output for the same input and never change anything outside themselves. This makes your code like a reliable recipe: you know exactly what to expect every time you run it. It's easier to test, understand, and fix.

Before vs After

✗ Before

def add_to_list(item, list)
  list << item
  return list
end

✓ After

def add_to_list(item, list)
  list + [item]
end

What It Enables

Pure functions let you build programs that are easier to trust, test, and change without unexpected surprises.

Real Life Example

Think of a calculator app: pressing 2 + 3 always shows 5, no matter what happened before. Pure functions help make that consistency happen in code.

Key Takeaways

Pure functions always return the same result for the same input.

They do not change anything outside themselves.

This makes code easier to understand and debug.

Practice

1. Which of the following best describes a pure function in Ruby?

easy

A. A function that reads user input and prints output directly.

B. A function that always returns the same output for the same input and has no side effects.

C. A function that modifies global variables to keep track of state.

D. A function that changes the value of its arguments.

Solution

1. Step 1: Understand pure function definition

   A pure function always returns the same result when given the same input and does not affect anything outside itself.

2. Step 2: Compare options to definition

   A function that always returns the same output for the same input and has no side effects. matches this definition exactly. Options A, B, and D involve side effects or changing external state.

3. Final Answer:

   A function that always returns the same output for the same input and has no side effects. -> Option B

4. Quick Check:

   Pure function = consistent output + no side effects [OK]

Hint: Pure functions never change outside data and always return same output [OK]

Common Mistakes:

• Thinking functions that modify variables are pure
• Confusing input/output consistency with side effects
• Assuming printing output is allowed in pure functions

2. Which Ruby function below is a pure function?

easy

A. def add(a, b); a + b; end

B. def add(a, b); puts a + b; end

C. def add(a, b); @sum = a + b; end

D. def add(a, b); print a + b; end

Solution

1. Step 1: Check each function for side effects and output consistency

   def add(a, b); a + b; end returns sum without changing anything outside or printing. def add(a, b); puts a + b; end prints (side effect). def add(a, b); @sum = a + b; end changes instance variable (side effect). def add(a, b); print a + b; end prints (side effect).

2. Step 2: Identify pure function

   Only def add(a, b); a + b; end meets pure function rules: same output for same input, no side effects.

3. Final Answer:

   def add(a, b); a + b; end -> Option A

4. Quick Check:

   Pure function = no side effects + returns value [OK]

Hint: Pure functions return values without printing or changing variables [OK]

Common Mistakes:

• Confusing printing output with returning value
• Assuming modifying instance variables is allowed
• Assuming print doesn't cause side effects

3. What is the output of this Ruby code?

def multiply(x, y)
  x * y
end

result = multiply(3, 4)
puts result

medium

A. Error

B. nil

C. 12

D. 7

Solution

1. Step 1: Understand the multiply function

   The function returns the product of x and y, so multiply(3, 4) returns 12.

2. Step 2: Check the output statement

   puts prints the returned value, so it prints 12.

3. Final Answer:

   12 -> Option C

4. Quick Check:

   multiply(3,4) = 12 [OK]

Hint: Pure functions return predictable output; multiply(3,4) = 12 [OK]

Common Mistakes:

• Thinking function returns nil if no explicit return
• Confusing addition with multiplication
• Expecting error due to missing return keyword

4. Identify the error in this Ruby function that is supposed to be pure:

def increment(n)
  n += 1
  puts n
end

medium

A. It modifies the input variable n.

B. It uses invalid syntax for increment.

C. It does not return any value explicitly.

D. It prints output, causing a side effect.

Solution

1. Step 1: Check for side effects

   The function prints the value of n, which is a side effect because it affects outside world.

2. Step 2: Verify other aspects

   Modifying n locally is allowed; syntax is correct; implicit return is allowed but printing breaks purity.

3. Final Answer:

   It prints output, causing a side effect. -> Option D

4. Quick Check:

   Pure functions must not print or cause side effects [OK]

Hint: Printing inside function breaks purity due to side effects [OK]

Common Mistakes:

• Thinking local variable change breaks purity
• Assuming implicit return is an error
• Ignoring side effects caused by printing

5. You want to write a pure function in Ruby that takes a list of numbers and returns a new list with each number doubled. Which implementation is pure?

hard

A. def double_list(nums)
nums.map { |n| n * 2 }
end

B. def double_list(nums)
nums.collect! { |n| n *= 2 }
nums
end

C. def double_list(nums)
nums.map! { |n| n * 2 }
end

D. def double_list(nums)
nums.clear
nums.push(2, 4, 6)
end

Solution

1. Step 1: Understand pure function requirements

   A pure function returns new data without changing input or external state.

2. Step 2: Analyze each option

   def double_list(nums) nums.collect! { |n| n *= 2 } nums end uses collect! which modifies original array (side effect). def double_list(nums) nums.map { |n| n * 2 } end returns a new array with doubled values without changing original. def double_list(nums) nums.map! { |n| n * 2 } end uses map! which modifies original array (side effect). def double_list(nums) nums.clear nums.push(2, 4, 6) end clears and modifies original array (side effect).

3. Final Answer:

   def double_list(nums) nums.map { |n| n * 2 } end -> Option A

4. Quick Check:

   Pure function = no input change + returns new data [OK]

Hint: Use non-destructive methods like map, avoid modifying input [OK]

Common Mistakes:

• Using destructive methods like map! or clear
• Modifying input array instead of returning new one
• Confusing each with map for transformation