Proc composition in Ruby - Mini Project: Build & Apply

Proc composition

📖 Scenario: You are working on a small Ruby program that processes numbers using reusable pieces of code called Procs. Procs are like little machines that take input and give output. You want to combine these machines to create a new machine that does two things in order.

🎯 Goal: Build a Ruby program that creates two simple Procs, then combines them into one Proc that applies both steps in sequence. Finally, run the combined Proc on a number and see the result.

📋 What You'll Learn

Create two Procs named double and increment.

double should multiply its input by 2.

increment should add 1 to its input.

Create a new Proc called double_then_increment that applies double first, then increment.

Call double_then_increment with the number 3 and print the result.

💡 Why This Matters

🌍 Real World

Proc composition is useful when you want to build complex operations from simple reusable steps, like processing data or applying filters.

💼 Career

Understanding Proc composition helps in writing clean, modular Ruby code often used in web development and scripting tasks.

1

Create two Procs

Create a Proc called double that multiplies its input by 2, and another Proc called increment that adds 1 to its input.

Ruby

# Create the Proc double and increment here
# Your code here

Hint

Use Proc.new { |x| ... } to create each Proc.

2

Create a composed Proc

Create a new Proc called double_then_increment that takes one argument x, applies double to x, then applies increment to the result.

Ruby

double = Proc.new { |x| x * 2 }
increment = Proc.new { |x| x + 1 }
# Create double_then_increment Proc here
# Your code here

Hint

Inside the new Proc, call double.call(x) first, then pass that result to increment.call(...).

3

Call the composed Proc

Call the Proc double_then_increment with the number 3 and store the result in a variable called result.

Ruby

double = Proc.new { |x| x * 2 }
increment = Proc.new { |x| x + 1 }
double_then_increment = Proc.new { |x| increment.call(double.call(x)) }
# Call double_then_increment with 3 and save to result
# Your code here

Hint

Use double_then_increment.call(3) and assign it to result.

4

Print the result

Print the value of the variable result.

Ruby

double = Proc.new { |x| x * 2 }
increment = Proc.new { |x| x + 1 }
double_then_increment = Proc.new { |x| increment.call(double.call(x)) }
result = double_then_increment.call(3)
# Print the result
# Your code here

Hint

Use puts result to display the output.

Practice

1. What does the >> operator do when used with two Procs in Ruby?

easy

A. It merges two Procs into one that runs them both in parallel.

B. It creates a new Proc that runs the first Proc, then passes its result to the second Proc.

C. It swaps the order of execution of two Procs.

D. It deletes the second Proc and keeps only the first.

Solution

1. Step 1: Understand Proc composition operator

   The >> operator chains two Procs so the output of the first becomes input of the second.

2. Step 2: Explain the effect of chaining

   Calling the combined Proc runs the first, then passes its result to the second, producing a final result.

3. Final Answer:

   It creates a new Proc that runs the first Proc, then passes its result to the second Proc. -> Option B

4. Quick Check:

   Proc composition = chaining with >> [OK]

Hint: Remember >> chains Procs passing output as input [OK]

Common Mistakes:

• Thinking >> runs Procs in parallel
• Assuming >> swaps execution order
• Believing >> deletes one Proc

2. Which of the following is the correct syntax to compose two Procs p1 and p2 in Ruby?

easy

A. combined = p1 >> p2

B. combined = p1 >>> p2

C. combined = p1 << p2

D. combined = p1 >> p2.call

Solution

1. Step 1: Recall correct Proc composition operator

   The operator to compose Procs is >>, which is written as >> but in Ruby code it is >> (two greater-than signs).

2. Step 2: Check each option

   combined = p1 >> p2 uses >> correctly. combined = p1 >>> p2 uses three > which is invalid. combined = p1 << p2 uses << which is not for composition. combined = p1 >> p2.call calls p2 immediately, which is wrong.

3. Final Answer:

   combined = p1 >> p2 -> Option A

4. Quick Check:

   Proc composition syntax = p1 >> p2 [OK]

Hint: Use exactly two > signs to compose Procs [OK]

Common Mistakes:

• Using >>> instead of >>
• Using << instead of >>
• Calling Proc instead of composing

3. What is the output of this Ruby code?

p1 = Proc.new { |x| x + 2 }
p2 = Proc.new { |y| y * 3 }
combined = p1 >> p2
puts combined.call(4)

medium

A. 18

B. 14

C. 20

D. 12

Solution

1. Step 1: Evaluate first Proc with input 4

   p1 adds 2 to input: 4 + 2 = 6.

2. Step 2: Pass result 6 to second Proc

   p2 multiplies input by 3: 6 * 3 = 18.

3. Final Answer:

   18 -> Option A

4. Quick Check:

   (4 + 2) * 3 = 18 [OK]

Hint: Apply first Proc, then second on result [OK]

Common Mistakes:

• Adding instead of multiplying in second Proc
• Multiplying first, then adding
• Confusing order of Procs

4. Identify the error in this Ruby code that tries to compose two Procs:

p1 = Proc.new { |x| x * 2 }
p2 = Proc.new { |y| y + 5 }
combined = p1 >> p2.call
puts combined.call(3)

medium

A. Syntax error due to missing parentheses

B. Using wrong operator for composition

C. Missing block parameters in p1

D. Calling p2 immediately instead of composing

Solution

1. Step 1: Check how p2 is used in composition

   The code uses p2.call which calls p2 immediately, returning a value, not a Proc.

2. Step 2: Understand correct composition usage

   To compose, use p1 >> p2 without calling p2. Calling p2 breaks composition.

3. Final Answer:

   Calling p2 immediately instead of composing -> Option D

4. Quick Check:

   Compose Procs without calling them [OK]

Hint: Do not call Procs when composing, just use >> [OK]

Common Mistakes:

• Calling Proc instead of composing
• Using wrong operator like <<
• Forgetting block parameters

5. Given these Procs:

p1 = Proc.new { |x| x.to_s }
p2 = Proc.new { |s| s + "!" }
p3 = Proc.new { |str| str.reverse }

How do you compose them so that calling the combined Proc with 5 returns "5!"?

hard

A. combined = p3 >> p2 >> p1

B. combined = p1 >> p2 >> p3

C. combined = p1 >> p3 >> p2

D. combined = p2 >> p1 >> p3

Solution

1. Step 1: Understand each Proc's effect

   p1 converts number to string, p2 adds "!", p3 reverses string.

2. Step 2: Find composition order to get "5!"

   "5".reverse = "5", so p1 >> p3 >> p2 gives: "5" -> "5" -> "5!". But p1 >> p2 >> p3: "5" -> "5!" -> "!5". Only A produces "5!".

3. Final Answer:

   combined = p1 >> p3 >> p2 -> Option C

4. Quick Check:

   to_s, reverse, then add ! = "5!" [OK]

Hint: Order Procs by desired transformations stepwise [OK]

Common Mistakes:

• Putting p2 before p3 changes output to "!5"
• Reversing order causes errors on non-string
• Ignoring effect of reverse on "5!"