Why Proc composition in Ruby? - Purpose & Use Cases

The Big Idea

What if you could connect small tasks like puzzle pieces to build powerful programs effortlessly?

The Scenario

Imagine you have several small tasks to do one after another, like making coffee: first boil water, then grind beans, then brew. Doing each step separately and manually every time can be tiring and confusing.

The Problem

Manually running each step means you might forget the order, repeat steps, or write the same code again and again. It's slow and easy to make mistakes, especially when tasks depend on each other.

The Solution

Proc composition lets you link small tasks into one smooth flow. You create little pieces of work (Procs) and then combine them so they run in order automatically. This saves time and keeps your code neat and clear.

Before vs After

✗ Before

step1 = ->(x) { x + 1 }
step2 = ->(x) { x * 2 }
result = step2.call(step1.call(3))

✓ After

step1 = ->(x) { x + 1 }
step2 = ->(x) { x * 2 }
combined = ->(x) { step2.call(step1.call(x)) }
result = combined.call(3)

What It Enables

It makes building complex tasks from simple parts easy and error-free, like creating a recipe that always works perfectly.

Real Life Example

In a web app, you might check user input, then clean it, then save it. Using proc composition, you can chain these steps so the input flows smoothly through all checks and changes.

Key Takeaways

Manual step-by-step code is slow and error-prone.

Proc composition links small tasks into one clear flow.

This makes code easier to write, read, and maintain.

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!"