DSL building patterns in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple Ruby DSL block

What is the output of this Ruby code that uses a simple DSL pattern?

Ruby

class GreetingDSL
  def initialize
    @messages = []
  end

  def say(text)
    @messages << text
  end

  def output
    @messages.join(", ")
  end
end

def greet(&block)
  dsl = GreetingDSL.new
  dsl.instance_eval(&block)
  dsl.output
end

puts greet do
  say "Hello"
  say "world"
end

A Hello, world

B Hello world

C say Hello, say world

D Error: undefined method 'say'

💡 Hint

Look at how the DSL collects messages and joins them with a comma and space.

The DSL collects the strings "Hello" and "world" in an array, then joins them with ", ". So the output is "Hello, world".

🧠 Conceptual

intermediate

Purpose of instance_eval in Ruby DSLs

In Ruby DSL building, what is the main purpose of using instance_eval inside a method that takes a block?

A To execute the block in the context of the DSL object, allowing direct method calls without receiver

B To run the block in a new thread for concurrency

C To convert the block into a string for parsing

D To prevent the block from accessing instance variables

💡 Hint

Think about how DSL methods are called inside the block without specifying an object.

Using instance_eval runs the block with self set to the DSL object, so methods defined on that object can be called directly inside the block.

🔧 Debug

advanced

Why does this Ruby DSL code raise NoMethodError?

Given this Ruby DSL code, why does it raise NoMethodError when calling title inside the block?

class BookDSL
  def initialize
    @title = ""
  end

  def title(text)
    @title = text
  end
end

def book(&block)
  dsl = BookDSL.new
  block.call
  dsl
end

book do
  title "Ruby Guide"
end

A The block is missing a parameter to receive the DSL object

B The title method is private and cannot be called

C The block is called without changing self, so title method is undefined in main context

D The BookDSL class does not define a title method

💡 Hint

Check how the block is called and what self is inside it.

The block is called with block.call, which runs it in the current context (main), not the DSL object. So title is undefined there.

📝 Syntax

advanced

Identify the syntax error in this Ruby DSL code

Which option correctly fixes the syntax error in this Ruby DSL snippet?

class ConfigDSL
  def set(key, value)
    @settings ||= {}
    @settings[key] = value
  end

  def settings
    @settings
  end
end

def config(&block)
  dsl = ConfigDSL.new
  dsl.instance_eval(&block)
  dsl.settings
end

config do
  set :mode, "production"
  set :debug true
end

A Remove the second set call entirely

B Change 'set :debug true' to 'set :debug, true'

C Add parentheses: 'set(:debug true)'

D Replace 'set :debug true' with 'set :debug => true'

💡 Hint

Look at how method arguments are separated in Ruby.

Ruby methods require commas between arguments. 'set :debug true' misses a comma, causing a syntax error.

🚀 Application

expert

How many keys are in the resulting hash from this Ruby DSL?

Consider this Ruby DSL code that builds a hash:

class HashBuilder
  def initialize
    @hash = {}
  end

  def add(key, value)
    @hash[key] = value
  end

  def build
    @hash
  end
end

def build_hash(&block)
  builder = HashBuilder.new
  builder.instance_eval(&block)
  builder.build
end

result = build_hash do
  add :a, 1
  add :b, 2
  add :a, 3
end

puts result.size

How many keys does result have?

A 3

B 0

C 1

D 2

💡 Hint

Think about what happens when you add a key twice in a Ruby hash.

Adding the same key twice overwrites the previous value. So keys :a and :b remain, total 2 keys.