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.

Practice

1. What is the main purpose of using a DSL (Domain Specific Language) in Ruby?

easy

A. To make code easier to read and write for specific tasks

B. To speed up Ruby interpreter execution

C. To replace Ruby's core syntax completely

D. To automatically generate user interfaces

Solution

1. Step 1: Understand DSL purpose

   A DSL is designed to simplify and clarify code for a specific domain or task.

2. Step 2: Compare options

   Only To make code easier to read and write for specific tasks correctly states the purpose of DSLs; others describe unrelated goals.

3. Final Answer:

   To make code easier to read and write for specific tasks -> Option A

4. Quick Check:

   DSL purpose = simplify domain code [OK]

Hint: DSLs focus on specific tasks for clearer code [OK]

Common Mistakes:

• Thinking DSLs speed up Ruby interpreter
• Believing DSLs replace Ruby syntax entirely
• Confusing DSLs with UI generators

2. Which Ruby method is commonly used to execute a block inside the context of a DSL object?

easy

A. eval_block

B. block_exec

C. class_eval

D. instance_eval

Solution

1. Step 1: Recall Ruby block execution methods

   Ruby provides methods like instance_eval and class_eval to run blocks in specific contexts.

2. Step 2: Identify the method for DSL context

   instance_eval runs the block inside the instance's context, which is typical for DSLs.

3. Final Answer:

   instance_eval -> Option D

4. Quick Check:

   Block runs in object context = instance_eval [OK]

Hint: Use instance_eval to run blocks inside DSL objects [OK]

Common Mistakes:

• Confusing class_eval with instance_eval
• Using non-existent methods like eval_block
• Assuming block_exec is a Ruby method

3. What will be the output of this Ruby DSL code?

class Builder
  def initialize
    @steps = []
  end
  def step(name)
    @steps << name
  end
  def build(&block)
    instance_eval(&block)
    @steps
  end
end

b = Builder.new
result = b.build do
  step 'clean'
  step 'compile'
  step 'test'
end
puts result.inspect

medium

A. [:clean, :compile, :test]

B. ["clean", "compile", "test"]

C. ["step", "step", "step"]

D. SyntaxError

Solution

1. Step 1: Understand the Builder class behavior

   The build method runs the block inside the Builder instance, collecting step names as strings in @steps.

2. Step 2: Trace the block execution

   Each call to step adds the string name to @steps: 'clean', 'compile', 'test'. The build method returns @steps.

3. Final Answer:

   ["clean", "compile", "test"] -> Option B

4. Quick Check:

   Collected steps = ["clean", "compile", "test"] [OK]

Hint: instance_eval runs block adding strings to array [OK]

Common Mistakes:

• Expecting symbols instead of strings
• Thinking step method returns step name
• Confusing array contents with method names

4. Identify the error in this DSL builder code:

class Configurator
  def set(name, value)
    @settings ||= {}
    @settings[name] = value
  end
  def configure(&block)
    instance_eval(block)
  end
end

c = Configurator.new
c.configure do
  set :mode, 'fast'
end

medium

A. Wrong argument passing to instance_eval

B. Missing @settings initialization

C. set method missing return value

D. Block not passed to configure method

Solution

1. Step 1: Check instance_eval usage

   instance_eval expects a block passed with &block, but here it is called as instance_eval(block) without &.

2. Step 2: Understand correct block passing

   The correct call is instance_eval(&block) to convert the block argument to a block for instance_eval.

3. Final Answer:

   Wrong argument passing to instance_eval -> Option A

4. Quick Check:

   Use &block with instance_eval [OK]

Hint: Pass block with & when calling instance_eval [OK]

Common Mistakes:

• Calling instance_eval without & for block
• Forgetting to initialize @settings
• Expecting set to return a value

5. You want to build a Ruby DSL that allows nested configuration like this:

config = Config.new do
  database do
    adapter 'sqlite3'
    pool 5
  end
  cache do
    enabled true
    size 64
  end
end

Which approach best supports this nested DSL pattern?

hard

A. Use global variables to store settings inside blocks

B. Use class variables to share state across blocks

C. Use instance_eval with separate classes for database and cache blocks

D. Use eval with string interpolation for nested blocks

Solution

1. Step 1: Understand nested DSL needs

   Nested blocks require separate context objects for database and cache to hold their settings.

2. Step 2: Identify best practice

   Using instance_eval on separate classes for each nested block cleanly isolates configuration and supports nested DSL.

3. Final Answer:

   Use instance_eval with separate classes for database and cache blocks -> Option C

4. Quick Check:

   Nested DSL = instance_eval + separate classes [OK]

Hint: Use instance_eval with helper classes for nested blocks [OK]

Common Mistakes:

• Using global variables causing conflicts
• Using class variables mixing state
• Using eval with strings risking security and complexity