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Ruby Dictionary

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Block / yield / block_given?

Block / yield / block_given?

A block is a chunk of code that can be passed to a method. Use yield to call a block, and block_given? to check whether a block was provided.

Syntax

# Call a method with a block (do...end form).
method_name do |argument|
  process
end

# Call a method with a block ({} form).
method_name { |argument| process }

# Call the block inside a method definition.
def method_name
  yield         # Executes the block.
  yield(argument)   # Executes the block, passing an argument.
end

# Check whether a block was provided.
def method_name
  if block_given?
    yield
  end
end

Syntax List

Syntax / Method	Description
do...end	Defines a multi-line block. Has lower precedence, so it is commonly used when combining with method arguments.
{ }	Defines a single-line block. Has higher precedence and is used for inline blocks.
yield	Executes the block passed to the method. You can also pass arguments to the block.
block_given?	Returns a boolean indicating whether a block was passed to the method.

Sample Code

# Define a method that calls a block with yield.
def greet
  puts "Starting."
  yield  # Executes the block.
  puts "Done."
end

greet do
  puts "Hello!"
end

# Pass an argument with yield.
def double(number)
  yield(number * 2)
end

double(5) { |n| puts "Result: #{n}" }  # Result: 10

# Use block_given? to check whether a block was provided.
def optional_process(value)
  if block_given?
    yield(value)
  else
    puts "No block: #{value}"
  end
end

optional_process(42)               # No block: 42
optional_process(42) { |n| puts "With block: #{n}" }  # With block: 42

# Use the return value of yield.
def transform(text)
  result = yield(text)
  puts "Transformed: #{result}"
end

transform("hello") { |s| s.upcase }  # Transformed: HELLO

Overview

Blocks are a core feature of Ruby that let you attach a piece of code to a method after the fact. Iterators such as each and map are also implemented as methods that accept blocks.

Both do...end and { } define blocks, but by convention do...end is used for multi-line blocks and { } for single-line blocks. Calling yield without a block raises a LocalJumpError, so always check with block_given? when a block is optional.

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