Blocks and Procs

Blocks and Procs

Methods can accept a block of code that is executed with the yield keyword. For example:

def twice
  yield
  yield
end

twice do
  puts "Hello!"
end

The above program prints "Hello!" twice, once for each yield.

To define a method that receives a block, simply use yield inside it and the compiler will know. You can make this more evident by declaring a dummy block argument, indicated as a last argument prefixed with ampersand (&):

def twice(&block)
  yield
  yield
end

To invoke a method and pass a block, you use do ... end or { ... }. All of these are equivalent:

twice() do
  puts "Hello!"
end

twice do
  puts "Hello!"
end

twice { puts "Hello!" }

The difference between using do ... end and { ... } is that do ... end binds to the left-most call, while { ... } binds to the right-most call:

foo bar do
  something
end

# The above is the same as
foo(bar) do
  something
end

foo bar { something }

# The above is the same as

foo(bar { something })

The reason for this is to allow creating Domain Specific Languages (DSLs) using do ... end to have them be read as plain English:

open file "foo.cr" do
  something
end

# Same as:
open(file("foo.cr")) do
end

You wouldn't want the above to be:

open(file("foo.cr") do
end)

Overloads

Two methods, one that yields and another that doesn't, are considered different overloads, as explained in the overloading section-

Yield arguments

The yield expression is similar to a call and can receive arguments. For example:

def twice
  yield 1
  yield 2
end

twice do |i|
  puts "Got #{i}"
end

The above prints "Got 1" and "Got 2".

A curly braces notation is also available:

twice { |i| puts "Got #{i}" }

You can yield many values:

def many
  yield 1, 2, 3
end

many do |x, y, z|
  puts x + y + z
end

# Output: 6

A block can specify less than the arguments yielded:

def many
  yield 1, 2, 3
end

many do |x, y|
  puts x + y
end

# Output: 3

It's an error specifying more block arguments than those yielded:

def twice
  yield
  yield
end

twice do |i| # Error: too many block arguments
end

Each block variable has the type of every yield expression in that position. For example:

def some
  yield 1, 'a'
  yield true, "hello"
  yield 2, nil
end

some do |first, second|
  # first is Int32 | Bool
  # second is Char | String | Nil
end

The block variable second also includes the Nil type because the last yield expression didn't include a second argument.

Short one-argument syntax

A short syntax exists for specifying a block that receives a single argument and invokes a method on it. This:

method do |argument|
  argument.some_method
end

Can be written as this:

method &.some_method

Or like this:

method(&.some_method)

The above is just syntax sugar and doesn't have any performance penalty.

Arguments can be passed to some_method as well:

method &.some_method(arg1, arg2)

And operators can be invoked too:

method &.+(2)
method &.[index]

yield value

The yield expression itself has a value: the last expression of the block. For example:

def twice
  v1 = yield 1
  puts v1

  v2 = yield 2
  puts v2
end

twice do |i|
  i + 1
end

The above prints "2" and "3".

A yield expression's value is mostly useful for transforming and filtering values. The best examples of this are Enumerable#map and Enumerable#select:

ary = [1, 2, 3]
ary.map { |x| x + 1 }         #=> [2, 3, 4]
ary.select { |x| x % 2 == 1 } #=> [1, 3]

A dummy transformation method:

def transform(value)
  yield value
end

transform(1) { |x| x + 1 } #=> 2

The result of the last expression is 2 because the last expression of the transform method is yield, whose value is the last expression of the block.

break

A break expression inside a block exits early from the method:

def thrice
  puts "Before 1"
  yield 1
  puts "Before 2"
  yield 2
  puts "Before 3"
  yield 3
  puts "After 3"
end

thrice do |i|
  if i == 2
    break
  end
end

The above prints "Before 1" and "Before 2". The thrice method didn't execute the puts "Before 3" expression because of the break.

break can also accept arguments: these become the method's return value. For example:

def twice
  yield 1
  yield 2
end

twice { |i| i + 1 } #=> 3
twice { |i| break "hello" } #=> "hello"

The first call's value is 3 because the last expression of the twice method is yield, which gets the value of the block. The second call's value is "hello" because a break was performed.

If there are conditional breaks, the call's return value type will be a union of the type of the block's value and the type of the many breaks:

value = twice do |i|
  if i == 1
    break "hello"
  end
  i + 1
end
value #:: Int32 | String

If a break receives many arguments, they are automatically transformed to a Tuple:

values = twice { break 1, 2 }
values #=> {1, 2}

If a break receives no arguments, it's the same as receiving a single nil argument:

value = twice { break }
value #=> nil

next

The next expression inside a block exits early from the block (not the method). For example:

def twice
  yield 1
  yield 2
end

twice do |i|
  if i == 1
    puts "Skipping 1"
    next
  end

  puts "Got #{i}"
end

# Ouptut:
# Skipping 1
# Got 2

The next expression accepts arguments, and these give the value of the yield expression that invoked the block:

def twice
  v1 = yield 1
  puts v1

  v2 = yield 2
  puts v2
end

twice do |i|
  if i == 1
    next 10
  end

  i + 1
end

# Output
# 10
# 3

If a next receives many arguments, they are automaticaly transformed to a Tuple. If it receives no arguments it's the same as receiving a single nil argument.

with ... yield

A yield expression can be modified, using the with keyword, to specify an object to use as the default receiver of method calls within the block:

class Foo
  def one
    1
  end

  def yield_with_self
    with self yield
  end

  def yield_normally
    yield
  end
end

def one
  "one"
end

Foo.new.yield_with_self { one } # => 1
Foo.new.yield_normally { one }  # => "one"

Unpacking block arguments

A block argument can specify sub-arguments enclosed in parentheses:

array = [{1, "one"}, {2, "two"}]
array.each do |(number, word)|
  puts "#{number}: #{word}"
end

The above is simply syntax sugar of this:

array = [{1, "one"}, {2, "two"}]
array.each do |arg|
  number = arg[0]
  word = arg[1]
  puts "#{number}: #{word}"
end

That means that any type that responds to [] with integers can be unpacked in a block argument.

Performance

When using blocks with yield, the blocks are always inlined: no closures, calls or function pointers are involved. This means that this:

def twice
  yield 1
  yield 2
end

twice do |i|
  puts "Got: #{i}"
end

is exactly the same as writing this:

i = 1
puts "Got: #{i}"
i = 2
puts "Got: #{i}"

For example, the standard library includes a times method on integers, allowing you to write:

3.times do |i|
  puts i
end

This looks very fancy, but is it as fast as a C for loop? The answer is: yes!

This is Int#times definition:

struct Int
  def times
    i = 0
    while i < self
      yield i
      i += 1
    end
  end
end

Because a non-captured block is always inlined, the above method invocation is exactly the same as writing this:

i = 0
while i < 3
  puts i
  i += 1
end

Have no fear using blocks for readability or code reuse, it won't affect the resulting executable performance.