The Ultimate Guide to Blocks, Procs & Lambdas

Ruby blocks, procs & lambdas.

What are they?

How do they work?

How are they different from each other?

You will learn that & a lot more by reading this post!

Contents

Understanding Ruby Blocks

Ruby blocks are little anonymous functions that can be passed into methods.

Blocks are enclosed in a do / end statement or between brackets {}, and they can have multiple arguments.

The argument names are defined between two pipe | characters.

If you have used each before, then you have used blocks!

Here is an example:

# Form 1: recommended for single line blocks
[1, 2, 3].each { |num| puts num }
                 ^^^^^ ^^^^^^^^
                 block   block
               arguments body

# Form 2: recommended for multi-line blocks
[1, 2, 3].each do |num|
  puts num
end

A Ruby block is useful because it allows you to save a bit of logic (code) & use it later.

This could be something like writing data to a file, comparing if one element is equal to another, or even printing an error message.

Ruby Yield Keyword

What does yield mean in Ruby?

Yield is a Ruby keyword that calls a block when you use it.

It’s how methods USE blocks!

When you use the yield keyword, the code inside the block will run & do its work.

Just like when you call a regular Ruby method.

Here’s an example:

def print_once
  yield
end

print_once { puts "Block is being run" }

This runs any block passed to print_once, as a result, "Block is being run" will be printed on the screen.

Did you know…

That yield can be used multiple times?

Every time you call yield, the block will run, so this is like calling the same method again.

Example:

def print_twice
  yield
  yield
end

print_twice { puts "Hello" }

# "Hello"
# "Hello"

And just like methods…

You can pass any number of arguments to yield .

Example:

def one_two_three
  yield 1
  yield 2
  yield 3
end

one_two_three { |number| puts number * 10 }
# 10, 20, 30

These arguments then become the block’s arguments.

In this example number.

Implicit vs Explicit Blocks

Blocks can be “explicit” or “implicit”.

Explicit means that you give it a name in your parameter list.

You can pass an explicit block to another method or save it into a variable to use later.

Here is an example:

def explicit_block(&block)
  block.call # same as yield
end

explicit_block { puts "Explicit block called" }

Notice the &block parameter…

That’s how you define the block’s name!

How To Check If A Block Was Given

If you try to yield without a block you will get a no block given (yield) error.

You can check if a block has been passed in with the block_given? method.

Example:

def do_something_with_block
  return "No block given" unless block_given?
  yield
end

This prevents the error if someone calls your method without a block.

What is a Lambda?

A lambda is a way to define a block & its parameters with some special syntax.

You can save this lambda into a variable for later use.

The syntax for defining a Ruby lambda looks like this:

say_something = -> { puts "This is a lambda" }

You can also use the alternative syntax: lambda instead of ->.

Defining a lambda won’t run the code inside it, just like defining a method won’t run the method, you need to use the call method for that.

Example:

say_something = -> { puts "This is a lambda" }
say_something.call

# "This is a lambda"

There are other ways to call a lambda, it’s good to know they exist, however, I recommend sticking with call for clarity.

Here’s the list:

my_lambda = -> { puts "Lambda called" }

my_lambda.call
my_lambda.()
my_lambda[]
my_lambda.===

Lambdas can also take arguments, here is an example:

times_two = ->(x) { x * 2 }
times_two.call(10)
# 20

If you pass the wrong number of arguments to a lambda, it will raise an exception, just like a regular method.

Lambdas vs Procs

Procs are a very similar concept…

One of the differences is how you create them.

Example:

my_proc = Proc.new { |x| puts x }

There is no dedicated Lambda class. A lambda is just a special Proc object. If you take a look at the instance methods from Proc, you will notice there is a lambda? method.

Now:

A proc behaves differently than a lambda, specially when it comes to arguments:

t = Proc.new { |x,y| puts "I don't care about arguments!" }
t.call
# "I don't care about arguments!"

Another difference between procs & lambdas is how they react to a return statement.

A lambda will return normally, like a regular method.

But a proc will try to return from the current context.

Here’s what I mean:

If you run the following code, you will notice how the proc raises a LocalJumpError exception.

The reason is that you can’t return from the top-level context.

Try this:

# Should work
my_lambda = -> { return 1 }
puts "Lambda result: #{my_lambda.call}"

# Should raise exception
my_proc = Proc.new { return 1 }
puts "Proc result: #{my_proc.call}"

If the proc was inside a method, then calling return would be equivalent to returning from that method.

This is demonstrated in the following example.

def call_proc
  puts "Before proc"
  my_proc = Proc.new { return 2 }
  my_proc.call
  puts "After proc"
end

p call_proc
# Prints "Before proc" but not "After proc"

Here is a summary of how procs and lambdas are different:

  • Lambdas are defined with -> {} and procs with Proc.new {}.
  • Procs return from the current method, while lambdas return from the lambda itself.
  • Procs don’t care about the correct number of arguments, while lambdas will raise an exception.

Taking a look at this list, we can see that lambdas are a lot closer to a regular method than procs are.

Closures

Ruby procs & lambdas also have another special attribute. When you create a Ruby proc, it captures the current execution scope with it.

This concept, which is sometimes called closure, means that a proc will carry with it values like local variables and methods from the context where it was defined.

They don’t carry the actual values, but a reference to them, so if the variables change after the proc is created, the proc will always have the latest version.

Let’s see an example:

def call_proc(my_proc)
  count = 500
  my_proc.call
end

count   = 1
my_proc = Proc.new { puts count }

p call_proc(my_proc) # What does this print?

In this example we have a local count variable, which is set to 1.

We also have a proc named my_proc, and a call_proc method which runs (via the call method) any proc or lambda that is passed in as an argument.

What do you think this program will print?

It would seem like 500 is the most logical conclusion, but because of the ‘closure’ effect this will print 1.

This happens because the proc is using the value of count from the place where the proc was defined, and that’s outside of the method definition.

The Binding Class

Where do Ruby procs & lambdas store this scope information?

Let me tell you about the Binding class…

When you create a Binding object via the binding method, you are creating an ‘anchor’ to this point in the code.

Every variable, method & class defined at this point will be available later via this object, even if you are in a completely different scope.

Example:

def return_binding
  foo = 100
  binding
end

# Foo is available thanks to the binding,
# even though we are outside of the method
# where it was defined.
puts return_binding.class
puts return_binding.eval('foo')

# If you try to print foo directly you will get an error.
# The reason is that foo was never defined outside of the method.
puts foo

In other words, executing something under the context of a binding object is the same as if that code was in the same place where that binding was defined (remember the ‘anchor’ metaphor).

You don’t need to use binding objects directly, but it’s still good to know this is a thing 🙂

Video Tutorial

Wrapping Up

In this post you learned how blocks work, the differences between Ruby procs & lambdas and you also learned about the “closure” effect that happens whenever you create a block.

One thing I didn’t cover is the curry method.

This method allows you to pass in some or all of the required arguments.

If you only pass in a partial number of arguments you will get a new proc with these arguments already ‘pre-loaded’, when all the arguments are supplied then the proc will be executed.

I hope you enjoyed this post!

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11 comments
Gagan says 7 years ago

Well explained!

    Jesus Castello says 7 years ago

    Thank you 🙂

Kaloyan Yordanov says 7 years ago

A wonderful article, Jesus, so nicely written!

    Jesus Castello says 7 years ago

    Thanks for reading 🙂

ttw (@ttwo32) says 7 years ago

Good article, Jesus. I think that it is important to understand differences between lambda and proc. This article describe differences politely by coding example, so readers easily understand.
And block is a feature to incarnate Closure.

    Jesus Castello says 7 years ago

    Thanks for your comment 🙂

vizvamitra says 7 years ago

There is also another interesting behavior of procs:

def proc_from
Proc.new
end

proc = proc_from { “hello” }
proc.call #=> “hello”

If Proc.new is called within a method with an attached block, that block is converted to the Proc object.
(example from http://ruby-doc.org/core-2.2.0/Proc.html)

Sangam Gupta says 7 years ago

Very helpful, thank you. However there is one think confusing, I checked Kernel module and I see lambda method which is creating Proc object but you mentioned, lambda is a special object of Proc class. Your thoughts?

    Jesus Castello says 7 years ago

    The lambda method on Kernel is the same as using the -> syntax for defining a lambda.

Sangam Gupta says 7 years ago

Thank you, I had the same assumption 🙂

Robert says 7 years ago

Good article: clear and gives a good overview!

Some minor nitpicking:
– Blocks are anonymous functions.
– Usually the alternative delimiters to do…end are called “curly braces” or “curly brackets” – “brackets” are (…) if I’m not mistaken.
– There is another, older and thus potentially more compatible syntax to create lambdas: lambda {|arg| …} and lambda do |arg| … end

Comments are closed