Ruby map(&:method) syntax - meaning & usage

One-line map in Ruby
| 8 min.

A few days ago, I worked on a customer project, and I wanted to use one line map but with an argument for a method inside this map. I never had this need before. Normally in one line map, I do something like .map(&:next). This time I needed something else. So, I did a short research on how can I do that. The results of this research were so interesting to me that I decided to create an article about this topic. So let’s get started!

Normally, when we use map or another iterator we use it in a block version:

[1, 2, 3, 4, 5].map do |item|
  item + 1
end
# => [2, 3, 4, 5, 6]

Sometimes calculations inside the block (like in the example above) are so simple that we can do it more shortly:

[1, 2, 3, 4, 5].map { |item| item + 1 }
# => [2, 3, 4, 5, 6]

Or even shorter:

[1, 2, 3, 4, 5].map(&:next)
# => [2, 3, 4, 5, 6]

The map(&...) looks nice and brings me to some questions. Is it a shortcut for already shown examples? Or maybe it’s something different? Can we put an argument to method inside map? How can we use it creatively? And the most important question is what’s going on there when we call map(&...)?

Disclaimers

  1. First of all, everything I will cover today, you can use for other Ruby iterators as well. Like: each, inject, select, and so on. Of course, not always do those tricks make sense, but they are possible.
  2. The main focus here is on understanding map(&...). Some of the solutions can be less readable than a normal map with a block. You, as a developer, decide what approach is the best for your code.
  3. Since I will cover a lot of Ruby proc, lambdas and block topic, I recommend you to read also my article about functional programming in Ruby. It will help you to better understand what is going on.
  4. I write this article for fun and to better understand the basic concepts of the Ruby language. I hope you will like it too.

What does the map(&...) do?

When we see:

[1, 2, 3, 4, 5].map(&:next)
# => [2, 3, 4, 5, 6]

at first, we can say that this is a shortcut for

[1, 2, 3, 4, 5].map { |item| item.next }
# => [2, 3, 4, 5, 6]

but it’s not true. It’s shorthand for

[1, 2, 3, 4, 5].map(&:next.to_proc)
# => [2, 3, 4, 5, 6]

The example above doesn’t give us a full understanding of this code, so let’s go step by step. When [1, 2, 3, 4, 5].map(&:next) is executed, the &:next is given to the map. Since the &:next is passed to the map as a block argument (I will explain it more in a moment) Ruby will try to make a Proc from it. It’s why the &:next.to_proc is called. The :next is a Symbol object so that we can call on it the Symbol#to_proc method. Then map sends call message to the &:next.to_proc with first parameter 1. The :next.to_proc.call(1) is executed. From the implementation of the to_proc method, we know to_proc procedure for Symbol object sends a send method to a 1 object with argument :next (so, 1.send(:next) is executed). The whole process is repeated on the rest of the elements in [1, 2, 3, 4, 5].

Block argument in map

Let’s go back to the block argument in map. In Ruby, we can put “function” (block) as an argument to any method (not only a map). See the example below:

def check_arguments_method(*args, &block)
  puts "args: #{args.inspect}"
  puts "block: #{block.inspect}"
end

When we call it with no block argument, we will see:

check_arguments_method(:next)

# args: [:next]
# block: nil

There is no block in this case. But when we run it with &:

check_arguments_method(&:next)

# args: []
# block: #<Proc:0x00005588aae7b4a8(&:next) (lambda)>

We see that Ruby converts :next by using the to_proc method into a Proc object, and we have a block. Why do I say that? Well, check the next example. When I try to provide String as a block, I will get:

check_arguments_method(&'next')

# (irb):13:in `<main>': wrong argument type String (expected Proc) (TypeError)

We see that Sting is not a Proc and cannot be converted into a Proc. Or in other words, String doesn’t respond to #to_proc. What happened when I put the real Proc object into this method?

my_proc = Proc.new { |item| item.next }

# => #<Proc:0x00005588aa81c468 (irb):14>

check_arguments_method(&my_proc)

# args: []
# block: #<Proc:0x00005588aa81c468 (irb):14>

We see that the my_proc object is passed as a block argument into our check_arguments_method, and inside this method, we see the same object (based on its object id). Let’s check one more thing. What happens if we omit the & before my_proc?

check_arguments_method(my_proc)

# args: [#<Proc:0x00005588aa81c468 (irb):14>]
# block: nil

In this case, Ruby treats the my_proc argument as a no-block argument. So to be sure that the argument, we put into a method, is treated as a block, we need to add & as a prefix for it. That’s why we put & in our [1, 2, 3, 4, 5].map(&:next) example.

How to provide an argument into a block argument in the map?

As you saw in the previous example, we can do:

[1, 2, 3, 4, 5].map(&:next)
# => [2, 3, 4, 5, 6]

But what if we want to add an argument to our block method? Something like:

[1, 2, 3, 4, 5].map(&:+(2))

# /home/agnieszka/.rvm/rubies/ruby-3.0.3/lib/ruby/3.0.0/irb/workspace.rb:116:in `eval': (irb):19: syntax error, unexpected '(', expecting ')' (SyntaxError)

We see that it’s not possible in that way. But can we do that differently? The answer is - yes.

[1, 2, 3, 4, 5].map(&2.method(:+))
# => [3, 4, 5, 6, 7]

WOW! What happened here? First 2.method(:+) is called. We take object 2 and we create Method Object on it. The Method Object responds to to_proc method so 2.method(:+).to_proc is run. Then for each element of the array method call is called on Proc object. For example: 2.method(:+).to_proc.call(1).

There is one more thing worth mentioning here. When we have a Method Object, we can call the call method directly. We don’t need to convert it to Proc.

2.method(:+).call(1)
# => 3

Other examples with additional argument

Convert Array to Enumerable object

[1, 2, 3, 4, 5].to_enum.with_object(2).map(&:+)
# => [3, 4, 5, 6, 7]

We convert Array to Enumerable object. Then inject the 2 object into it. In the end, we iterate through each element of the Array and do the addition.

Lambda in map method

[1, 2, 3, 4, 5].map(&->(item) { item + 2 })
# => [3, 4, 5, 6, 7]

Where ->(item) { item + 2 } is a lambda and it’s similar to Proc version.

[1, 2, 3, 4, 5].map(&(Proc.new { |item| item + 2 }))
# => [3, 4, 5, 6, 7]

Declare our own method

This example is similar to the first one with a method, but now we have full control over what is going on in the double method. We don’t rely on the Ruby 2 object interface.

def double(x)
  x + 2
end

[1, 2, 3, 4, 5].map(&method(:double))
# => [3, 4, 5, 6, 7]

Use curry method

[1, 2, 3, 4, 5].map(&:+.to_proc.curry(2).call(2))
# => [3, 4, 5, 6, 7]

We will stop for a moment here to understand, what is going on in this code. First, we put the :+ method into a map and convert it to Proc &:+.to_proc. Then &:+.to_proc.curry(2) tells Ruby that this Proc can be called only when has 2 arguments. I will talk a bit more about the curry method in the next section. Next we provide first argument 2 to this Proc :+.to_proc.curry(2).call(2). Now is the part we know already. We call to_proc on our Proc and call the call method with each element of the array.

Curring - quick overview

Last but not least curring. Curring is a mathematical term used in programming too. It’s the technique a function with multiple arguments is converted into a sequence of functions that each have only one argument. Let’s see that in our example. In normal life, method :+ needs 2 arguments. We will convert it into two methods (functions) with only one argument.

adding_method = :+.to_proc.curry(2)
# => #<Proc:0x000056401dc42fa8 (lambda)>

first_function = adding_method.call(2)
# => #<Proc:0x000056401e27ad08 (lambda)>

first_function.call(1)
# => 3

First we create our function (Proc) :+.to_proc.curry(2). Then we call it with the first argument and assign it to the variable first_function = adding_method.call(2). It will return a new function (Proc) with only one argument. After calling the new method first_function.call(1) we will get result of adding to numbers 2 + 1. Without the curry method, we need to provide all arguments at the same time to the :+ method.

:+.to_proc.call(2, 1)
# => 3

In case we will try to postpone putting the second argument to the method, we will get an error.

:+.to_proc.call(2)
# (irb):51:in `+': wrong number of arguments (given 0, expected 1) (ArgumentError)

We can say that curring allows us to postpone the final call of the method with multiple arguments in time. But it’s worth mentioning that we don’t need to postpone it.

adding_method.call(2, 1)
# => 3

To summarize what we saw. When we use the curry method on Proc, the curried Proc-based method is returned. When the call with a number of arguments that is lower than the method’s arity is called on returned Proc, then another curried Proc is returned. Only when enough arguments have been supplied to satisfy the method signature, the actual method will be called. This behavior is useful, for example, in a block like map.

The map method in Ruby is definitely an interesting one, similar to other block methods. Hopefully, I was able to explain the details of map with a block in a simple way. If you enjoyed it, make sure to give a 👏 so I know that it helped!


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