What does map(&:name) mean in Ruby?

I found this code in a RailsCast:

def tag_names
  @tag_names || tags.map(&:name).join(' ')
end

What 开发者_如何学运维does the (&:name) in map(&:name) mean?

---

It's shorthand for tags.map(&:name.to_proc).join(' ')

If foo is an object with a to_proc method, then you can pass it to a method as &foo, which will call foo.to_proc and use that as the method's block.

The Symbol#to_proc method was originally added by ActiveSupport but has been integrated into Ruby 1.8.7. This is its implementation:

class Symbol
  def to_proc
    Proc.new do |obj, *args|
      obj.send self, *args
    end
  end
end

---

Another cool shorthand, unknown to many, is

array.each(&method(:foo))

which is a shorthand for

array.each { |element| foo(element) }

By calling method(:foo) we took a Method object from self that represents its foo method, and used the & to signify that it has a to_proc method that converts it into a Proc.

This is very useful when you want to do things point-free style. An example is to check if there is any string in an array that is equal to the string "foo". There is the conventional way:

["bar", "baz", "foo"].any? { |str| str == "foo" }

And there is the point-free way:

["bar", "baz", "foo"].any?(&"foo".method(:==))

The preferred way should be the most readable one.

---

It's equivalent to

def tag_names
  @tag_names || tags.map { |tag| tag.name }.join(' ')
end

---

tags.map(&:name)

is The same as

tags.map{|tag| tag.name}

&:name just uses the symbol as the method name to be called.

---

While let us also note that ampersand #to_proc magic can work with any class, not just Symbol. Many Rubyists choose to define #to_proc on Array class:

class Array
  def to_proc
    proc { |receiver| receiver.send *self }
  end
end

# And then...

[ 'Hello', 'Goodbye' ].map &[ :+, ' world!' ]
#=> ["Hello world!", "Goodbye world!"]

Ampersand & works by sending to_proc message on its operand, which, in the above code, is of Array class. And since I defined #to_proc method on Array, the line becomes:

[ 'Hello', 'Goodbye' ].map { |receiver| receiver.send( :+, ' world!' ) }

---

It's shorthand for tags.map { |tag| tag.name }.join(' ')

---

Two things are happening here, and it's important to understand both.

As described in other answers, the Symbol#to_proc method is being called.

But the reason to_proc is being called on the symbol is because it's being passed to map as a block argument. Placing & in front of an argument in a method call causes it to be passed this way. This is true for any Ruby method, not just map with symbols.

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

some_method(:whatever)
# args: [:whatever]
# block: nil

some_method(&:whatever)
# args: []
# block: #<Proc:0x007fd23d010da8>

some_method(&"whatever")
# TypeError: wrong argument type String (expected Proc)
# (String doesn't respond to #to_proc)

The Symbol gets converted to a Proc because it's passed in as a block. We can show this by trying to pass a proc to .map without the ampersand:

arr = %w(apple banana)
reverse_upcase = proc { |i| i.reverse.upcase }
reverse_upcase.is_a?(Proc)
=> true

arr.map(reverse_upcase)
# ArgumentError: wrong number of arguments (1 for 0)
# (map expects 0 positional arguments and one block argument)

arr.map(&reverse_upcase)
=> ["ELPPA", "ANANAB"]

Even though it doesn't need to be converted, the method won't know how to use it because it expects a block argument. Passing it with & gives .map the block it expects.

---

Josh Lee's answer is almost correct except that the equivalent Ruby code should have been as follows.

class Symbol
  def to_proc
    Proc.new do |receiver|
      receiver.send self
    end
  end
end

not

class Symbol
  def to_proc
    Proc.new do |obj, *args|
      obj.send self, *args
    end
  end
end

With this code, when print [[1,'a'],[2,'b'],[3,'c']].map(&:first) is executed, Ruby splits the first input [1,'a'] into 1 and 'a' to give obj 1 and args* 'a' to cause an error as Fixnum object 1 does not have the method self (which is :first).

---

When [[1,'a'],[2,'b'],[3,'c']].map(&:first) is executed;

1. :first is a Symbol object, so when &:first is given to a map method as a parameter, Symbol#to_proc is invoked.

2. map sends call message to :first.to_proc with parameter [1,'a'], e.g., :first.to_proc.call([1,'a']) is executed.

3. to_proc procedure in Symbol class sends a send message to an array object ([1,'a']) with parameter (:first), e.g., [1,'a'].send(:first) is executed.

4. iterates over the rest of the elements in [[1,'a'],[2,'b'],[3,'c']] object.

This is the same as executing [[1,'a'],[2,'b'],[3,'c']].map(|e| e.first) expression.

---

(&:name) is short for (&:name.to_proc) it is same as tags.map{ |t| t.name }.join(' ')

to_proc is actually implemented in C

---

map(&:name) takes an enumerable object (tags in your case) and runs the name method for each element/tag, outputting each returned value from the method.

It is a shorthand for

array.map { |element| element.name }

which returns the array of element(tag) names

---

First, &:name is a shortcut for &:name.to_proc, where :name.to_proc returns a Proc (something that is similar, but not identical to a lambda) that when called with an object as (first) argument, calls the name method on that object.

Second, while & in def foo(&block) ... end converts a block passed to foo to a Proc, it does the opposite when applied to a Proc.

Thus, &:name.to_proc is a block that takes an object as argument and calls the name method on it, i. e. { |o| o.name }.

---

Although we have great answers already, looking through a perspective of a beginner I'd like to add the additional information:

What does map(&:name) mean in Ruby?

This means, that you are passing another method as parameter to the map function. (In reality you're passing a symbol that gets converted into a proc. But this isn't that important in this particular case).

What is important is that you have a method named name that will be used by the map method as an argument instead of the traditional block style.

---

It basically execute the method call tag.name on each tags in the array.

It is a simplified ruby shorthand.

---

There isn't a &: operator in Ruby. What you are seeing is the & operator applied to a :symbol.

In a method argument list, the & operator takes its operand, converts it to a Proc object if it isn't already (by calling to_proc on it) and passes it to the method as if a block had been used.

my_proc = Proc.new { puts "foo" }

my_method_call(&my_proc) # is identical to: my_method_call { puts "foo" }

---

Here :name is the symbol which point to the method name of tag object. When we pass &:name to map, it will treat name as a proc object. For short, tags.map(&:name) acts as:

tags.map do |tag|
  tag.name
end

---

it means

array.each(&:to_sym.to_proc)

---

It is same as below:

def tag_names
  if @tag_names
    @tag_names
  else
    tags.map{ |t| t.name }.join(' ')
end