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