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Unit 1, Lesson 21
In Progress

Proc and Threequals

Video transcript & code

You know the threequals operator? The one that acts as a "smart" matching operator in Ruby? It sure is handy. You can match regular expressions, ranges, even classes with it.

/\A\d+\z/ === "54321" # => true
(0..10) === 5         # => true
Numeric === 123       # => true

You probably also know that the threequals is used under the hood by case statements.

case obj
when /\A\d+\z/ 
  puts "A numeric string"
when 0..10
  puts "A positive integer under 10"
when Numeric
  puts "A number"
end

Here's something you might not have known: the Proc class aliases #=== to the #call method. Let's see what this means. We define a lambda that will determine if a number is even. So if we call it on the number "2", for example, it returns true, and when we call it on "3" it returns false.

There's nothing new here so far. But instead of sending the #call message, we can substitute the threequals operator, and get the exact same result!

even = ->(x){ (x % 2) == 0 }
even.call(2)                    # => true
even.call(3)                    # => false

even === 2                      # => true
even === 3                      # => false

Since it implements the threequals, we can use this lambda as a predicate in a case statement.

case num
when even then puts "Even"
else puts "Odd"
end

The implications of this are pretty big. They mean we can define arbitrary "matchers" for any kind of complex condition we can think of. As a more practical example, here's some code that uses a case statement to determine if an HTTP response was successful or not.

require 'net/http'

SUCCESS = ->(response) { (200..299) === response.code.to_i }
CLIENT_ERROR = ->(response) { (400..499) === response.code.to_i }

response = Net::HTTP.get_response(URI.parse("http://www.google.com"))
case response
when SUCCESS then puts "Success!"
when CLIENT_ERROR then puts "Client error."
else puts "Other"
end
# >> Success!  

I don't know about you, but I think this is kinda cool.

That's it for today. Happy hacking!

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