Learn about metaprogramming and common meta programming methods

Sunday, 12/07/2020

Tram Ho

I have just reviewed some in-depth concepts in ruby after a while of inactivity, today I would like to write an article about metaprogramming.

Metaprogramming concept:

According to Wikipedia:

 Metaprogramming is the writing of computer programs that write or manipulate other programs (or themselves) as their data, or that do part of the work at compile time that would otherwise be done at runtime. In many cases, this allows programmers to get more done in the same amount of time as they would take to write all the code manually, or it gives programs greater flexibility to efficiently handle new situations without recompilation. Or, more simply put: Metaprogramming is writing code that writes code during runtime to make your life easier.

Metaprogramming is the writing of computer programs that write or manipulate other programs (or themselves) as their data, or that do part of the work at compile time that would otherwise be done at runtime. In many cases, this allows programmers to get more done in the same amount of time as they would take to write all the code manually, or it gives programs greater flexibility to efficiently handle new situations without recompilation. Or, more simply put: Metaprogramming is writing code that writes code during runtime to make your life easier.

In a nutshell, metaprogramming is a technique that makes our code more flexible and dynamic. That is, it will allow the definition of methods and classes at runtime, keeping the code short and avoiding duplication.

The common method in metaprogramming

Send

Whenever you call a method on an object, you’re sending a “message” to that object. And “message” is the name of the method. The first parameter is the name of the method that we want to call, the other arguments are parameters corresponding to the parameters of the function we defined below. For example:


class LearnSendMethod
  def dogs name
    puts "My name is " + name 
  end
end

a = LearnSendMethod.new
a.send(:dogs, "alsaka")

class LearnSendMethod

def dogs name

puts "My name is " + name

end

a = LearnSendMethod.new

a.send(:dogs, "alsaka")

respond_to?

This method will return true if the object responds with the input method name, it is a symbol (if you pass the string, it will also convert to symbol itself).


class LearnSendMethod
  def dogs name
    puts "My name is " + name 
  end
end

a = LearnSendMethod.new

if a.respond_to? :dogs
  a.send(:dogs, "alsaka")
else
  puts "No method invalid"
end

class LearnSendMethod

def dogs name

puts "My name is " + name

end

a = LearnSendMethod.new

if a.respond_to? :dogs

a.send(:dogs, "alsaka")

else

puts "No method invalid"

end

However, if written as above, it can only find public methods, to find protected and private methods, you must pass the second parameter. Note that the second parameter of the method respond_to? Very important here, it denotes that respond_to? You can find all methods whose scope includes both protected and private methods. In the above example, if dogs is a private method, we would call the following:

class LearnSendMethod

private
  def dogs name
    puts "My name is " + name 
  end
end

a = LearnSendMethod.new

if a.respond_to? :dogs, true
  a.send(:dogs, "alsaka")
else
  puts "No method invalid"
end

class LearnSendMethod

private

def dogs name

puts "My name is " + name

end

a = LearnSendMethod.new

if a.respond_to? :dogs, true

a.send(:dogs, "alsaka")

else

puts "No method invalid"

end

instance_eval

I rewrite the above code a little to the full as follows:


class InstanceEvalMethod
  def initialize name
    @name = name
  end
  def dogs
    puts "My name is " + name 
  end

  attr_reader :name
end

dog = InstanceEvalMethod.new "alaska"

if dog.respond_to? :dogs, true
  dog.send(:dogs)
else
  puts "No method invalid"
end

class InstanceEvalMethod

def initialize name

@name = name

end

def dogs

puts "My name is " + name

end

attr_reader :name

end

dog = InstanceEvalMethod.new "alaska"

if dog.respond_to? :dogs, true

dog.send(:dogs)

else

puts "No method invalid"

end

Currently I have called the dogs method to print out the name of the dog, but suppose in case I do not want to call the dogs method, I just want to know what the instance variable name of object a is. dog breeds were infused. Now I will use instance_eval. This method is called either block or string.

class InstanceEvalMethod
  def initialize name
    @name = name
  end
  def dogs
    puts "My name is " + name 
  end

  attr_reader :name
end

dogs = InstanceEvalMethod.new "alaska"

if dog.respond_to? :dogs, true
   dog.instance_eval do 
       puts @name
   end
else
  puts "No method invalid"
end

class InstanceEvalMethod

def initialize name

@name = name

end

def dogs

puts "My name is " + name

end

attr_reader :name

end

dogs = InstanceEvalMethod.new "alaska"

if dog.respond_to? :dogs, true

dog.instance_eval do

puts @name

end

else

puts "No method invalid"

end

module_eval, class_eval

The module_eval and class_eval methods are used on modules and classes, not objects. class_eval is an alias of module_eval. These methods are used to add / remove values of class variables from outside the class.

class Dogs
  @@name = "alaska"
end
puts Dogs.class_eval("@@name") # =&gt; alaska

class Dogs

@@name = "alaska"

end

puts Dogs.class_eval("@@name") # => alaska

Module_eval and class_eval can be used to add instance methods to a module or class. Although the two methods have different names, the functions are identical.

class Dogs
end
Dogs.class_eval do
  def who
    "Pecgie"
  end
end
obj = Dogs.new
puts obj.who # =&gt; Pecgie

class Dogs

end

Dogs.class_eval do

def who

"Pecgie"

end

obj = Dogs.new

puts obj.who # => Pecgie

Note: class_eval defines instance methods, and instance_eval defines class_methods

method_missing

Method_missing is a method that ruby allows you to access inside an object and handle in case you call a method that doesn’t exist. It is a built-in method of the BasicObject class in Ruby Method_mising that takes three parameters:

The first parameter is the name of the method you are trying to call
The second parameter is the argument (* args) passed when calling to that method
The third argument is a block (& block) the parameters of the method The second and third parameters can be empty if the method called does not pass the parameters.

 class MethodMissing
  def method_missing(m, *args, &amp;block)
    puts "Called #{m} with #{args.inspect} and #{block}"
  end
end

MethodMissing.new.anything # =&gt; Called anything with [] and
MethodMissing.new.anything(3, 4) { something } # =&gt; Called anything with [3, 4] and #&lt;Proc: <a class="__cf_email__" href="/cdn-cgi/l/email-protection">[email protected]</a> :7&gt;

class MethodMissing

def method_missing(m, *args, &block)

puts "Called #{m} with #{args.inspect} and #{block}"

end

MethodMissing.new.anything # => Called anything with [] and

MethodMissing.new.anything(3, 4) { something } # => Called anything with [3, 4] and #<Proc: <a class="__cf_email__" href="/cdn-cgi/l/email-protection">[email protected]</a> :7>

summary

Above is an important introduction about metaprogramming and some methods, there will be many other methods but within this article I only write some common methods when doing projects. Hope everyone can contribute ideas to improve the article!

References

http://ruby-metaprogramming.rubylearning.com/html/ruby_metaprogramming_3.html

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Source : Viblo

Learn about metaprogramming and common meta programming methods

Metaprogramming concept:

The common method in metaprogramming

Send

respond_to?

instance_eval

module_eval, class_eval

method_missing

summary

References

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