ruby - how to dynamically call instance method in a module

Question

I'm trying to dynamically call instance methods. I found send, call, eval to do that. There are examples of dynamically calling class methods, but I haven't figured out how to make it work for instance methods.

e.g.

module MyModule
  def Foo
    puts "hello"
  end
end

the instance method names can be listed by:

MyModule.instance_methods
#=> [:Foo]

But I can't figure out how to call the method:

MyModule.send("Foo")
#=> NoMethodError: undefined method `Foo' for MyModule:Module

MyModule.method("Foo").call
#=> NameError: undefined method `Foo' for class `Module'

eval 'MyModule.Foo'
#=> NoMethodError: undefined method `Foo' for MyModule:Module

How can I call the instance methods, like Foo, by the method name?

Note that eval 'MyModule.Foo' is the same as MyModule.Foo. — Stefan
– Stefan, Commented Apr 20, 2017 at 10:29

Andrey Deineko · Accepted Answer · 2017-04-20 11:11:26Z

11

Disclaimer: Bad practice and it makes little to no sense:

MyModule.instance_method(:Foo) # #<UnboundMethod: MyModule#Foo>
        .bind(Object)          # #<Method: MyModule#Foo>
        .call                  #=> hello

You can't call an unbound method, thus you have to first bind it to some object.

References:

It would be much easier if you make the method singleton method:

module MyModule
  def self.Foo
    puts "hello"
  end
end

MyModule.Foo
#=> "hello"

Another option is to use module_function (read up on the method's visibility for the including object when using this option):

module MyModule
  def Foo
    puts "hello"
  end
  module_function :Foo
end

MyModule.Foo
#=> "hello"

And yet another one is using extend self:

module MyModule
  def Foo
    puts "hello"
  end

  extend self
end

MyModule.Foo
#=> "hello"

P.S. It is not conventional to use capital letters in method names. It is very rare case in Ruby where it can be used and your case is 100% not the one :)

edited Apr 20, 2017 at 11:11

answered Apr 20, 2017 at 9:04

Andrey Deineko

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6 Comments

Aleksei Matiushkin Over a year ago

Both module_function and extend self are worth to mention here.

jerry Over a year ago

Thanks a lot! the singleton method approach works for me!

Ilya Over a year ago

Without module body editing Class.new.include(MyModule).new.Foo just as ugly but still possible way.

Stefan Over a year ago

@Ilya you don't have to create a class: Object.new.extend(MyModule).Foo

Andrey Deineko Over a year ago

@Stefan Object.extend(MyModule).Foo is even shorter (despite the pollution caused by it) :)

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