Ruby instance_eval and const_missing

Question

I would like to call const_missing in an instance_eval, but seem to be missing something:

class A
  def self.const_missing(c)
    puts "Constant missing: #{c}"
  end

  def method_missing(m, *args, &block)
    puts "Method missing: #{m}"
  end       
end

# Works:
a = A.new
a.myMethod # OK: "Method missing: myMethod" 
a.MYCONST  # Sort of OK: "Method missing: MYCONST"

# Doesn't work
a.instance_eval {
  myMethod # OK: "Method missing: myMethod"
  MYCONST  # uninitialized constant MYCONST (NameError)
}

Q: Where does the const_missing call in the instance_eval go? How can redirect it to class A? (Ruby 1.9.3 if this matters)

Context: I am writing a small DSL, where upper-case letters are used as state names.

DSL.evaluate {
  ENTER_STATE_A
  WAIT_FOR_INPUT
}

These should be mapped to methods, but I would like to keep them upper case.

If your Ruby DSL requires you to contravene the established Ruby syntax for defining constants, then you are digging yourself into a giant hole. — anothermh
– anothermh, Commented Jul 31, 2017 at 21:44
That is why I would like to have any change really just inside an instance_eval. The DSL files are loaded at a well defined stage and don't mix with code. — Christopher Oezbek
– Christopher Oezbek, Commented Jul 31, 2017 at 21:49

Kyle Decot · Accepted Answer · 2017-07-31 21:43:42Z

2

You're trying to access a constant w/ a.MYCONST which ruby is interpreting as a method. You instead need to use :: along w/ access the class itself and not the instance of that class:

a = A.new
a.instance_eval {
  self::class::MYCONST # => Constant missing: MYCONST
}

It seems that when calling MYCONST inside of instance_eval it's being caught by Object.const_missing. For example if you override it you'll see the behavior you're after:

def Object.const_missing(c)
  puts "Constant missing: #{c}"
end 

a = A.new
a.instance_eval {
  MYCONST # => Constant missing: MYCONST
}

I don't recommend actually doing this though. I'm only showing this to illustrate what's happening.

edited Jul 31, 2017 at 21:43

answered Jul 31, 2017 at 21:35

Kyle Decot

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3 Comments

Christopher Oezbek Over a year ago

Yes, I got that wrong. But I would like to write just MYCONST and delegate to a.

Christopher Oezbek Over a year ago

Thanks Kyle! Is there a way to monkey_patch const_missing on Object, while I am in the instance_eval?

Kyle Decot Over a year ago

@ChristopherOezbek I'm honestly not sure. There may be a way to do it without having to monkey patch Object. Not sure if this would work but you may want to look into Refinements in Ruby.

Lucas Costa · Accepted Answer · 2017-07-31 21:20:30Z

1

Note that when you do

a.MYCONST

you got Method missing: MYCONST (and not Constant missing: MYCONST).

You should do the following:

a.instance_eval {
  myMethod
  A::MYCONST
}

answered Jul 31, 2017 at 21:20

Lucas Costa

1,1299 silver badges16 bronze badges

3 Comments

Christopher Oezbek Over a year ago

Not helping, I don't want to put DSL::ENTER_STATE_A

engineerDave Over a year ago

That's a bit harsh. His answer would help you if you'd look at it. Constants exist on the Class or Singleton level. You're expecting an instance level method call to trigger a class level constant. You have to call the constant through the class hence why you need A::MYCONST

Christopher Oezbek Over a year ago

True, I understand, sorry for not being more open to the advice. But A::MYCONST is really not what I am looking for as a solution. Technically, I don't think it is a big problem to go from class level to my instance (Thread local storage was my thought)

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