Access variables programmatically by name in Ruby

Question

I'm not entirely sure if this is possible in Ruby, but hopefully there's an easy way to do this. I want to declare a variable and later find out the name of the variable. That is, for this simple snippet:

foo = ["goo", "baz"]

How can I get the name of the array (here, "foo") back? If it is indeed possible, does this work on any variable (e.g., scalars, hashes, etc.)?

Edit: Here's what I'm basically trying to do. I'm writing a SOAP server that wraps around a class with three important variables, and the validation code is essentially this:

  [foo, goo, bar].each { |param|
      if param.class != Array
        puts "param_name wasn't an Array. It was a/an #{param.class}"
        return "Error: param_name wasn't an Array"
      end
      }

My question is then: Can I replace the instances of 'param_name' with foo, goo, or bar? These objects are all Arrays, so the answers I've received so far don't seem to work (with the exception of re-engineering the whole thing ala dbr's answer)

Answer 1

Foo is only a location to hold a pointer to the data. The data has no knowledge of what points at it. In Smalltalk systems you could ask the VM for all pointers to an object, but that would only get you the object that contained the foo variable, not foo itself. There is no real way to reference a vaiable in Ruby. As mentioned by one answer you can stil place a tag in the data that references where it came from or such, but generally that is not a good apporach to most problems. You can use a hash to receive the values in the first place, or use a hash to pass to your loop so you know the argument name for validation purposes as in DBR's answer.

Answer 2

The closest thing to a real answer to you question is to use the Enumerable method each_with_index instead of each, thusly:

my_array = [foo, baz, bar]
my_array.each_with_index do |item, index|
  if item.class != Array
    puts "#{my_array[index]} wasn't an Array. It was a/an #{item.class}"
  end
end

I removed the return statement from the block you were passing to each/each_with_index because it didn't do/mean anything. Each and each_with_index both return the array on which they were operating.

There's also something about scope in blocks worth noting here: if you've defined a variable outside of the block, it will be available within it. In other words, you could refer to foo, bar, and baz directly inside the block. The converse is not true: variables that you create for the first time inside the block will not be available outside of it.

Finally, the do/end syntax is preferred for multi-line blocks, but that's simply a matter of style, though it is universal in ruby code of any recent vintage.

Answer 3

OK, it DOES work in instance methods, too, and, based on your specific requirement (the one you put in the comment), you could do this:

local_variables.each do |var|
  puts var if (eval(var).class != Fixnum)
end

Just replace Fixnum with your specific type checking.

Answer 4

Great question. I fully understand your motivation. Let me start by noting, that there are certain kinds of special objects, that, under certain circumstances, have knowledge of the variable, to which they have been assigned. These special objects are eg. Module instances, Class instances and Struct instances:

Dog = Class.new
Dog.name # Dog

The catch is, that this works only when the variable, to which the assignment is performed, is a constant. (We all know that Ruby constants are nothing more than emotionally sensitive variables.) Thus:

x = Module.new # creating an anonymous module
x.name #=> nil # the module does not know that it has been assigned to x
Animal = x # but will notice once we assign it to a constant
x.name #=> "Animal"

This behavior of objects being aware to which variables they have been assigned, is commonly called constant magic (because it is limited to constants). But this highly desirable constant magic only works for certain objects:

Rover = Dog.new
Rover.name #=> raises NoMethodError

Fortunately, I have written a gem y_support/name_magic, that takes care of this for you:

 # first, gem install y_support
require 'y_support/name_magic'

class Cat
  include NameMagic
end

The fact, that this only works with constants (ie. variables starting with a capital letter) is not such a big limitation. In fact, it gives you freedom to name or not to name your objects at will:

tmp = Cat.new # nameless kitty
tmp.name #=> nil
Josie = tmp # by assigning to a constant, we name the kitty Josie
tmp.name #=> :Josie

Unfortunately, this will not work with array literals, because they are internally constructed without using #new method, on which NameMagic relies. Therefore, to achieve what you want to, you will have to subclass Array:

require 'y_support/name_magic'
class MyArr < Array
  include NameMagic
end

foo = MyArr.new ["goo", "baz"] # not named yet
foo.name #=> nil
Foo = foo # but assignment to a constant is noticed
foo.name #=> :Foo

# You can even list the instances
MyArr.instances #=> [["goo", "baz"]]
MyArr.instance_names #=> [:Foo]

# Get an instance by name:
MyArr.instance "Foo" #=> ["goo", "baz"]
MyArr.instance :Foo #=> ["goo", "baz"]

# Rename it:
Foo.name = "Quux"
Foo.name #=> :Quux

# Or forget the name again:
MyArr.forget :Quux
Foo.name #=> nil

# In addition, you can name the object upon creation even without assignment
u = MyArr.new [1, 2], name: :Pair
u.name #=> :Pair
v = MyArr.new [1, 2, 3], ɴ: :Trinity
v.name #=> :Trinity

I achieved the constant magic-imitating behavior by searching all the constants in all the namespaces of the current Ruby object space. This wastes a fraction of second, but since the search is performed only once, there is no performance penalty once the object figures out its name. In the future, Ruby core team has promised const_assigned hook.

Answer 5

Building on joshmsmoore, something like this would probably do it:

# Returns the first instance variable whose value == x
# Returns nil if no name maps to the given value
def instance_variable_name_for(x)
  self.instance_variables.find do |var|
    x == self.instance_variable_get(var)
  end
end