Q:

Get the value of an instance variable given its name

In general, how can I get a reference to an object whose name I have in a string?
More specifically, I have a list of the parameter names (the member variables - built dynamically so I can't refer to them directly).
Each parameter is an object that also has an from_s method.
I want to do something like the following (which of course doesn't work...):
define_method(:from_s) do | arg |
    @ordered_parameter_names.each do | param |
        instance_eval "field_ref = @#{param}"
        field_ref.from_s(param)
    end
end

A:

The most idiomatic way to achieve this is:
some_object.instance_variable_get("@#{name}")

There is no need to use + or intern; Ruby will handle this just fine. However, if you find yourself reaching into another object and pulling out its ivar, there's a reasonably good chance that you have broken encapsulation.
If you explicitly want to access an ivar, the right thing to do is to make it an accessor. Consider the following:
class Computer
  def new(cpus)
    @cpus = cpus
  end
end

In this case, if you did Computer.new, you would be forced to use instance_variable_get to get at @cpus. But if you're doing this, you probably mean for @cpus to be public. What you should do is:
class Computer
  attr_reader :cpus
end

Now you can do Computer.new(4).cpus.
Note that you can reopen any existing class and make a private ivar into a reader. Since an accessor is just a method, you can do Computer.new(4).send(var_that_evaluates_to_cpus)

A:

To get an instance variable from the name of an instance variable do:
name = "paramName"
instance_variable_get(("@" + name).intern)

This will return the value of the instance variable @paramName

