Hello, Michael! For the sake of explanation, I'll assume that you're a Ruby beginner (sorry if you're not!).
First, be aware that the borders of metaprogramming are somewhat fuzzy in Ruby. As you learn about metaprogramming, you'll change your mind about which techniques are "meta" and which are not. In fact, one of the biggest hurdles I had while writing the book was trying to decide which techniques belonged to "metaprogramming" and which are just plain vanilla Ruby. In the end, there is no hard and fast distinction.
For example, most Ruby programmers are used to Class Macros, and don't think of them as especially "meta". If you don't know what a Class Macro is, think of
attr_accessor:
So this is technically an example of metaprogramming: under the hood,
attr_accessor wrote two new methods for you, named
my_attribute() and
my_attribute=(). Even if you never heard about metaprogramming at all, you're supposed to know about
attr_accessor if you're an experienced rubyist. If you're advanced, you're also expected to be able to come up with your own Class Macros, like ActiveRecord does when it defines macros such as
belongs_to or
validates_length_of. To write Class Macros, you probably need to understand how Ruby classes, objects and modules are organized - the so-called "object model".
If you chose to willfully ignore these techniques, you'll probably have trouble understanding the way Ruby works, and you'll have trouble understanding the typical Ruby libraries. This is what I mean when I say that metaprogramming is a core part of Ruby. The language expects that you know how the object model works.
So, how is metaprogramming "bad"? Like all powerful tools, it can be abused. Once you know the ins and outs of the Ruby object model, you have enough of a gun to shoot your own foot. For example, instead of just resorting to a Class Macro, you can open the eigenclass to define a class macro that defines a method by evaluating a
string that conditionally composes code that is nearly impossible to understand. (If that's gibberish to you, just replace it with "you can do something that's overly complicated") ;) .
I think that this kind of abuse is what Martin Fowler was talking about. It's a basic trade-off of Ruby: you have a lot of power at your disposal, and you have to draw your own line between using not enough of it, and too much of it. Wherever you put the line, you're supposed to know these techniques before you can take an informed decision.
