Question

I want a function that keeps local state in Ruby. Each time I call the function I want to return a result that depends both on a calling argument and on the function’s stored state. Here’s a simple example:

def inc_mult(factor)
  @state ||= 0   # initialize the state the first time.
  @state += 1    # adjust the internal state.
  factor * @state
end

Note that the state is initialized the first time, but subsequent calls access stored state. This is good, except that @state leaks into the surrounding context, which I don’t want.

What is the most elegant way of rewriting this so that @state doesn’t leak?

(Note: My actual example is much more
complicated, and initializing the
state is expensive.)

Answer 1

You probably want to encapsulate inc_mult into its own class, since you want to encapsulate its state separately from its containing object. This is how generators (the yield statement) work in Python and C#.

Something as simple as this would do it:

class Foo 
  state = 0 
  define_method(:[]) do |factor|
    state += 1
    factor * state
  end 
end

Philosophically, I think what you’re aiming for is incompatible with Ruby’s view of methods as messages, rather than as functions that can somewhat stand alone.