Dynamic Variable Scoping in Ruby 1.9

Question

I'm interested in using dynamic (as opposed to lexical) scoped variables in Ruby.

It's seems there isn't a direct built-in way, as with let in Lisp. One possible way to do dynamic scoped variable is suggested by Christian Neukirchen. He creates a "thread local hash" in his Dynamic class. I wasn't too crazy about that.

Then I remembered that Ruby 1.9 has a tap method. I see a lot of people using tap to print debugging values in a chain of commands. I think it can be used to very nicely mimic a dynamically scoped variable.

Below is an an example of a situation in which one would want to use a dynamical scoped variable, and a solution using tap.

If I had a blog to post this on, and get some feedback, I would do it there. Instead I come to S/O for criticism of this idea. Post your critique, I'll give the correct answer to the one with the most upvotes.

---

Situation

You have an ActiveRecord object representing an Account, each account has_many Transactions. A Transaction has two attributes:

• description
• amount

You want to find the sum of all transactions on the account, keeping in mind that amount can be either nil or a Float (no you can't critique that).

Your first idea is:

def account_value
  transactions.inject(0){|acum, t| acum += t.amount}
end

This bombs the first time you have a nil amount:

TypeError: nil can't be coerced into Fixnum

Clean Solution

Use tap to temporarily define amount = 0. We only want this to be temporary in case we forget to set it back and save the transaction with the 0 value still in place.

def account_value
  transactions.inject(0){|acm, t| t.amount.tap{|amount| amount ||=0; acm+=amount}; acm}
end

Since the assignment-to-zero-if-nil of amount is within the tap bock, we don't have to worry about forgetting to set it back to nil.

What are your thoughts?

Answer 1

Well, I think you're aiming for something else, but the following code fixes your example and is actually easier to understand:

transactions.inject(0) { |acum, t| acum += t.amount || 0 }

But I don't don't think the method summing up the amounts should know about the default value for nil amounts, so (even if your question states that I can't argue with it) I would change the amount method to return the default instead:

def amount
  @amount || 0
end

Nevertheless I think your example is just too easy to solve and you're actually aiming for answers to a more complex question. Looking forward to all the other answers.