How can I execute a block to populate a variable only if the variable is blank?

Question

I have some instance methods in a class that must be called in a sequence. The failure of any method in a sequence will require the previous method to be re-called. I'm storing the result of each successful method call in a class variable:

class User
  @@auth_hash = {}
  def get_auth_token
    result = MyApi.get_new_auth_token(self) if @@auth_hash[self]['auth_token'].blank?
    if result['Errors']
      raise Exception, "You must reauthorize against the external application."
    else
      @@auth_hash[self]['auth_token'] = result['auth_token']
    end
    @@auth_hash[self]['auth_token']
  end
  def get_session_id
    result = MyApi.get_new_session_id if @@auth_hash[self]['session_id'].blank?
    if result['Errors']
      get_auth_token
      # Recursion
      get_session_id
    else
      @@auth_hash[self]['session_id'] = result['session_id']
    end
    @@auth_hash[self]['session_id']
  end
end

I would like to get rid of those conditionals, but don't know how to perform the block only if there are errors present in the returned hash.

It looks like the first line of `get_session_id` will generate an infinite recursion if your condition is met - is that how it's actually written? — Zach Kemp, Oct 10 '13 at 19:33
I just cobbled this together to simulate what I really have, but that conditional will not execute if result['Errors'] is nil. — RubyRedGrapefruit, Oct 10 '13 at 19:42
What is `@@auth_hash` all about? Using class variables is usually a sign something's very wrong with your design. Are you using it as some kind of cache here? That's usually counter-productive, the cache only applies in one process, of which there are usually many, and you have no invalidation code here. — tadman, Oct 10 '13 at 19:47
I'm trying to use it as some kind of cache from AR. This all runs from one background process. I would love to know a better way, believe me. I've got writer's block after looking at this too long. — RubyRedGrapefruit, Oct 10 '13 at 19:49
@tadman I am against "Using class variables is usually a sign something's very wrong with your design", but in the OPs case, it does smell wrong because the OP has not shown it used in any class method. — sawa, Oct 10 '13 at 19:50
@AKWF Have you considered Zach Kemp's comment? That infinite recursion seems useless. — sawa, Oct 10 '13 at 19:54
@sawa Virtually every time someone's using class instance variables directly, they should be using class methods that access them and act as a thin abstraction layer. Your class should not have its state manipulated by instance methods themselves, it just makes things messy and too inter-dependent. — tadman, Oct 10 '13 at 19:58
`result => nil` when `if @@auth_hash[self]['session_id'].blank? => false`. Do you want that? If not, fix that and we may be able to help. — Cary Swoveland, Oct 10 '13 at 20:17

dancow · Accepted Answer · 2013-10-10T21:48:31.353

1

This whole method requires a rewrite...but to answer your question, why wouldn't the following work?

my_proc = ->(){ fill_me_with_something }
my_var ||= my_proc.call

raise StandardError if my_var.nil?

Editing my answer to better answer the question...The syntax:

 my_proc  =   ->(a,b){ a + b }

Is another way to express a block, for all intents and purposes:

 my_proc(1, 5)   #=> 6

You can also express Procs in this format:

 my_proc = Proc.new{ |a, b| a + b }

The advantage of using a Proc is that you can set your block-like behavior to some variable, and then invoke call on that proc whenever you want, which, in your case, is when some other variable is blank.

edited Oct 10 '13 at 21:48

answered Oct 10 '13 at 20:37

dancow

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Thanks Dan! There are things on that first line I don't recognize: If you had to write that first line in English, how would you say it? I understand the assignment and the block, but the "->()" is a mystery to me. – RubyRedGrapefruit Oct 10 '13 at 21:21
That's short hand for a Proc...so another way to write it is: my_proc = Proc.new{|val| puts val} (eh screw it, just going to edit my answer) – dancow Oct 10 '13 at 21:46
I love this. I never knew that shorthand, thanks a whole lot! – RubyRedGrapefruit Oct 11 '13 at 14:32

How can I execute a block to populate a variable only if the variable is blank?

1 Answers1