Ruby one time assignment on instance variable

Question

I usually use "one time assignment" (don't really know how to call this anw) in ruby like this:

class FooBar
    def foo
       @foo ||= begin
           # Costly operation putting here
       end 
    end
end

This will allow the variable @foo to be computed only once, then being used directly in subsequent calls to method foo. My question are:

Is using begin/end block in this case good or not, as the documentation states that it is used for encapsulating exceptions, doesn't mention any other uses.
Can I use other block types (Proc, Lambda, do/end) with this syntax? If yes then how?

That's normally done in initialize, but could be done anywhere. Yes, @foo could point to the result of any type of operation: @foo = arr.map..., @foo = if..else...end, @foo = lambda..., etc. — Cary Swoveland
– Cary Swoveland, Commented Jul 23, 2014 at 5:43
Please don't change your code once comments or answers have been given, as it can render the latter meaningless. It's better to leave it as is and add a note (as an edit, not a comment) explaining what you intended, preferably with the word "edit", e.g., "Edit:...". — Cary Swoveland
– Cary Swoveland, Commented Jul 23, 2014 at 5:50

Windor C · Accepted Answer · 2014-07-23 14:23:24Z

2

I haven't seen any usage of begin/end in the memorization, but I think that's fine.
Of course, you can use lambda like this @foo ||= lambda { "foo" }.call, just remember that the one time assignment(||=) only evaluate the proc when @foo is evaluated to false. It's equivalent to the following:
```
if @foo
  @foo
else
  @foo = lambda { "foo" }.call
end
```

edited Jul 23, 2014 at 14:23

answered Jul 23, 2014 at 8:55

Windor C

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2 Comments

herophuong Over a year ago

But doesn't that mean I have to use @foo.call to get the value of @foo?

herophuong Over a year ago

I see. So I can do thing like this also: Proc.new {}.call

Nick Veys · Accepted Answer · 2014-07-23 05:46:11Z

1

I typically do it something like this:

class MyClass

  def foo
    @foo ||= build_foo
  end

  private

  def build_foo
    puts "I'm expensive"
    5
  end

end

I don't see why you'd need to make it any more complicated. To my eyes having a method named something like build_XXX or process_XXX clearly explains it's doing something more involved, and having the memoization take place in the publicly-accessible method makes it clear you are avoiding doing that more than once if needed.

edited Jul 23, 2014 at 5:46

answered Jul 23, 2014 at 5:35

Nick Veys

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1 Comment

Cary Swoveland Over a year ago

We don't know if herophuong is calling foo more than once. It could be called just once (as with initialize) then other methods are called that use @foo.

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