From: James Coglan <jcoglan@...>
Date: 2009-04-08T23:43:05+09:00
Subject: Re: performing an action only the first time a function called

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> Could anyone suggest an elegant way of doing something like:
>
> $directXSDKBootstrapped = false
>
> def bootstrapDirectXSDK
>  if !$directXSDKBootstrapped
>    bootstrapBuildToolsDirectory("DirectX/#{DIRECTX_VERSION}")
>    $directXSDKBootstrapped = true
>    $directXSDKBootstrapped.freeze
>  end
> end



One way of doing this is as follows:

class Proc
  def runs(n)
    wrapped, count = self, 0
    lambda { |*args|
      count += 1
      count <= n ? wrapped.call(*args) : nil
    }
  end
end

>> foo = lambda { |arg| puts arg }.runs(3)
#=> #<Proc:0xb7d310dc@(irb):33>

>> foo['look!']
look!
=> nil
>> foo['look!']
look!
=> nil
>> foo['look!']
look!
=> nil
>> foo['look!']
=> nil

Notice nothing is printed the fourth time. So using this, you could write
you function as:

bootstrapDirectXSDK = lambda {
  bootstrapBuildToolsDirectory("DirectX/#{DIRECTX_VERSION}")
}.runs(1)

Which will only allow its contents to be executed once.

--
James Coglan
http://github.com/jcoglan

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