From: James French <James.French@...>
Date: 2009-04-09T00:34:16+09:00
Subject: Re: performing an action only the first time a function called

I have to admit though, I don't understand the 'wrapped, count = self, 0' line...

-----Original Message-----
From: James French [mailto:James.French@naturalmotion.com] 
Sent: 08 April 2009 16:13
To: ruby-talk ML
Subject: Re: performing an action only the first time a function called

I see I still have some way to go with ruby :) Nice! Can't believe how quick you knocked that up...

-----Original Message-----
From: James Coglan [mailto:jcoglan@googlemail.com] 
Sent: 08 April 2009 15:43
To: ruby-talk ML
Subject: Re: performing an action only the first time a function called

> 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