From: Timothy Hunter <TimHunter@...>
Date: 2006-07-31T05:19:14+09:00
Subject: Re: mimic File.open pattern

Daniel Schierbeck wrote:
> Sch�le Daniel wrote:
>> Hello all,
>>
>> open is singleton method of File and can be used in two ways
>>
>> # 1 way
>> f = File.open("data")
>> f.readline until f.lineno == 10
>> puts f.readline
>> f.close
>>
>> # 2 way
>> File.open("data") do |f|
>>     f.readline until f.lineno == 10
>>     puts f.readline
>> end
>>
>> I am trying to mimic this with my own classes
>>
>> irb(main):001:0> class Resource
>> irb(main):002:1> @@cnt=0
>> irb(main):003:1> def initialize
>> irb(main):004:2> @id =  @@cnt
>> irb(main):005:2> @@cnt += 1
>> irb(main):006:2> end
>> irb(main):007:1> def id;@id;end
>> irb(main):008:1> end
>> => nil
>> irb(main):009:0> class Q
>> irb(main):010:1> def Q.get &block
>> irb(main):011:2> r = Resource.new
>> irb(main):012:2> if Kernel::block_given?
>> irb(main):013:3> block.call(r)
>> irb(main):014:3> else
>> irb(main):015:3* return r
>> irb(main):016:3> end
>> irb(main):017:2> end
>> irb(main):018:1> end
>> => nil
>> irb(main):019:0> Q.get
>> => #<Resource:0x40206fe0 @id=0>
>> irb(main):020:0> Q.get {|r| puts r.id}
>> 1
>> => nil
>> irb(main):021:0>
>
> First off, you can make it more efficient if your use `yield' instead 
> of calling the block explicitly:
>
>   class Q
>     def self.get
>       if block_given?
>         yield Resource.new
>       else
>         Resource.new
>       end
>     end
>   end
>
> The above is what I'd go with, but there a lots of ways to accomplish 
> what you want. Take this for example:
>
>   class Q
>     def self.get
>       yield resource = Resource.new
>     rescue LocalJumpError
>       resource
>     end
>   end
>
> or even shorter (though it'll catch other exceptions as well, so you 
> have to be careful)
>
>   class Q
>     def self.get
>       yield(resource = Resource.new) rescue resource
>     end
>   end
>
>
> Cheers,
> Daniel
>
There's no need to golf this. There's a well-known idiom for coding 
block-scoped resources. See the section entitled "Destroy this object 
when it goes out of scope" at 
http://wiki.rubygarden.org/Ruby/page/show/RubyIdioms. Here's the example 
from that page:

 def Resource.open( identifier )  # :yield: resource
     resource = Resource.new( identifier )
     if block_given?
         begin
             yield resource
         ensure
             resource.close
         end
     else
         return resource
     end
 end


The benefit of using the standard idiom is that any Ruby programmer that 
reads your code will instantly understand what it's doing. Coding it any 
other way will likely cause a bit of confusion. And, of course, if you 
don't use the idiom you might forget the "ensure" part.