From: Ezra Zygmuntowicz <ezmobius@...>
Date: 2006-10-19T11:48:47+09:00
Subject: Re: whats this lambda code doing?


On Oct 18, 2006, at 7:38 PM, Ezra Zygmuntowicz wrote:
> Yes thats essentially it. Maybe this helps clear it up a bit. Lets  
> forget about the with_scope AR stuff for a minute and just mock  
> this out:
>
> class Scoper
>   def self.with_block(hsh={}, &block)
>     p hsh.merge( {:klass => name})
>     block.call
>   end
> end
>
> class Foo < Scoper
> end
>
> class Bar < Scoper
> end
>
> class Baz < Scoper
> end
>
> def set_scopers(klasses=[Foo, Bar, Baz], &block)
>    hash = {:test => 'test'}
>    klasses.inject(block) do |blk, klass|
>      lambda { klass.with_block hash, &blk }
>    end.call
> end
>
> puts set_scopers { puts 'done'}
>
> # outputs
> {:test=>"test", :klass=>"Baz"}
> {:test=>"test", :klass=>"Bar"}
> {:test=>"test", :klass=>"Foo"}
> done
>
>
>
> Now if you comment out the block.call line in the Scoper.with_block  
> method:
>
> class Scoper
>   def self.with_block(hsh={}, &block)
>     p hsh.merge( {:klass => name})
>     #block.call
>   end
> end
>
> <snip>
>
> puts set_scopers { puts 'done' }
> #ouput
> {:test=>"test", :klass=>"Baz"}
> nil
>
>
> and run the same code again it only calls the last lambda that gets  
> made. It is a twisty little mess of block and wrappers that is hard  
> to follow. But essentially inject is chaning those lambda's  
> together and the final .call calls the chain which calls the final  
> &blk in the end.
> nil


	What it is doing with the inject is essentially building up a block  
with multiple procs inside it and then calling that. So the block  
that gets called looks something like this by the time it gets call'ed:

lambda {
   lambda { Foo.with_block hash, &blk }
   lambda { Bar.with_block hash, &blk }
   lambda { Baz.with_block hash, &blk }
}.call


Make sense?

-- Ezra Zygmuntowicz 
-- Lead Rails Evangelist
-- ez@engineyard.com
-- Engine Yard, Serious Rails Hosting
-- (866) 518-YARD (9273)