From: Rob Biedenharn <Rob@...>
Date: 2009-02-02T06:04:55+09:00
Subject: Re: Could someone please explain the magic in this code

On Feb 1, 2009, at 3:15 PM, John Small wrote:

> I need to use erb templates so I went to look at the documentation. It
> all seems straight forwards until I get to the example where a lot of
> new coding concepts are pulled in and I'm left totally mystified about
> what's going on.
>
> The code, from the api documentation is
> <pre>
> require "erb"
>
> # build data class
> class Listings
>   PRODUCT = { :name => "Chicken Fried Steak",
>               :desc => "A well messages pattie, breaded and fried.",
>               :cost => 9.95 }
>
>   attr_reader :product, :price
>
>   def initialize( product = "", price = "" )
>     @product = product
>     @price = price
>   end
>
>   def build
>     b = binding
>     # create and run templates, filling member data variebles
>     ERB.new(<<-'END_PRODUCT'.gsub(/^\s+/, ""), 0, "",
> "@product").result b
>       <%= PRODUCT[:name] %>
>       <%= PRODUCT[:desc] %>
>     END_PRODUCT
>     ERB.new(<<-'END_PRICE'.gsub(/^\s+/, ""), 0, "", "@price").result b
>       <%= PRODUCT[:name] %> -- <%= PRODUCT[:cost] %>
>       <%= PRODUCT[:desc] %>
>     END_PRICE
>   end
> end
>
> # setup template data
> listings = Listings.new
> listings.build
>
> puts listings.product + "\n" + listings.price
>
> </pre>
> My questions are;-
>
> (1) what does "<<-" do just before 'END_PRODUCT'

That's a "here document".  <<-'END_PRODUCT' means to take the  
following lines up until a line begins with END_PRODUCT as if they  
were a string.  The - let's END_PRODUCT be indented from the beginning  
of the line.  The single quotes say that no interpolation is to be  
done on the string (that is, it should act like a string literal with  
single quotes).

>
> (2) how do the lines after .result b get substituted into END_PRODUCT

I hope (1) answers that ...

>
> and why is there a gsub there.

The gsub is removing the leading whitespace /^\s+/ from each line.   
This lets the here-document contents be intended to where code would  
normally be.

> My understanding of the order things are
> happening is clearly deficient because the gsub to remove white space
> will have no effect on 'END_PRODUCT' since there's no white space in  
> it.
> So somehow 'END_PRODUCT' is getting replaced by the lines below, which
> aren't in quotes, before the gsub happens. wtf is going on here?

Do you get it now?

The equivalent would look like:
     ERB.new("<%= PRODUCT[:name] %>\n<%= PRODUCT[:desc] %>\n",
             0, "", "@product").result b
If the here-document were not used.

>
> (3) How does binding work? The documentation is pretty opaque. What is
> it doing in this context and why is it required?

You want the PRODUCT, @product, and @price to be evaluated in the  
context of the Listings object rather than the new ERB object just  
created.

>
>
> All explanations welcome
> -- 
> Posted via http://www.ruby-forum.com/.
>


I hope this helps.

-Rob

Rob Biedenharn		http://agileconsultingllc.com
Rob@AgileConsultingLLC.com