From: "Jonas Pfenniger (zimbatm)" <jonas@...>
Date: 2011-01-07T03:48:36+09:00
Subject: Re: Block variable - How is it read in English?

2011/1/6 SW Engineer <abder.rahman.ali@gmail.com>:
> Following the "Ruby on Rails Tutorial", and under section "6.1.1
> Database migrations"
> http://railstutorial.org/chapters/modeling-and-viewing-users-one#top
>
> There is the following migration:
>
> class CreateUsers < ActiveRecord::Migration
>  def self.up
>    create_table :users do |t|
>      t.string :name
>      t.string :email
> .
> .
> .
>
> How do we read this line in English?
>
> create_table :users do |t|
>
> Why should we write the following line with "t."?
>
> t.string :name
>
> I know that block variables are used to pass parameters, but what is it
> trying to tell us here?
>
> So the "do |t|" part is actually the part that is confusing me.
>
> Any clarifications would be appreciated.
>
> Thanks.

The do..end notation is the syntax to describe a block. Blocks can
also be written with the {...} notation, which is equivalent.

create_table :users {|t|
  t.string :name
  t.string :email
}

create_table will accept two arguments, the :users symbol, and the block.
notice there is no comma between the two arguments, this is because blocks have
a special status, they are only described as the last argument of a
function-call.

the block in turn, is like a function, but that has no name. instead
of defining arguments
with the (...) notation, it uses this special |...| notation.

when create_table is called with those two arguments, it will itself call
the block, and provide an object, that we named 't'.
t.string further calls the string function on the t argument.

I hope I made it a bit clearer, but if I didn't persist. Once you
understand this,
you will feel much more empowered :)
Also, I see you took the example from the old Pickaxe book. It doesn't
do a really good
job of explaining blocks, so maybe it's worth to buy the new edition.

This seems to be a good introduction too:
http://eli.thegreenplace.net/2006/04/18/understanding-ruby-blocks-procs-and-methods/