From: Brian Candler <B.Candler@...>
Date: 2003-07-13T02:08:11+09:00
Subject: Re: do...end vs. begin..end

On Sat, Jul 12, 2003 at 04:46:21AM +0900, Kurt M. Dresner wrote:
> I realize what the difference between these two block delimiters is, but
> I can't seem to put it in words.  I realize that you only use do..end
> when you are passing a block to a method, and that you can use
> begin..end to create a block expression with a specific return value
> (and rescue/ensure clauses).  How would I explain to someone what the
> difference is, though?  I've been reading through "Programming Ruby" and
> they just call both of them "block"s.  Anyone with a better way of
> explaining it?

do...end  delimits a block, just like { ... }. As far as I know, it can't be
used as a stand-alone construct; the block has to be passed to a method.

irb(main):002:0> do
irb(main):003:1* puts "hello"
irb(main):004:1> end
SyntaxError: compile error

irb(main):005:0> 1.times do
irb(main):006:1* puts "hello"
irb(main):007:1> end
hello

begin...end  groups together some expressions/statements. I've never seen
it used except when 'rescue' and/or 'ensure' are also required, because
Ruby's other control structures (e.g. if .. else .. end, while ... end) also
implicitly group statements together.

Note that 'def' also behaves like 'begin':

  def mymethod(x)
    puts "hello #{x}"
    raise "roof"
  rescue
    puts "got exception"
  end

  mymethod("world")

  #>> hello world
  #>> got exception

Regards,

Brian.