From: Patrick Hurley <phurley@...>
Date: 2006-04-01T11:04:08+09:00
Subject: Re: question on blocks

On 3/31/06, Patrick Hurley <phurley@gmail.com> wrote:
> On 3/31/06, Jeff Thorne <jeff_thorne@yahoo.com> wrote:
> > I am new to ruby from java and had a question on blocks.
> >
> >
> >
> > Can a block be applied to a writeable attribute or setter? I keep getting
> > errors so I
> >
> > am assuming no but I wanted to double check in case my syntax is incorrect.
> >
> >
> >
> > Thanks for the help,
> >
> > Jeff
> >
> >
> >
> >
> >
> > class Test
> >
> >
> >
> >   def initialize(one, two)
> >
> >     @one = one
> >
> >     @two = two
> >
> >   end
> >
> >
> >
> >   def one=(one)
> >
> >     @one = one;
> >
> >     Yield;
> >
> >   end
> >
> >
> >
> >  attr_reader :one, :two
> >
> >  attr_writer :two
> >
> >
> >
> > end
> >
> >
> >
> >
> >
> >
> >
> > test = Test.new("Hello", 2);
> >
> > test.one = 3 { puts "Test Block" }
> >
> >
> >
> >
> >
>
> You are correct it is not allowed, but I am curious why you want to do
> this? Also if I were to go and write a setter that took a block,
> wouldn't you either pass the new value into the block or use the
> result of the block to set the variable?
>
> class Test
>   def a=(a)
>     @a = a
>     yield @a if block_given?
>   end
>
>   #or
>   def b=(b)
>     @b = (block_given?) ? yield b : b
>   end
> end
>
> pth
>
>

Sorry just to be clear, I do not think that method= methods are
allowed to take anything more than a single parameter -- when using
the normal "call" syntax.

Of course you can send a message to it with a block:

class A
  attr_reader :a
  def a=(a,&b)
    @a = block_given? ? yield(a) : a
  end
end

a=A.new
a.a = 7
p a.a
a.a = 1
bl = Proc.new { |v| v * 6 }
a.send "a=", 7, &bl
p a.a