Question on blocks

ruby-talk
1

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" }

2

Can anyone explain this: ?

class A
     def x
         yield
     end

     def x=a
         yield
     end
end

a=A.new
a.x{p "called"}
a.x=1 do
     p "called"
end

../test.rb:13: parse error, unexpected kDO, expecting $

lopex

3

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

···

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" }

4

Patrick,

Thanks for the quick response. I agree that it doesn't make sense
unless I was passing the new value back to the block. I am about
15 minutes into the Picaxe and was just trying to get a handle on the
syntax. The language looks great. My brain is just hardwired with java
syntax at this point :slight_smile:

Cheers,
Jeff

···

-----Original Message-----
From: Patrick Hurley [mailto:phurley@gmail.com]
Sent: Friday, March 31, 2006 5:49 PM
To: ruby-talk ML
Subject: Re: question on blocks

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

5

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

···

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