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comp.lang.ruby

question on blocks

Jeff Thorne

4/1/2006 1:06:00 AM

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 Answers

Marcin Mielzynski

4/1/2006 1:45:00 AM

0

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

Patrick Hurley

4/1/2006 1:49:00 AM

0

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

Jeff Thorne

4/1/2006 2:00:00 AM

0

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 :-)

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

Patrick Hurley

4/1/2006 2:04:00 AM

0

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