Calling a block with parameters in a new scope

ruby-talk
1

Calling a block in a new scope is easy enough:

self.instance_eval &block

And calling a block, passing it parameters is easy enough:

block.call(*params)

But is there some way to do both? To both change the scope of evaluation and pass variables into the block.

2

Ruby Talk wrote:

Calling a block in a new scope is easy enough:

self.instance_eval &block

And calling a block, passing it parameters is easy enough:

block.call(*params)

But is there some way to do both? To both change the scope of evaluation
and pass variables into the block.

Hello,

I'm not sure I fully understand the question being asked here. Can you
provide a small bit of code to highlight what you are trying to acheive.

Do you wish to pass vaiables into the instance_eval block?

Any variables in scope when the instance_eval block is called can be
used in the instance_eval block itself. However, if you were to pass a
proc object into the instance_eval it would retain the context it was
created at.

See below for an example of this.

class Test
  def initialize(arg)
    @instance_variable = arg
  end
end

test_obj = Test.new("initializing_arg")

outside = "outside instance_eval"

proc = Proc.new {|arg| puts self} #self main here

test_obj.instance_eval do
   puts @instance_variable #test_obj instance variable.
   puts outside #any variable from outside the block can be passed in.
   puts proc #will return main rather than the object
end

Hmm, I get the feeling I have ranted about stuff that does not help you
one bit with your question...

Get back to me with a bit more detail and hopefully I'll be able to
help!

Cheers,

Matt.

···

--
Posted via http://www.ruby-forum.com/\.

3

http://eigenclass.org/hiki.rb?instance_exec

···

On May 19, 10:07 pm, Ruby Talk <rubyt...@postful-inc.com> wrote:

self.instance_eval &block

And calling a block, passing it parameters is easy enough:

block.call(*params)

But is there some way to do both? To both change the scope of evaluation
and pass variables into the block.

4

Matt Gretton wrote:

Ruby Talk wrote:

Calling a block in a new scope is easy enough:

self.instance_eval &block

And calling a block, passing it parameters is easy enough:

block.call(*params)

But is there some way to do both? To both change the scope of evaluation
and pass variables into the block.

Hello,

I'm not sure I fully understand the question being asked here. Can you
provide a small bit of code to highlight what you are trying to acheive.

Do you wish to pass vaiables into the instance_eval block?

Any variables in scope when the instance_eval block is called can be
used in the instance_eval block itself. However, if you were to pass a
proc object into the instance_eval it would retain the context it was
created at.

See below for an example of this.

class Test
  def initialize(arg)
    @instance_variable = arg
  end
end

test_obj = Test.new("initializing_arg")

outside = "outside instance_eval"

proc = Proc.new {|arg| puts self} #self main here

test_obj.instance_eval do
   puts @instance_variable #test_obj instance variable.
   puts outside #any variable from outside the block can be passed in.
   puts proc #will return main rather than the object
end

Hmm, I get the feeling I have ranted about stuff that does not help you
one bit with your question...

Get back to me with a bit more detail and hopefully I'll be able to
help!

Cheers,

Matt.

Oops, that last 'puts proc' should be proc.call("whatever")

If full:

class Test
  def initialize(arg)
    @instance_variable = arg
  end
end

test_obj = Test.new("initializing_arg")

outside = "outside instance_eval"

proc = Proc.new {|arg| puts self} #self main here

test_obj.instance_eval do
   puts @instance_variable #test_obj instance variable.
   puts outside #any variable from outside the block can be passed in.
   proc.call("whatever") #will return main rather than the object
end

···

--
Posted via http://www.ruby-forum.com/\.

5

OP We have just discussed this twice recently, maybe you should browse
the recent posts...

Phrogz: Your link seems outdated,
c.f.http://eigenclass.org/hiki/bounded+space+instance_exec
for a discussion of #undef_method (bad) vs. #remove_method (good) and
please note the potential danger of endless recursion.

Cheers
Robert

···

On 5/20/07, Phrogz <gavin@refinery.com> wrote:

On May 19, 10:07 pm, Ruby Talk <rubyt...@postful-inc.com> wrote:
> self.instance_eval &block
>
> And calling a block, passing it parameters is easy enough:
>
> block.call(*params)
>
> But is there some way to do both? To both change the scope of evaluation
> and pass variables into the block.

http://eigenclass.org/hiki.rb?instance_exec

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You see things; and you say Why?
But I dream things that never were; and I say Why not?
-- George Bernard Shaw