Executing a block in the context of receiver

Ruby
1

In the following code the do_this instance method of the foo object
calls yield to run the block that is passed to it.

foo.do_this(p1, p2) {
do_that(p3, p4)
}

Is there any way to make it so that do_that is executed in the context
of the foo object, i.e. it acts as though it was invoked with
foo.do_that(p3, p4)? I know I could just say “foo.do_that(p3, p4)”,
but I was wondering if I could avoid that.

2

On Wed, 2006-03-15 at 00:20 +0900, Mark V. wrote:

but I was wondering if I could avoid that.
If you’re writing ‘foo’ then you can do:

class Foo
  def do_this(&blk)
    instance_eval &blk
  end

  def do_that
    puts "Doing that..."
  end
end

f.do_this { do_that }
# (prints) Doing that...

# can use this form, too
f.do_this { |foo| foo.do_that }
# (prints) Doing that...

There’s a few subtleties to using instance_eval you might want to look
up, but maybe it does the job…

3

Also,
send(:do_that, p3, p4)

4

On Thu, 2006-03-16 at 05:57 +0900, [email protected] wrote:

}
instance_eval &blk
# can use this form, too
f.do_this { |foo| foo.do_that }
# (prints) Doing that…

But only if you yield self from do_this (just to clarify). And
initialize f, but I guess that was implied :slight_smile:

(Oops, overzealous cut/paste job :slight_smile: )

Probably irrelevant, but from what I can gather, instance_eval will pass
the new self in as a block parameter, too:

Object.new.instance_eval { |me| self == me }
# => true

Though this doesn’t seem to be documented…?

I like the second form better. Just having self change mysteriously
for the duration of a block always strikes me as a bit of an
obfuscation.

I like instance_eval for those cool DSL style things, but usually I’m
with you on that one.

5

Hi –

On Wed, 15 Mar 2006, Ross B. wrote:

foo.do_that(p3, p4)? I know I could just say “foo.do_that(p3, p4)”,
puts “Doing that…”
end
end

f.do_this { do_that }

(prints) Doing that…

can use this form, too

f.do_this { |foo| foo.do_that }

(prints) Doing that…

But only if you yield self from do_this (just to clarify). And
initialize f, but I guess that was implied :slight_smile:

I like the second form better. Just having self change mysteriously
for the duration of a block always strikes me as a bit of an
obfuscation.

David

–
David A. Black ([email protected])
Ruby Power and Light, LLC (http://www.rubypowerandlight.com)

“Ruby for Rails” chapters now available
from Manning Early Access Program! Ruby for Rails

6

Hi –

On Thu, 16 Mar 2006, Ross B. wrote:

Probably irrelevant, but from what I can gather, instance_eval will pass
the new self in as a block parameter, too:

Object.new.instance_eval { |me| self == me }

=> true

Though this doesn’t seem to be documented…?

Interesting – definitely relevant, too, in the sense that my
correction was wrong. I don’t remember seeing that documented either.

David

–
David A. Black ([email protected])
Ruby Power and Light, LLC (http://www.rubypowerandlight.com)

“Ruby for Rails” chapters now available
from Manning Early Access Program! Ruby for Rails