Blocks and context

[Dokkalfr]

Hi,

When learning ruby blocks I've read that the context of the block, as the example in http://www.rubycentral.com/book/tut_containers.html

def nTimes(aThing)
  return proc { |n| aThing * n }
end
p1 = nTimes(23)
p1.call(3) 	» 	69
p1.call(4) 	» 	92
p2 = nTimes("Hello ")
p2.call(3) 	» 	"Hello Hello Hello "

But now I wanna to write GUIs and reading the QtRuby documentation I saw this use of blocks:

 w = MyWidget.new { setCaption("foobar") }

. The document explains the sample saying that " The block will be called in the context of the newly created instance". Is it a misuse of ruby blocks??

[Fenrir2]

I don't think so. If you didn't use blocks for that you would have this code:

w = MyWidget.new
w.setCaption("foobar")

And you couln't pass it as a parameter then:

w = MyWidget.new
w.setCaption("foobar")
foo(w)

vs.

foo(MyWidget.new {
                  setCaption("foobar")
})

[Dokkalfr]

It could be done with blocks like this

w = MyWidget.new { |widget| widget.setCaption("foobar") }

w/a change the context into the new instance. This change on context doesn't break the concept of closure??

[stereofrog]

Normally blocks are executed in a definition (caller) context

class A
	def initialize
		yield
	end
end
A.new { p self } #main

With instance_eval it's possible to execute block in callee's (or any other) context:

class B
	def initialize(&block)
		instance_eval(&block)
	end
end
B.new { p self } #B...

and this is indeed a method tk widgets use:

require 'tk'
TkLabel.new { p self } # TkLabel...

I don't know if that is a good or bad "practice", but at the first glance it looks confusing. For example, you're unable to use caller's methods inside the block:

class X
	def foo
		p "foo"
	end
	def bar
		TkLabel.new { foo() } #doesn't work!
	end
end

[Dokkalfr]

thanks for the answer stereofrog ... I was wondering wether to use this "approach" in my code or not, but I agree it's confusing, so I will keep it away