Making things available only inside Ruby blocks

Question

Is there any way to make methods and functions only available inside blocks? What I'm trying to do:

some_block do
    available_only_in_block
    is_this_here?
    okay_cool
end

But the is_this_here?, okay_cool, etc. only being accessible inside that block, not outside it. Got any ideas?

Answer 1

Pass an object with the methods that you want to be available into the block as an argument. This is a pattern that's widely used in Ruby, such as in IO.open or XML builder.

some_block do |thing|
    thing.available_only_in_block
    thing.is_this_here?
    thing.okay_cool
end

Note that you can get closer to what you asked for using instance_eval or instance_exec, but that's generally a bad idea as it can have fairly surprising consequences.

class Foo
  def bar
    "Hello"
  end
end

def with_foo &block
  Foo.new.instance_exec &block
end

with_foo { bar }      #=> "Hello"
bar = 10
with_foo { bar }      #=> 10
with_foo { self.bar } #=> "Hello

While if you pass an argument in, you always know what you'll be referring to:

def with_foo
  yield Foo.new
end

with_foo { |x| x.bar }      #=> "Hello"
bar = 10
x = 20
with_foo { |x| x.bar }      #=> "Hello"