Setting method local variables from a proc

Question

If I have a class with two instance variables @x and @y, I can change them from a proc using self.instance_exec:

class Location
  attr_accessor :x, :y
  def initialize
    @x = 0
    @y = 0
  end
  def set &block
    self.instance_exec(&block)
  end
end

location = Location.new
location.set do
  @x = rand(100)
  @y = rand(100)
end

puts location.x
puts location.y

If I have a class with a method set with two local variables x and y, I can use proc return values to set them:

class Location
  def set &block
    x = 0;
    y = 0;
    x, y = block.call()
    # do something with x and y
    puts x
    puts y
  end
end

location = Location.new
location.set do
  x = rand(100)
  y = rand(100)
  [x, y]
end

Is there a way to access the set method local variables x and y from the proc without using return values?

No, at least I hope not. Blocks can't muck about with their calling scope like that. Generally that's the wrong direction for data flow. Procs and functions should accept inputs and return outputs, without side effects like overwriting the value of variables in their parent scope. — user229044
– user229044 ♦, Commented Jun 8, 2014 at 13:34
When you are saying a variable as local, how then you expect it to behave like non-local.. No you can't use local variables of one scope to the other scope.( closure is an exception, but it doesn't fit into your need) — Arup Rakshit
– Arup Rakshit, Commented Jun 8, 2014 at 13:36

Community · Accepted Answer · 2017-05-23 12:05:08Z

2

You can do it, sort of, but it isn't pretty

There is a way for block to set a variable in a calling method, but it isn't pretty. You can pass in a binding, then eval some code using the binding:

def foo(binding)
  binding.eval "x = 2"
end

x = 1
foo(binding)
p x     # => 2

Blocks also carry with them the binding in which they were defined, so if a block is being passed, then:

def foo(&block)
  block.binding.eval "x = 2"
end

x = 1
foo {}
p x     # => 2

What's in the block doesn't matter, in this case. It's just being used as a carrier for the binding.

Other ways to achieve the same goal

Yield an object

A more pedestrian way for a block to interact with it's caller is to pass an object to the block:

class Point
  attr_accessor :x
  attr_accessor :y
end

class Location

  def set
    point = Point.new
    yield point
    p point.x    # => 10
    p point.y    # => 20
  end

end

location = Location.new
location.set do |point|
  point.x = 10
  point.y = 20
end

This is often preferred to fancier techniques: It's easy to understand both its implementation and its use.

instance_eval an object

If you want to (but you probably shouldn't want to), the block's caller can use instance_eval/instance_exec to call the block. This sets self to the object, for that block.

class Location

  def set(&block)
    point = Point.new
    point.instance_eval(&block)
    p point.x    # => 10
    p point.y    # => 20
  end

end

location = Location.new
location.set do
  self.x = 10
  self.y = 20
end

You see that the block had to use use self. when calling the writers, otherwise new, local variables would have been declared, which is not what is wanted here.

Yield or instance_eval an object

Even though you still probably shouldn't use instance_eval, sometimes it's useful. You don't always know when it's good, though, so let's let the method's caller decide which technique to use. All the method has to do is to check that the block has parameters:

class Location

  def set(&block)
    point = Point.new
    if block.arity == 1
      block.call point
    else
      point.instance_eval(&block)
    end
    p point.x
    p point.y
  end

end

Now the user can have the block executed in the scope of the point:

location = Location.new
location.set do
  self.x = 10
  self.y = 20
end
# => 10
# => 20

or it can have the point passed to it:

location.set do |point|
  point.x = 30
  point.y = 40
end
# => 30
# => 40

edited May 23, 2017 at 12:05

CommunityBot

11 silver badge

answered Jun 8, 2014 at 15:15

Wayne Conrad

109k26 gold badges158 silver badges195 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

uros calakovic Over a year ago

Thanks for the detailed explanation, the 'yield an object' looks good.

Wayne Conrad Over a year ago

@uroscalakovic You are welcome. I'm glad it was helpful, and that the plain approach will work for you.

Arup Rakshit Over a year ago

@WayneConrad Ahh! Nice effort to club them together.. :)

Collectives™ on Stack Overflow

Setting method local variables from a proc

1 Answer 1

You can do it, sort of, but it isn't pretty

Other ways to achieve the same goal

Yield an object

instance_eval an object

Yield or instance_eval an object

3 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

You can do it, sort of, but it isn't pretty

Other ways to achieve the same goal

Yield an object

instance_eval an object

Yield or instance_eval an object

3 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related