instance variable in block when using define_method

Question

I'm trying to do a DSL, in which the user can pass a block and expect an instance variable @arg to be defined. This is a full example with a unit test failing:

# Implementation
class Filter
  def initialize
    @arg = 'foo'
  end

  def self.filters &block
    define_method :filter do |els|
      els.select &block
    end
  end
end

# Usage
class Foo < Filter
  filters {|el| el == @arg}
end

# Expected behavior
describe 'filters created with the DSL' do
  subject { Foo.new }
  it 'can use @arg in the filters block' do
    els = %w[notthearg  either  foo  other]
    expect(subject.filter els).to be_eql(['foo'])
  end
end

Using pry or putting puts statements inside the block, I can see that @arg is nil. But Foo.new.instance_variable_get :@arg correctly outputs foo, so it must be related to some scoping rules.

What do I need to change in the implementation to make the test pass and the DSL to work?

@Stefan The @arg of the class in which I'm calling filter. How could I tweak the implementation then so the @arg in the block gets evaluated in the scope of the instantiated class? — RedPointyJackson
– RedPointyJackson, Commented May 28, 2019 at 10:43
@Stefan: filter is an instance method. filters, which sets up filter, is a class method. However, filters captures a block in a class context and passes it on to filter; OP wishes to have the block executed in the instance context. (Not saying it's possible or impossible just yet, my brain hurts.) — Amadan
– Amadan, Commented May 28, 2019 at 11:07
@Amadan Stefan nailed it in the last comment, that's my problem exactly. I need a class method for filters so I can create the DSL in the post (which is part of a bigger project), or at least I think I do. The end objective here is to tweak the implementation so the test passes for the example filter Foo that I have created. — RedPointyJackson
– RedPointyJackson, Commented May 28, 2019 at 11:15
@Amadan oh I see, I thought filter and filters were the same. I missed the define_method call. — Stefan
– Stefan, Commented May 28, 2019 at 11:44

Amadan · Accepted Answer · 2019-05-28 11:26:09Z

3

instance_exec to rescue!

class Filter
  def initialize
    @arg = 'foo'
  end

  def self.filters &block
    define_method :filter do |els|
      els.select { |e| self.instance_exec(e, &block) }
    end
  end
end

class Foo < Filter
  filters {|el| el == @arg }
end

Foo.new.filter(%w[notthearg  either  foo  other])
# => ["foo"]

Caution: Make sure this is very well documented, since any shenanigans involving instance_exec or its cousins are breaking programmer expectations left and right - by design, you're destroying the concept of "scope". I'm pretty sure OP knows this, but it is worth putting down on the proverbial paper.

Also, consider using accessors rather than plain instance variables - accessors are checked, and variables are not. i.e. { |el| el == urg } will result in an error, but { |el| el == @urg } will silently fail (and filter for nil).

edited May 28, 2019 at 11:26

answered May 28, 2019 at 11:15

Amadan

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1 Comment

RedPointyJackson Over a year ago

You are right about the accessors, I'll do that. Thanks!

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