accessing instance methods from class method proc

Question

I am trying to call a proc on a class but access instance methods from it with an inherited class. I think some mock code will make more sense :)

class Bar
  def self.foo &block
    @foo ||= block
  end
  def foo; self.class.foo.call(); end
  def bar; 'bar'; end
end

class Foo < Bar
  foo do
    bar
  end
end

Foo.new.foo
# NameError: undefined local variable or method `bar' for Foo:Class

I want to be able to access the bar instance method on the Bar class. The reason for calling the foo class method with a block from the inherited class is part of the DSL requirement, but any suggestions for a better design would be appreciated.

@SergioTulentsev HA! sorry... im not very creative with my contrived code :) — brewster
– brewster, Commented Nov 23, 2013 at 19:04

Jörg W Mittag · Accepted Answer · 2013-11-23 12:06:55Z

1

Blocks are lexically scoped, including the value of self. At the point where the block is defined, self is Bar and Bar doesn't respond to bar. You need to evaluate the block in the context of the object (in this case an instance of Bar and not Bar itself) whose methods you want to call. That's what instance_eval does:

class Bar
  def self.foo(&block) @foo ||= block end
  def foo; instance_eval(&self.class.foo) end
  def bar; 'bar' end
end

class Foo < Bar; foo do bar end end

Foo.new.foo
# => 'bar'

Note that all the usual disclaimers about instance_eval apply: since you change the value of self methods and instance variables that the block author may expect to be available won't be.

answered Nov 23, 2013 at 12:06

Jörg W Mittag

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1 Comment

brewster Over a year ago

perfect! i was actually playing with instance_eval earlier, but it was not working since I was not using the ampersand to convert it to a proc. But now it makes sense. Thanks you!

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