Why does instance_eval succeed with a Proc but not with a Lambda?

Question

I have the following class:

class User
  code1 = Proc.new { }
  code2 = lambda { }

  define_method :test do
    self.class.instance_eval &code1
    self.class.instance_eval &code2
  end
end

User.new.test

Why does the secondinstance_eval fail with a wrong number of arguments (1 for 0) error?

Answer 1

instance_eval is yielding self (User) to the lambda. Lambdas are particular about their parameters - in the same way methods are - and will raise an ArgumentError if there are too few/many.

class User
  code1 = Proc.new { |x| x == User } # true
  code2 = lambda { |x| x == User }   # true

  define_method :test do
    self.class.instance_eval &code1
    self.class.instance_eval &code2
  end
end

Relevant: What's the difference between a proc and a lambda in Ruby?

Answer 2

If you still want to use lambda, this code will work:

block = lambda { "Hello" } # or -> { "Hello" }
some_obj.instance_exec(&block)

instance_exec contrary to instance_eval will not supply self as an argument to the given block, so wrong number of arguments (1 for 0) won't be thrown.

Look here for more info.